c3p0 - JDBC3 Connection and Statement Pooling

by Steve Waldman <swaldman@mchange.com>

© 2015 Machinery For Change, Inc.

This software is made available for use, modification, and redistribution, under the terms of the Lesser GNU Public License, v.2.1 (LGPL) or the Eclipse Public License, v.1.0 (EPL), at your option. You should have received copies of both licenses with this distribution.

Note: Coordinates of this version on the Maven central repository: [groupId: com.mchange, artifactId: c3p0, version:]


  1. Contents
  2. Quickstart
  3. What is c3p0?
  4. Prerequisites
  5. Installation
  6. Using c3p0
    1. Using ComboPooledDataSource
    2. Using the DataSouces factory class
    3. Querying Pool Status
    4. Cleaning Up Pool Resources
    5. Advanced: Building Your Own PoolBackedDataSource
    6. Advanced: Raw Connection and Statement Operations
  7. Configuration
    1. Introduction
    2. Basic Pool Configuration
    3. Managing Pool Size and Connection Age
    4. Configuring Connection Testing
    5. Configuring Statement Pooling
    6. Configuring Recovery From Database Outages
    7. Managing Connection Lifecycles with Connection Customizers
    8. Configuring Unresolved Transaction Handling
    9. Configuring To Debug and Workaround Broken Client Applications
    10. Configuring To Avoid Memory Leaks On Redeploy
    11. Other DataSource Configuration
    12. Configuring and Managing c3p0 via JMX
    13. Configuring Logging
    14. Named configurations
    15. Per-user configurations
    16. User extensions to configuration
    17. Mixing named, per-user, and user-defined configuration extensions
  8. Performance
  9. Known shortcomings
  10. Feedback and support
  11. Appendix A: Configuration Properties
  12. Appendix B: Configuation Files, etc.
    1. Overriding c3p0 defaults with a c3p0.properties file
    2. Overriding c3p0 defaults with "HOCON" (typesafe-config) configuration files
    3. Overriding c3p0 defaults with System properties
    4. Named and Per-User configuration: Overriding c3p0 defaults via c3p0-config.xml
    5. Precedence of Configuration Settings
  13. Appendix C: Hibernate-specific notes
  14. Appendix D: Configuring c3p0 pooled DataSources for Apache Tomcat
  15. Appendix E: JBoss-specific notes
  16. Appendix F: Oracle-specific API: createTemporaryBLOB() and createTemporaryCLOB()
(See also the API Documentation here)

QuickstartGo To Top

c3p0 was designed to be butt-simple to use. Just put the files lib/c3p0- and lib/mchange-commons-java-0.2.10.jar in your application's effective CLASSPATH, then make a DataSource like this:

import com.mchange.v2.c3p0.*; ... ComboPooledDataSource cpds = new ComboPooledDataSource(); cpds.setDriverClass( "org.postgresql.Driver" ); //loads the jdbc driver cpds.setJdbcUrl( "jdbc:postgresql://localhost/testdb" ); cpds.setUser("dbuser"); cpds.setPassword("dbpassword");

[Optional] If you want to turn on PreparedStatement pooling, you must also set maxStatements and/or maxStatementsPerConnection (both default to 0):

cpds.setMaxStatements( 180 );

Do whatever you want with your DataSource, which will be backed by a Connection pool set up with default parameters. You can bind the DataSource to a JNDI name service, or use it directly, as you prefer.

When you are done, you can clean up the DataSource you've created like this:


That's it! The rest is detail.

What is c3p0?Go To Top

c3p0 is an easy-to-use library for making traditional JDBC drivers "enterprise-ready" by augmenting them with functionality defined by the jdbc3 spec and the optional extensions to jdbc2. As of version 0.9.5, c3p0 fully supports the jdbc4 spec.

In particular, c3p0 provides several useful services:

The library tries hard to get the details right:

c3p0 hopes to provide DataSource implementations more than suitable for use by high-volume "J2EE enterprise applications". Please provide feedback, bug-fixes, etc!

PrerequisitesGo To Top

c3p0- requires a level 1.6.x or above Java Runtime Environment.

InstallationGo To Top

Place the files lib/c3p0- and lib/mchange-commons-java-0.2.10.jar somewhere in your CLASSPATH (or any other place where your application's classloader will find it). That's it!

Using c3p0Go To Top

From a users' perspective, c3p0 simply provides standard jdbc DataSource objects. When creating these DataSources, users can control pooling-related, naming-related, and other properties. (See Appendix A for a comprehensive list of configuration properties.) All pooling is entirely transparent to users once a DataSource has been created.

There are three ways of acquiring c3p0 pool-backed DataSources: 1) directly instantiate and configure a ComboPooledDataSource bean; 2) use the DataSources factory class; or 3) "build your own" pool-backed DataSource by directly instantiating PoolBackedDataSource and setting its ConectionPoolDataSource. Most users will probably find instantiating ComboPooledDataSource to be the most convenient approach. Once instantiated, c3p0 DataSources can be bound to nearly any JNDI-compliant name service.

Regardless of how you create your DataSource, c3p0 will use defaults for any configuration parameters that you do not specify programmatically. c3p0 has built-in, hard-coded defaults, but you can override these with configuration files, placed as top-level resources in the same CLASSPATH (or ClassLoader) that loads c3p0's jar file.

c3p0 DataSources may be configured via simple java.util.Properties files called c3p0.properties, via more advanced HOCON configuration files (e.g. application.conf, application.json), or in an XML format, c3p0-config.xml. See Configuration below.

Instantiating and Configuring a ComboPooledDataSource Go To Top

Perhaps the most straightforward way to create a c3p0 pooling DataSource is to instantiate an instance of com.mchange.v2.c3p0.ComboPooledDataSource. This is a JavaBean-style class with a public, no-arg constructor, but before you use the DataSource, you'll have to be sure to set at least the property jdbcUrl. You may also want to set user and password, and, if you use an old-style JDBC driver that you will not externally preload, you should set the driverClass.

ComboPooledDataSource cpds = new ComboPooledDataSource(); cpds.setDriverClass( "org.postgresql.Driver" ); //loads the jdbc driver cpds.setJdbcUrl( "jdbc:postgresql://localhost/testdb" ); cpds.setUser("swaldman"); cpds.setPassword("test-password"); // the settings below are optional -- c3p0 can work with defaults cpds.setMinPoolSize(5); cpds.setAcquireIncrement(5); cpds.setMaxPoolSize(20); // The DataSource cpds is now a fully configured and usable pooled DataSource ...

The default values of any c3p0 DataSource property are determined by configuration you supply, or else revert to hard-coded defaults [see configuration properties]. c3p0 supports named configurations so that you can configure multiple DataSources. If you wish to use a named configuration, construct your com.mchange.v2.c3p0.ComboPooledDataSource with the configuration name as a constructor agument:

ComboPooledDataSource cpds = new ComboPooledDataSource("intergalactoApp");

Of course, you can still override any configuration properties programmatically, as above.

Using the DataSources factory class Go To Top

Alternatively, you can use the static factory class com.mchange.v2.c3p0.DataSources to build unpooled DataSources from traditional JDBC drivers, and to build pooled DataSources from unpooled DataSources:

DataSource ds_unpooled = DataSources.unpooledDataSource("jdbc:postgresql://localhost/testdb", "swaldman", "test-password"); DataSource ds_pooled = DataSources.pooledDataSource( ds_unpooled ); // The DataSource ds_pooled is now a fully configured and usable pooled DataSource. // The DataSource is using a default pool configuration, and Postgres' JDBC driver // is presumed to have already been loaded via the jdbc.drivers system property or an // explicit call to Class.forName("org.postgresql.Driver") elsewhere. ...

If you use the DataSources factory class, and you want to programmatically override default configuration parameters, you can supply a map of override properties:

DataSource ds_unpooled = DataSources.unpooledDataSource("jdbc:postgresql://localhost/testdb", "swaldman", "test-password"); Map overrides = new HashMap(); overrides.put("maxStatements", "200"); //Stringified property values work overrides.put("maxPoolSize", new Integer(50)); //"boxed primitives" also work // create the PooledDataSource using the default configuration and our overrides ds_pooled = DataSources.pooledDataSource( ds_unpooled, overrides ); // The DataSource ds_pooled is now a fully configured and usable pooled DataSource, // with Statement caching enabled for a maximum of up to 200 statements and a maximum // of 50 Connections. ...

If you are using named configurations, you can specify the configuration that defines the default configuration for your DataSource:

// create the PooledDataSource using the a named configuration and specified overrides // "intergalactoApp" is a named configuration ds_pooled = DataSources.pooledDataSource( ds_unpooled, "intergalactoApp", overrides );

Show deprecated PoolConfig approach...

RARE: Forcing authentication information, regardless of (mis)configuration of the underlying (unpooled) DataSource

You can wrap any DataSouce using DataSource.pooledDataSource( ... ), usually with no problem whatsoever. DataSources are supposed to indicate the username and password associated by default with Connections via standard properties user and password. Some DataSource implementations do not offer these properties. Usually this is not at all a problem. c3p0 works around this by acquiring "default" Connections from the DataSource if it can't find default authentication information, and a client has not specified the authentification information via getConnection( user, password ).

However, in rare circumstances, non-c3p0 unpooled DataSources provide a user property, but not a password property, or you have access to a DataSource that you wish to wrap behind a pool, but you wish to override its build-in authentification defaults without actually modifying the user or password properties.

c3p0 provides configuation properties overrideDefaultUser and overrideDefaultPassword. If you set these properties, programmatically as above, or via any of c3p0's configuration mechanisms, c3p0 PooledDataSources will ignore the user and password property associated with the underlying DataSource, and use the specified overrides instead.

Querying a PooledDataSource's current status Go To Top

c3p0 DataSources backed by a pool, which include implementations of ComboPooledDataSource and the objects returned by DataSources.pooledDataSource( ... ), all implement the interface com.mchange.v2.c3p0.PooledDataSource, which makes available a number of methods for querying the status of DataSource Connection pools. Below is sample code that queries a DataSource for its status:

// fetch a JNDI-bound DataSource InitialContext ictx = new InitialContext(); DataSource ds = (DataSource) ictx.lookup( "java:comp/env/jdbc/myDataSource" ); // make sure it's a c3p0 PooledDataSource if ( ds instanceof PooledDataSource) { PooledDataSource pds = (PooledDataSource) ds; System.err.println("num_connections: " + pds.getNumConnectionsDefaultUser()); System.err.println("num_busy_connections: " + pds.getNumBusyConnectionsDefaultUser()); System.err.println("num_idle_connections: " + pds.getNumIdleConnectionsDefaultUser()); System.err.println(); } else System.err.println("Not a c3p0 PooledDataSource!");

The status querying methods all come in three overloaded forms, such as:

c3p0 maintains separate pools for Connections with distinct authentications. The various methods let you query the status of pools individually, or aggregate statistics for all authentifications for which your DataSource is maintaining pools. Note that pool configuration parmeters such as maxPoolSize are enforced on a per-authentification basis! For example, if you have set maxPoolSize to 20, and if the DataSource is managing connections under two username-password pairs [the default, and one other pair established via a call to getConnection(user, password), you should expect to see as many as 40 Connections from getNumConnectionsAllUsers().

Most applications only acquire default-authenticated Connections from DataSources, and can typically just use the getXXXDefaultUser() to gather Connection statistics.

As well as Connection pool realted statistics, you can retrieve status information about each DataSource's Thread pool. Please see PooledDataSource for a complete list of available operations.

Using C3P0Registry to get a reference to a DataSource

If it's inconvenient or impossible to get a reference to your DataSource via JNDI or some other means, you can find all live c3p0 DataSources using the C3P0Registry class, which includes three static methods to help you out:

The first method will hand you the Set of all live c3p0 PooledDataSources. If you are sure your application only makes one PooledDataSources, or you can distinguish between the DataSources by their configuration properties (inspected via "getters"), the first method may be sufficient. Because this will not always be the case, c3p0 PooledDataSources have a special property called dataSourceName. You can set the dataSourceName property directly when you construct your DataSource, or dataSourceName can be set like any other property in a named or the default config. Otherwise, dataSourceName will default to either 1) the name of your DataSource's configuration, if you constructed it with a named configuration; or 2) a unique (but unpredicatble) name if you are using the default configuration.

There is no guarantee that a dataSourceName will be unique. For example, if two c3p0 DataSources share the same named configuration, and you have not set the dataSourceName programmatically, the two data sources will both share the name of the configuration. To get all of the DataSources with a particular dataSourceName, use pooledDataSourcesByName( ... ). If you've ensured that your DataSource's name is unique (as you will generally want to do, if you intend to use C3P0Registry to find your DataSources), you can use the convenience method pooledDataSourceByName( ... ), which will return your DataSource directly, or null if no DataSource with that name is available. If you use pooledDataSourceByName( ... ) and more than one DataSource shares the name supplied, which one it will return is undefined.

Cleaning up after c3p0 PooledDataSources Go To Top

The easy way to clean up after c3p0-created DataSources is to use the static destroy method defined by the class DataSources. Only PooledDataSources need to be cleaned up, but DataSources.destroy( ... ) does no harm if it is called on an unpooled or non-c3p0 DataSource.

DataSource ds_pooled = null; try { DataSource ds_unpooled = DataSources.unpooledDataSource("jdbc:postgresql://localhost/testdb", "swaldman", "test-password"); ds_pooled = DataSources.pooledDataSource( ds_unpooled ); // do all kinds of stuff with that sweet pooled DataSource... } finally { DataSources.destroy( ds_pooled ); }

Alternatively, c3p0's PooledDataSource interface contains a close() method that you can call when you know you are finished with a DataSource. So, you can cast a c3p0 derived DataSource to a PooledDataSource and close it:

static void cleanup(DataSource ds) throws SQLException { // make sure it's a c3p0 PooledDataSource if ( ds instanceof PooledDataSource) { PooledDataSource pds = (PooledDataSource) ds; pds.close(); } else System.err.println("Not a c3p0 PooledDataSource!"); }

ComboPooledDataSource is an instance of PooledDataSource, and can be closed directly via its close() method. PooledDataSource implements java.lang.AutoCloseable, so they may be managed by Java 7+ try-with-resources blocks.

Unreferenced instances of PooledDataSource that are not close()ed by clients close() themselves prior to garbage collection in their finalize() methods. As always, finalization should be considered a backstop and not a prompt or sure approach to resource cleanup.

Advanced: Building your own PoolBackedDataSource Go To Top

There is little reason for most programmers to do this, but you can build a PooledDataSource in a step-by-step way by instantiating and configuring an unpooled DriverManagerDataSource, instantiating a WrapperConnectionPoolDataSource and setting the unpooled DataSource as its nestedDataSource property, and then using that to set the connectionPoolDataSource property of a new PoolBackedDataSource.

This sequence of events is primarily interesting if your driver offers an implementation of ConnectionPoolDataSource, and you'd like c3p0 to use that. Rather than using c3p0's WrapperConnectionPoolDataSource, you can create a PoolBackedDataSource and set its connectionPoolDataSource property. Statement pooling, ConnectionCustomizers, and many c3p0-specific properties are unsupported with third party implementations of ConnectionPoolDataSource. (Third-party DataSource implementations can be substituted for c3p0's DriverManagerDataSource with no significant loss of functionality.)

Advanced: Raw Connection and Statement Operations Go To Top

Note: As of c3p0-0.9.5, c3p0 supports the standard JDBC4 unwrap() methods for looking through proxies. Note that if you use the unwrap() methods, c3p0 can not clean up any Statement or ResultSet objects you may generate from raw Connections or Statements. Users must take care to clean up these objects directly. Further, users should take care not to modify underlying Connections in some manner that would render them no longer interchangeable with other Connections, as they must be to remain suitable for pooling.

JDBC drivers sometimes define vendor-specific, non-standard API on Connection and Statement implementations. C3P0 wraps these Objects behind a proxies, so you cannot cast C3P0-returned Connections or Statements to the vendor-specific implementation classes. C3P0 does not provide any means of accessing the raw Connections and Statements directly, because C3P0 needs to keep track of Statements and ResultSets created in order to prevent resource leaks and pool corruption.

C3P0 does provide an API that allows you to invoke non-standard methods reflectively on an underlying Connection. To use it, first cast the returned Connection to a C3P0ProxyConnection. Then call the method rawConnectionOperation, supplying the java.lang.reflect.Method object for the non-standard method you wish to call as an argument. The Method you supply will be invoked on the target you provide on the second argument (null for static methods), and using the arguments you supply in the third argument to that function. For the target, and for any of the method arguments, you can supply the special token C3P0ProxyConnection.RAW_CONNECTION, which will be replaced with the underlying vendor-specific Connection object before the Method is invoked.

C3P0ProxyStatement offers an exactly analogous API.

Any Statements (including Prepared and CallableStatements) and ResultSets returned by raw operations will be c3p0-managed, and will be properly cleaned-up on close() of the parent proxy Connection. Users must take care to clean up any non-standard resources returned by a vendor-specific method.

Here's an example of using Oracle-specific API to call a static method on a raw Connection:

C3P0ProxyConnection castCon = (C3P0ProxyConnection) c3p0DataSource.getConnection(); Method m = CLOB.class.getMethod("createTemporary", new Class[]{Connection.class, boolean.class, int.class}); Object[] args = new Object[] {C3P0ProxyConnection.RAW_CONNECTION, Boolean.valueOf( true ), new Integer( 10 )}; CLOB oracleCLOB = (CLOB) castCon.rawConnectionOperation(m, null, args);

C3P0 includes special support for some Oracle-specific methods. See Appendix F.

ConfigurationGo To Top

Introduction Go To Top

While c3p0 does not require very much configuration, it is very tweakable. Most of the interesting knobs and dials are represented as JavaBean properties. Following JavaBean conventions, we note that if an Object has a property of type T called foo, it will have methods that look like...

public T getFoo();
public void setFoo(T foo);
...or both, depending upon whether the property is read-only, write-only, or read-writable.

There are several ways to modify c3p0 properties: You can directly alter the property values associated with a particular DataSource in your code, or you can configure c3p0 externally...

Configuration files are normally looked up under standard names (c3p0.properties or c3p0-config.xml) at the top level of an application's classpath, but the XML configuration can be placed anywhere in an application's file system or classpath, if the system property com.mchange.v2.c3p0.cfg.xml is set.

DataSources are usually configured before they are used, either during or immediately following their construction. c3p0 does support property modifications midstream, however.

If you obtain a DataSource by instantiating a ComboPooledDataSource, configure it by simply calling appropriate setter methods offered by that class before attempting a call to getConnection(). See the example above.

If you obtain a DataSource by using factory methods of the utility class com.mchange.v2.c3p0.DataSources, and wish to use a non-default configuration, you can supply a Map of property names (beginning with lower-case letters) to property values (either as Strings or "boxed" Java primitives like Integer or Boolean).

All tweakable properties are documented for reference in Appendix A. Most c3p0 configuration topics are discussed in detail below.

Basic Pool Configuration Go To Top

c3p0 Connection pools are very easy to configure via the following basic parameters:

initialPoolSize, minPoolSize, maxPoolSize define the number of Connections that will be pooled. Please ensure that minPoolSize <= maxPoolSize. Unreasonable values of initialPoolSize will be ignored, and minPoolSize will be used instead.

Within the range between minPoolSize and maxPoolSize, the number of Connections in a pool varies according to usage patterns. The number of Connections increases whenever a Connection is requested by a user, no Connections are available, and the pool has not yet reached maxPoolSize in the number of Connections managed.

Since Connection acquisition is very slow, it is almost always useful to increase the number of Connections eagerly, in batches, rather than forcing each client to wait for a new Connection to provoke a single acquisition when the load is increasing. acquireIncrement determines how many Connections a c3p0 pool will attempt to acquire when the pool has run out of Connections. (Regardless of acquireIncrement, the pool will never allow maxPoolSize to be exceeded.)

The number of Connections in a pool decreases whenever a pool tests a Connection and finds it to be broken (see Configuring Connection Testing below), or when a Connection is expired by the pool after sitting idle for a period of time, or for being too old (See Managing Pool Size and Connection Age.)

Managing Pool Size and Connection Age Go To Top

Different applications have different needs with regard to trade-offs between performance, footprint, and reliability. C3P0 offers a wide variety of options for controlling how quickly pools that have grown large under load revert to minPoolSize, and whether "old" Connections in the pool should be proactively replaced to maintain their reliablity.

By default, pools will never expire Connections. If you wish Connections to be expired over time in order to maintain "freshness", set maxIdleTime and/or maxConnectionAge. maxIdleTime defines how many seconds a Connection should be permitted to go unused before being culled from the pool. maxConnectionAge forces the pool to cull any Connections that were acquired from the database more than the set number of seconds in the past.

maxIdleTimeExcessConnections is about minimizing the number of Connections held by c3p0 pools when the pool is not under load. By default, c3p0 pools grow under load, but only shrink if Connections fail a Connection test or are expired away via the parameters described above. Some users want their pools to quickly release unnecessary Connections after a spike in usage that forces a large pool size. You can achieve this by setting maxIdleTimeExcessConnections to a value much shorter than maxIdleTime, forcing Connections beyond your set minimum size to be released if they sit idle for more than a short period of time.

Some general advice about all of these timeout parameters: Slow down! The point of Connection pooling is to bear the cost of acquiring a Connection only once, and then to reuse the Connection many, many times. Most databases support Connections that remain open for hours at a time. There's no need to churn through all your Connections every few seconds or minutes. Setting maxConnectionAge or maxIdleTime to 1800 (30 minutes) is quite aggressive. For most databases, several hours may be more appropriate. You can ensure the reliability of your Connections by testing them, rather than by tossing them. (see Configuring Connection Testing.) The only one of these parameters that should generally be set to a few minutes or less is maxIdleTimeExcessConnections.

Configuring Connection Testing Go To Top

c3p0 can be configured to test the Connections that it pools in a variety of ways, to minimize the likelihood that your application will see broken or "stale" Connections. Pooled Connections can go bad for a variety of reasons -- some JDBC drivers intentionally "time-out" long-lasting database Connections; back-end databases or networks sometimes go down "stranding" pooled Connections; and Connections can simply become corrupted over time and use due to resource leaks, driver bugs, or other causes.

c3p0 provides users a great deal of flexibility in testing Connections, via the following configuration parameters:

idleConnectionTestPeriod, testConnectionOnCheckout, and testConnectionOnCheckin control when Connections will be tested. automaticTestTable, connectionTesterClassName, and preferredTestQuery control how they will be tested.

When configuring Connection testing, first try to minimize the cost of each test. If you are using a JDBC driver that you are certain supports the new(ish) jdbc4 API — and if you are using c3p0-0.9.5 or higher! — let your driver handle this for you. jdbc4 Connections include a method called isValid() that should be implemented as a fast, reliable Connection test. By default, c3p0 will use that method if it is present.

However, if your driver does not support this new-ish API, c3p0's default behavior is to test Connections by calling the getTables() method on a Connection's associated DatabaseMetaData object. This has the advantage of being very robust and working with any database, regardless of the database schema. However, a call to DatabaseMetaData.getTables() is often much slower than a simple database query, and using this test may significantly impair your pool's performance.

The simplest way to speed up Connection testing under a JDBC 3 driver (or a pre-0.9.5 version of c3p0) is to define a test query with the preferredTestQuery parameter. Be careful, however. Setting preferredTestQuery will lead to errors as Connection tests fail if the query target table does not exist in your database prior to initialization of your DataSource. Depending on your database and JDBC driver, a table-independent query like SELECT 1 may (or may not) be sufficient to verify the Connection. If a table-independent query is not sufficient, instead of preferredTestQuery, you can set the parameter automaticTestTable. Using the name you provide, c3p0 will create an empty table, and make a simple query against it to test the database.

The most reliable time to test Connections is on check-out. But this is also the most costly choice from a client-performance perspective. Most applications should work quite reliably using a combination of idleConnectionTestPeriod and testConnectionOnCheckin. Both the idle test and the check-in test are performed asynchronously, which can lead to better performance, both perceived and actual.

For some applications, high performance is more important than the risk of an occasional database exception. In its default configuration, c3p0 does no Connection testing at all. Setting a fairly long idleConnectionTestPeriod, and not testing on checkout and check-in at all is an excellent, high-performance approach.

It is possible to customize how c3p0's DefaultConnectionTester tests when no preferredTestQuery or automaticTestTable are available. Please see Configuring DefaultConnectionTester.isValidTimeout and Configuring DefaultConnectionTester.QuerylessTestRunner.

Advanced users may define any kind of Connection testing they wish, by implementing a ConnectionTester and supplying the fully qualified name of the class as connectionTesterClassName. If you'd like your custom ConnectionTesters to honor and support the preferredTestQuery and automaticTestTable parameters, implement UnifiedConnectionTester, most conveniently by extending AbstractConnectionTester. See the api docs for more information.

If you know you want to use the jdbc4 Connection.isValid() method, but you want to set a timeout, consider writing a trivial extension of IsValidConnectionTester.

package com.mchange.v2.c3p0.example; import com.mchange.v2.c3p0.util.IsValidOnlyConnectionTester; public final class IsValidOnlyConnectionTester30 extends IsValidOnlyConnectionTester { protected int getIsValidTimeout() { return 30; } }

See? These really are trivial to write.

Simple advice on Connection testing

If you don't know what to do, try this:

  1. If you know your driver supports the JDBC 4 Connection.isValid(...) method and you are using c3p0-0.9.5 or above, don't set a preferredTestQuery. If your driver does not support this method (or if you are not sure), try SELECT 1 for your preferredTestQuery, if you are running MySQL or Postgres. For other databases, look for suggestions here. Leave automatedTestTable undefined.

  2. Begin by setting testConnectionOnCheckout to true and get your application to run correctly and stably. If you are happy with your application's performance, you can stop here! This is the simplest, most reliable form of Connection-testing, but it does have a client-visible performance cost.

  3. If you'd like to improve performance by eliminating Connection testing from clients' code path:

    1. Set testConnectionOnCheckout to false

    2. Set testConnectionOnCheckin to true

    3. Set idleConnectionTestPeriod to 30, fire up you application and observe. This is a pretty robust setting, all Connections will tested on check-in and every 30 seconds thereafter while in the pool. Your application should experience broken or stale Connections only very rarely, and the pool should recover from a database shutdown and restart quickly. But there is some overhead associated with all that Connection testing.

    4. If database restarts will be rare so quick recovery is not an issue, consider reducing the frequency of tests by idleConnectionTestPeriod to, say, 300, and see whether clients are troubled by stale or broken Connections. If not, stick with 300, or try an even bigger number. Consider setting testConnectionOnCheckin back to false to avoid unnecessary tests on checkin. Alternatively, if your application does encounter bad Connections, consider reducing idleConnectionTestPeriod and set testConnectionOnCheckin back to true. There are no correct or incorrect values for these parameters: you are trading off overhead for reliability in deciding how frequently to test. The exact numbers are not so critical. It's usually easy to find configurations that perform well. It's rarely worth spending time in pursuit of "optimal" values here.

So, when should you stick with simple and reliable (Step 2 above), and when is it worth going for better performance (Step 3)? In general, it depends on how much work clients typically do with Connections once they check them out. If clients usually make complex queries and/or perform multiple operations, adding the extra cost of one fast test per checkout will not much affect performance. But if your application typically checks out a Connection and performs one simple query with it, throwing in an additional test can really slow things down.

That's nice in theory, but often people don't really have a good sense of how much work clients perform on average. The best thing to do is usually to try Step 3, see if it helps (however you measure performance), see if it hurts (is your application troubled by broken Connections? does it recover from database restarts well enough?), and then decide. You can always go back to simple, slow, and robust. Just set testConnectionOnCheckout to true, testConnectionOnCheckin to false, and set idleConnectionTestPeriod to 0.

But do, always, be sure that your tests themselves are performant, either because your JDBC driver supports Connection.isValid(...) or because you have set an efficient preferredTestQuery !!!

Configuring Statement Pooling Go To Top

c3p0 implements transparent PreparedStatement pooling as defined by the JDBC spec. Under some circumstances, statement pooling can dramatically improve application performance. Under other circumstances, the overhead of statement pooling can slightly harm performance. Whether and how much statement pooling will help depends on how much parsing, planning, and optimizing of queries your databases does when the statements are prepared. Databases (and JDBC drivers) vary widely in this respect. It's a good idea to benchmark your application with and without statement pooling to see if and how much it helps.

You configure statement pooling in c3p0 via the following configuration parameters:

maxStatements is JDBC's standard parameter for controlling statement pooling. maxStatements defines the total number PreparedStatements a DataSource will cache. The pool will destroy the least-recently-used PreparedStatement when it hits this limit. This sounds simple, but it's actually a strange approach, because cached statements conceptually belong to individual Connections; they are not global resources. To figure out a size for maxStatements that does not "churn" cached statements, you need to consider the number of frequently used PreparedStatements in your application, and multiply that by the number of Connections you expect in the pool (maxPoolSize in a busy application).

maxStatementsPerConnection is a non-standard configuration parameter that makes a bit more sense conceptually. It defines how many statements each pooled Connection is allowed to own. You can set this to a bit more than the number of PreparedStatements your application frequently uses, to avoid churning.

If either of these parameters are greater than zero, statement pooling will be enabled. If both parameters are greater than zero, both limits will be enforced. If only one is greater than zero, statement pooling will be enabled, but only one limit will be enforced.

If statementCacheNumDeferredCloseThreads is greater than zero, the Statement pool will defer physically close()ing cached Statements until its parent Connection is not in use by any client or internally (in e.g. a test) by the pool itself. For some JDBC drivers (especially Oracle), attempts to close a Statement freeze if the parent Connection is in use. This parameter defaults to 0. Set it to a positive value if you observe "APPARENT DEADLOCKS" realted to Connection close tasks. Almost always, that value should be one: if you need more than one Thread dedicated solely to Statement destruction, you probably should set maxStatements and/or maxStatementsPerConnection to higher values so you don't churn through cached Statements so quickly.

Configuring Recovery From Database Outages Go To Top

c3p0 DataSources are designed (and configured by default) to recover from temporary database outages, such as those which occur during a database restart or brief loss of network connectivity. You can affect how c3p0 handles errors in acquiring Connections via the following configurable properties:

When a c3p0 DataSource attempts and fails to acquire a Connection, it will retry up to acquireRetryAttempts times, with a delay of acquireRetryDelay between each attempt. If all attempts fail, any clients waiting for Connections from the DataSource will see an Exception, indicating that a Connection could not be acquired. Note that clients do not see any Exception until a full round of attempts fail, which may be some time after the initial Connection attempt. If acquireRetryAttempts is set to 0, c3p0 will attempt to acquire new Connections indefinitely, and calls to getConnection() may block indefinitely waiting for a successful acquisition.

Once a full round of acquisition attempts fails, there are two possible policies. By default, the c3p0 DataSource will remain active, and will try again to acquire Connections in response to future requests for Connections. If you set breakAfterAcquireFailure to true, the DataSource will consider itself broken after a failed round of Connection attempts, and future client requests will fail immediately.

Note that if a database restart occurs, a pool may contain previously acquired but now stale Connections. By default, these stale Connections will only be detected and purged lazily, when an application attempts to use them, and sees an Exception. Setting maxIdleTime or maxConnectionAge can help speed up the replacement of broken Connections. (See Managing ConnectionAge.) If you wish to avoid application Exceptions entirely, you must adopt a connection testing strategy that is likely to detect stale Connections prior to their delivery to clients. (See "Configuring Connection Testing".) Even with active Connection testing (testConnectionOnCheckout set to true, or testConnectionOnCheckin and a short idleConnectionTestPeriod), your application may see occasional Exceptions on database restart, for example if the restart occurs after a Connection to the database has already been checked out.

Managing Connection Lifecycles with Connection Customizer Go To Top

Application frequently wish to set up Connections in some standard, reusable way immediately after Connection acquisitions. Examples of this include setting-up character encodings, or date and time related behavior, using vendor-specific APIs or non-standard SQL statement executions. Occasionally it is useful to override the default values of standard Connection properties such as transactionIsolation, holdability, or readOnly. c3p0 provides a "hook" interface that you can implement, which gives you the opportunity to modify or track Connections just after they are checked out from the database, immediately just prior to being handed to clients on checkout, just prior to being returned to the pool on check-in, and just prior to final destruction by the pool. The Connections handed to ConnectionCustomizers are raw, physical Connections, with all vendor-specific API accessible. See the API docs for ConnectionCustomizer.

To install a ConnectionCustomizer just implement the interface, make your class accessible to c3p0's ClassLoader, and set the configuration parameter below:

ConnectionCustomizers are required to be immutable classes with public no argument constructors. They shouldn't store any state. For (rare) applications that wish to track the behavior of individual DataSources with ConnectionCustomizers, the lifecycle methods each accept a DataSource-specific "identityToken", which is unique to each PooledDataSource. ConnectionCustomizers can be configured via user-defined configuration extensions.

Below is a sample ConnectionCustomizer. Implementations that do not need to override all four ConnectionCustomizer methods can extend AbstractConnectionCustomizer to inherit no-op implementations of all methods.

import com.mchange.v2.c3p0.*; import java.sql.Connection; public class VerboseConnectionCustomizer { public void onAcquire( Connection c, String pdsIdt ) { System.err.println("Acquired " + c + " [" + pdsIdt + "]"); // override the default transaction isolation of // newly acquired Connections c.setTransactionIsolation( Connection.REPEATABLE_READ ); } public void onDestroy( Connection c, String pdsIdt ) { System.err.println("Destroying " + c + " [" + pdsIdt + "]"); } public void onCheckOut( Connection c, String pdsIdt ) { System.err.println("Checked out " + c + " [" + pdsIdt + "]"); } public void onCheckIn( Connection c, String pdsIdt ) { System.err.println("Checking in " + c + " [" + pdsIdt + "]"); } }

For an example ConnectionCustomizer that employs user-defined configuration properties, see below.

Configuring Unresolved Transaction Handling Go To Top

Connections checked into a pool cannot have any unresolved transactional work associated with them. If users have set autoCommit to false on a Connection, and c3p0 cannot guarantee that there is no pending transactional work, c3p0 must either rollback() or commit() on check-in (when a user calls close()). The JDBC spec is (unforgivably) silent on the question of whether unresolved work should be committed or rolled back on Connection close. By default, c3p0 rolls back unresolved transactional work when a user calls close().

You can adjust this behavior via the following configuration properties:

If you wish c3p0 to allow unresolved transactional work to commit on checkin, set autoCommitOnClose to true. If you wish c3p0 to leave transaction management to you, and neither commit nor rollback (nor modify the state of Connection autoCommit), you may set forceIgnoreUnresolvedTransactions to true. Setting forceIgnoreUnresolvedTransactions is strongly discouraged, because if clients are not careful to commit or rollback themselves prior to close(), or do not set Connection autoCommit consistently, bizarre unreproduceable behavior and database lockups can occur.

Configuring to Debug and Workaround Broken Client Applications Go To Top

Sometimes client applications are sloppy about close()ing all Connections they check out. Eventually, the pool grows to maxPoolSize, and then runs out of Connections, because of these bad clients.

The right way to address this problem is to fix the client application. c3p0 can help you debug, by letting you know where Connections are checked out that occasionally don't get checked in. In rare and unfortunate situations, development of the client application is closed, and even though it is buggy, you cannot fix it. c3p0 can help you work around the broken application, preventing it from exhausting the pool.

The following parameters can help you debug or workaround broken client applications.

unreturnedConnectionTimeout defines a limit (in seconds) to how long a Connection may remain checked out. If set to a nozero value, unreturned, checked-out Connections that exceed this limit will be summarily destroyed, and then replaced in the pool. Obviously, you must take care to set this parameter to a value large enough that all intended operations on checked out Connections have time to complete. You can use this parameter to merely workaround unreliable client apps that fail to close() Connections.

Much better than working-around is fixing. If, in addition to setting unreturnedConnectionTimeout, you set debugUnreturnedConnectionStackTraces to true, then a stack trace will be captured each time a Connection is checked-out. Whenever an unreturned Connection times out, that stack trace will be printed, revealing where a Connection was checked out that was not checked in promptly. debugUnreturnedConnectionStackTraces is intended to be used only for debugging, as capturing a stack trace can slow down Connection check-out.

Configuring To Avoid Memory Leaks On Hot Redeploy Of Clients Go To Top

c3p0 spawns a variety of Threads (helper threads, java.util.Timer threads), and does so lazily in response to the first client request experienced by a PooledDataSource. By default, the Threads spawned by c3p0 inherit a java.security.AccessControlContext and a contextClassLoader property from this first-calling Thread. If that Thread came from a client that may need to be hot-undeployed, references to these objects may prevent the undeployed application, often partitioned into a ClassLoader, from being garbage collected. (See for example this description of Tomcat memory leaks on redeployment.)

c3p0 provides two configuration parameters that can help with this:

contextClassLoaderSource should be set to one of caller, library, or none. The default (which yields the default behavior described above) is caller. Set this to library to use c3p0's ClassLoader, so that no reference is maintained to a client that may need to be redeployed.

privilegeSpawnedThreads is a boolean, false by default. Set this to true so that c3p0's Threads use the the c3p0 library's AccessControlContext, rather than an AccessControlContext that may be associated with the client application and prevent its garbage collection.

Other DataSource Configuration Go To Top

See Appendix A for information about the following configuration properties:

numHelperThreads and maxAdministrativeTaskTime help to configure the behavior of DataSource thread pools. By default, each DataSource has only three associated helper threads. If performance seems to drag under heavy load, or if you observe via JMX or direct inspection of a PooledDataSource, that the number of "pending tasks" is usually greater than zero, try increasing numHelperThreads. maxAdministrativeTaskTime may be useful for users experiencing tasks that hang indefinitely and "APPARENT DEADLOCK" messages. (See Appendix A for more.)

Ordinarily check-ins are performed asynchronously so that clients do not experience the overhead of on-check-in Connection tests and/or operations specified in ConnectionCustomizer.onCheckIn(...). However, asynchronous checkins add to Thread pool congestion. Under loads so large that it is impractical to expand numHelperThreads to reduce congestion, forceSynchronousCheckins will cause client Threads to perform the checkin operations, adding to the load of the Thread pool and precluding any delays in termination of checkin due to Thread pool congestion. As long as you neither perform Connection tests on check-in (see testConnectionOnCheckin) nor perform database operations or other slow work in ConnectionCustomizer.onCheckIn(...), this setting is likely to improve performance. However, if Connections are tested on check-in, or custom work is performed, setting forceSynchronousCheckins will cause clients to experience delays associated with that work when they call Connection.close().

checkoutTimeout limits how long a client will wait for a Connection, if all Connections are checked out and one cannot be supplied immediately. usesTraditionalReflectiveProxies is of little practical use and is now formally deprecated. It permits you to use an old, now superceded implementation of C3P0-generated proxy objects. (C3P0 used to use reflective, dynamic proxies. Now, for enhanced performance, it uses code-generated, nonrefective implementations.) factoryClassLocation can be used to indicate where a URL from which c3p0 classes can be downloaded, if c3p0 DataSources will be retrieved as References from a JNDI DataSource by clients who do not have c3p0 locally installed.

Configuring and Managing c3p0 via JMX Go To Top

If JMX libraries and a JMX MBeanServer are available in your environment (they are include in JDK 1.5 and above), you can inspect and configure your c3p0 datasources via a JMX administration tool (such as jconsole, bundled with jdk 1.5). You will find that c3p0 registers MBeans under the domain com.mchange.v2.c3p0, one with statistics about the library as a whole (called C3P0Registry), and an MBean for each PooledDataSource you deploy. You can view and modify your DataSource's configuration properties, track the activity of Connection, Statement, and Thread pools, and reset pools and DataSources via the PooledDataSource MBean. (You may wish to view the API docs of PooledDataSource for documentation of the available operations.)

Configuring JMX Names

Each PooledDataSource within your application may have the following attributes embedded within its ObjectName:

The type will always be PooledDataSource. The identityToken is a unique String associated with each c3p0 DataSource. The name will be the value of the property dataSourceName, which you can set yourself to ensure that semanically equivalent data sources are identifiable across application restarts. If you do not set a dataSourceName, the name attribute may not be defined at all, or it may take some default value.

For example, the following might be the full JMX ObjectName, in String format, of a c3p0 DataSource whose dataSourceName is intergalactoApp:


c3p0 prefers to include identity tokens in JMX ObjectNames to ensure that every ObjectName is unique. If you can, stick with c3p0's default behavior. But if you really need to, you can configure c3p0 to exclude the identityToken attribute from ObjectNames, so that your PooledDataSources have predictable, reproducible names. Set the following, as a System property, in c3p0.properties, or in HOCON config:


This will lead to names missing the long identity token, names like


If you exclude identity tokens from JMX names you must ensure that each PooledDataSource always has a unique dataSourceName value! Otherwise, only one of the PooledDataSources with identical names will be accessible by JMX, and which one will be undefined. Excluding identity tokens from JMX names is particularly hazardous if you will initialize multiple DataSource from the same named configuration. By default, dataSourceName takes the value of the configuration name. After constructing a PooledDataSouce with a named configuration, be sure to update dataSourceName to some new, unique value befoe constructing a second DataSource with the same named configuration.

The singleton C3P0Registry is also repesented by an MBean. It may have the following attributes embedded within its JMX ObjectName:

The value of the name attribute is determined by the following property, which may be set as a System property, in c3p0.properties, or in HOCON config.


With the RegistryName shown above, the full JMX ObjectName, in String format, would be


If you do not explicitly set a RegistryName, no default value is used, no name attribute is embedded. The full JMX ObjectName in String format would be


Disabling JMX Support

If you do not want c3p0 to register MBeans with your JMX environment, you can suppress JMX support entirely. Set the following, as a System property, in c3p0.properties, or in HOCON config:


Configuring Logging Go To Top

c3p0 uses a custom logging library similar to jakarta commons-logging. Log messages can be directed to the to the popular slf4j (with its logback backend), to the venerable log4j library, to the standard logging facility introduced with jdk1.4, or to System.err. Nearly all configuration should be done at the level of your preferred logging library. There are a very few configuration options specific to c3p0's logging, and usually the defaults will be fine. Logging-related parameters may be placed in your c3p0.properties file, in HOCON configuration files, in a file called mchange-log.properties at the top-level of your classpath, or they may be defined as System properties. (The logging properties defined below may not be defined in c3p0-config.xml!) See the box below.

c3p0's logging behavior is affected by certain build-time options. If build-option c3p0.debug is set to false, all messages at a logging level below INFO will be suppressed. Build-option c3p0.trace controls how fine-grained c3p0's below INFO level reporting will be. For the moment, distributed c3p0 binaries are compiled with debug set to true and trace set to its maximum level of 10. But binaries may eventually be distributed with debug set to false. (For the moment, the performance impact of the logging level-checks seems very small, and it's most flexible to compile in all the messages, and let your logging library control which are emitted.) When c3p0 starts up, it emits the build-time values of debug and trace, along with the version and build time.

Determines which library c3p0 will output log messages to. By default, if slf4j is available, it will use that library, otherwise log4j if available, otherwise jdk1.4 logging apis, and if all are unavailable, it will fallback to logging via System.err. If you want to directly control which library is used, you may set this property to one of:
  • com.mchange.v2.log.slf4j.Slf4jMLog
  • com.mchange.v2.log.log4j.Log4jMLog
  • com.mchange.v2.log.jdk14logging.Jdk14MLog
  • com.mchange.v2.log.FallbackMLog
Alternatively, the following abbreviations are supported:
  • slf4j
  • log4j
  • jul, jdk14, java.util.logging
  • fallback
You may also set this property to a comma separated list of any mix the above alternatives, to define an order of preference among logging libraries.
Under JDK standard logging, the logging library may inspect stack traces to determine the class and method from which a log message was generated. That can be helpful, but it is also slow. Setting this configuration parameter to true will suppress this stack walk, and reduce the overhead of logging. This property now defaults to true, and logger names are logged in place of class names. To return to the original slower but more informative approach, explicitly set the property to false.
By default, c3p0 uses very fine-grained logging, in general with one logger for each c3p0 class. For a variety of reasons, some users may prefer fewer, more global loggers. You may opt for one-logger-per-package by setting com.mchange.v2.log.NameTransformer to the value com.mchange.v2.log.PackageNames. Advanced users can also define other strategies for organizing the number and names of loggers by setting this variable to the fully-qualified class name of a custom implementation of the com.mchange.v2.log.NameTransformer interface.
If, whether by choice or by necessity, you are using c3p0's System.err fallback logger, you can use this parameter to control how detailed c3p0's logging should be. Any of the following values (taken from the jdk1.4 logging library) are acceptable:
  • OFF
  • INFO
  • FINE
  • ALL
This property defaults to INFO.

Named configurations Go To Top

You can define named configurations which augment and override the default configuration that you define. When you instantiate a c3p0 PooledDataSource, whether via the ComboPooledDataSource constructor or via the DataSources factory class, you can supply a configuration name. For example, using ComboPooledDataSource:

ComboPooledDataSource cpds = new ComboPooledDataSource("intergalactoApp");

Or using the DataSources factory class:

DataSource ds_pooled = DataSources.pooledDataSource( ds_unpooled, "intergalactoApp" );

To define named configurations...

In a properties-style config file...

# define default-config param values c3p0.maxPoolSize=30 c3p0.minPoolSize=10 # define params for a named config called intergalactoApp c3p0.named-configs.intergalactoApp.maxPoolSize=1000 c3p0.named-configs.intergalactoApp.minPoolSize=100 c3p0.named-configs.intergalactoApp.numHelperThreads=50 # define params for a named config called littleTeenyApp c3p0.named-configs.littleTeenyApp.maxPoolSize=5 c3p0.named-configs.littleTeenyApp.minPoolSize=2

In a HOCON config file...

c3p0 { maxPoolSize=30 minPoolSize=10 named-configs { intergalactoApp { maxPoolSize=1000 minPoolSize=100 numHelperThreads=50 } littleTeenyApp { maxPoolSize=5 minPoolSize=2 } } }

In an XML config file...

<c3p0-config> <default-config> <property name="maxPoolSize">30</property> <property name="minPoolSize">10</property> </default-config> <named-config name="intergalactoApp"> <property name="maxPoolSize">1000</property> <property name="minPoolSize">100</property> <property name="numHelperThreads">50</property> </named-config> <named-config name="littleTeenyApp"> <property name="maxPoolSize">5</property> <property name="minPoolSize">2</property> </named-config> </c3p0-config>

Per-user configurations Go To Top

You can define overrides of default or named configurations that apply only to pools of Connections authenticated for a particular user. Not all configuration parameters support per-user overrides. See Appendix A for details.

To define per-user configurations...

In a properties-style config file...

# define default-config param values c3p0.maxPoolSize=30 c3p0.minPoolSize=10 # define params for a user called 'steve' c3p0.user-overrides.steve.maxPoolSize=15 c3p0.user-overrides.steve.minPoolSize=5 # define params for a user called 'ramona' c3p0.user-overrides.steve.maxPoolSize=50 c3p0.user-overrides.steve.minPoolSize=20

In a HOCON config file...

c3p0 { maxPoolSize=30 minPoolSize=10 user-overrides { steve { maxPoolSize=15 minPoolSize=5 } ramona { maxPoolSize=50 minPoolSize=20 } } }

In an XML config file...

<c3p0-config> <default-config> <property name="maxPoolSize">30</property> <property name="minPoolSize">10</property> <user-overrides user="steve"> <property name="maxPoolSize">15</property> <property name="minPoolSize">5</property> </user-overrides> <user-overrides user="ramona"> <property name="maxPoolSize">50</property> <property name="minPoolSize">20</property> </user-overrides> </default-config> </c3p0-config>

User extensions to configuration Go To Top

Users can add their own configuration information, usually to customize the behavior of ConnectionCustomizers. User configuration is stored as a Map containing String keys and values, stored under the following configuration parameter:

The extensions Map can be set programatically like any other configuration parameter. However, there is special support for defining keys and values for the extensions Map in configuration files. In a properties-style config file...

c3p0.extensions.initSql=SET SCHEMA 'foo' c3p0.extensions.timezone=PDT ...

In a HOCON config file...

c3p0 { extensions { initSql=SET SCHEMA 'foo' timezone=PDT } }

In an XML config file...

<c3p0-config> <default-config> <extensions> <property name="initSql">SET SCHEMA 'foo'</property> <property name="timezone">PDT</property> </extensions> </default-config> </c3p0-config>

To find the extensions defined for a PooledDataSource, you must have access to its identityToken, which is supplied as an argument to all ConnectionCustomizer methods. Given an identityToken, you can use the method C3P0Registry.extensionsForToken(...) to access the extensions Map.

Because extensions are primary designed to be used within ConnectionCustomizer implementations, the AbstractConnectionCustomizer class also defines a protected extensionsForToken(...) method as a convenience.

Here is an example ConnectionCustomizer implementation that makes use of user-defined configuration extensions. It defines an initSql extension, whose value should be a String containing SQL that should be executed when a Connection is checked out from the pool:

package mypkg; import java.sql.*; import com.mchange.v2.c3p0.AbstractConnectionCustomizer; public class InitSqlConnectionCustomizer extends AbstractConnectionCustomizer { private String getInitSql( String parentDataSourceIdentityToken ) { return (String) extensionsForToken( parentDataSourceIdentityToken ).get ( "initSql" ); } public void onCheckOut( Connection c, String parentDataSourceIdentityToken ) throws Exception { String initSql = getInitSql( parentDataSourceIdentityToken ); if ( initSql != null ) { Statement stmt = null; try { stmt = c.createStatement(); stmt.executeUpdate( initSql ); } finally { if ( stmt != null ) stmt.close(); } } } }
Note: There's no need to implement your own ConnectionCustomizer if you just want initSql. This example, plus a bit of extra TRACE-level logging, is implemented within the library as com.mchange.v2.c3p0.example.InitSqlConnectionCustomizer.

Mixing named, per-user, and user-defined configuration extensions Go To Top

Named configurations, per-user overrides, and user-defined configuration extensions can easily be mixed.

In a properties-style config file...

c3p0.maxPoolSize=30 c3p0.extensions.initSql=SET SCHEMA 'default' c3p0.named-configs.intergalactoApp.maxPoolSize=1000 c3p0.named-configs.intergalactoApp.extensions.initSql=SET SCHEMA 'intergalacto' c3p0.named-configs.user-overrides.steve.maxPoolSize=20

In a HOCON config file...

c3p0 { maxPoolSize=30 extensions { initSql=SET SCHEMA 'default' } named-configs { intergalactoApp { maxPoolSize=1000 user-overrides { steve { maxPoolSize=20 } } extensions { initSql=SET SCHEMA 'intergalacto' } } } }

In an XML config file...

<c3p0-config> <default-config> <property name="maxPoolSize">30</property> <extensions> <property name="initSql">SET SCHEMA 'default'</property> </extensions> </default-config> <named-config name="intergalactoApp"> <property name="maxPoolSize">1000</property> <user-overrides name="steve"> <property name="maxPoolSize">20</property> </user-overrides> <extensions> <property name="initSql">SET SCHEMA 'intergalacto'</property> </extensions> </named-config> </c3p0-config>

PerformanceGo To Top

Enhanced performance is the purpose of Connection and Statement pooling, and a major goal of the c3p0 library. For most applications, Connection pooling will provide a significant performance gain, especially if you are acquiring an unpooled Connection for each client access. If you are letting a single, shared Connection serve many clients to avoid Connection acquisition overhead, you may suffer performance issues and problems managing transactions when your Connection is under concurrent load; Connection pooling will enable you to switch to a one Connection-per-client model with little or no cost. If you are writing Enterprise Java Beans, you may be tempted to acquire a Connection once and not return it until the bean is about to be destroyed or passivated. But this can be resource-costly, as dormant pooled beans needlessly hold the Connection's network and database resources. Connection pooling permits beans to only "own" a Connection while they are using it.

But, there are performance costs to c3p0 as well. In order to implement automatic cleanup of unclosed ResultSets and Statements when parent resources are returned to pools, all client-visible Connections, ResultSets, Statements are really wrappers around objects provided by an underlying unpooled DataSource or "traditional" JDBC driver. Thus, there is some extra overhead to all JDBC calls.

Some attention has been paid to minimizing the "wrapper" overhead of c3p0. In my environment, the wrapper overhead amounts from several hundreths to several thousandths of the cost of Connection acquisition, so unless you are making many, many JDBC calls in fast succession, there will be a net gain in performance and resource-utilization efficiency. Significantly, the overhead associated with ResultSet operations (where one might iterate through a table with thousands of records) appears to be negligibly small.

Known ShortcomingsGo To Top

Feedback and SupportGo To Top

Please provide any and all feedback to <swaldman@mchange.com>! Also, feel free to join and ask questions on the c3p0-users mailing list. Sign up at http://sourceforge.net/projects/c3p0/

Thank you for using c3p0!!!

Appendix A: Configuration PropertiesGo To Top

c3p0 configuration properties can be divided into JavaBeans-style Properties and Other Properties.

JavaBeans-style Properties Go To Top

The following properties can be set directly in code as JavaBeans properties, via a System properties or a c3p0.properties file (with c3p0. prepended to the property name), in HOCON (typesafe-config) files, or in a c3p0-config.xml file. See the section on Configuration above. Click on the property name for a full description.

Default: 3
Default: 30
Default: 1000
Default: false
Default: null
Default: false
Default: 0
The number of milliseconds a client calling getConnection() will wait for a Connection to be checked-in or acquired when the pool is exhausted. Zero means wait indefinitely. Setting any positive value will cause the getConnection() call to time-out and break with an SQLException after the specified number of milliseconds.
Default: null
The fully qualified class-name of an implememtation of the ConnectionCustomizer interface, which users can implement to set up Connections when they are acquired from the database, or on check-out, and potentially to clean things up on check-in and Connection destruction. If standard Connection properties (holdability, readOnly, or transactionIsolation) are set in the ConnectionCustomizer's onAcquire() method, these will override the Connection default values.
Default: com.mchange.v2.c3p0.impl.DefaultConnectionTester
The fully qualified class-name of an implememtation of the ConnectionTester interface, or QueryConnectionTester if you would like instances to have access to a user-configured preferredTestQuery. This can be used to customize how c3p0 DataSources test Connections, but with the introduction of automaticTestTable and preferredTestQuery configuration parameters, "rolling your own" should be overkill for most users. [See "Configuring Connection Testing"]
Default: caller
Must be one of caller, library, or none. Determines how the contextClassLoader (see java.lang.Thread) of c3p0-spawned Threads is determined. If caller, c3p0-spawned Threads (helper threads, java.util.Timer threads) inherit their contextClassLoader from the client Thread that provokes initialization of the pool. If library, the contextClassLoader will be the class that loaded c3p0 classes. If none, no contextClassLoader will be set (the property will be null), which in practice means the system ClassLoader will be used. The default setting of caller is sometimes a problem when client applications will be hot redeployed by an app-server. When c3p0's Threads hold a reference to a contextClassLoader from the first client that hits them, it may be impossible to garbage collect a ClassLoader associated with that client when it is undeployed in a running VM. Setting this to library can resolve these issues. [See "Configuring To Avoid Memory Leaks On Hot Redeploy Of Client"]
Does Not Support Per-User Overrides.
Default: if configured with a named config, the config name, otherwise the pool's "identity token"
Every c3p0 pooled data source is given a dataSourceName, which serves two purposes. It helps users find DataSources via C3P0Registry, and it is included in the name of JMX mBeans in order to help track and distinguish between multiple c3p0 DataSources even across application or JVM restarts. dataSourceName defaults to the pool's configuration name, if a named config was used, or else to an "identity token" (an opaque, guaranteed unique String associated with every c3p0 DataSource). You may update this property to any name you find convenient. dataSourceName is not guaranteed to be unique — for example, multiple DataSource created from the same named configuration will share the same dataSourceName. But if you are going to make use of dataSourceName, you will probably want to ensure that all pooled DataSources within your JVM do have unique names.
Default: false
If true, and if unreturnedConnectionTimeout is set to a positive value, then the pool will capture the stack trace (via an Exception) of all Connection checkouts, and the stack traces will be printed when unreturned checked-out Connections timeout. This is intended to debug applications with Connection leaks, that is applications that occasionally fail to return Connections, leading to pool growth, and eventually exhaustion (when the pool hits maxPoolSize with all Connections checked-out and lost). This parameter should only be set while debugging, as capturing the stack trace will slow down every Connection check-out. [See "Configuring to Debug and Workaround Broken Client Applications"]
Default: null
Does Not Support Per-User Overrides.
Default: an empty java.util.Map
Does Not Support Per-User Overrides.
Default: null
DataSources that will be bound by JNDI and use that API's Referenceable interface to store themselves may specify a URL from which the class capable of dereferencing a them may be loaded. If (as is usually the case) the c3p0 libraries will be locally available to the JNDI service, leave this set as null.
Does Not Support Per-User Overrides.
Default: false
Default: false
Default: false
Setting the parameter driverClass causes that class to preload and register with java.sql.DriverManager. However, it does not on its own ensure that the driver used will be an instance of driverClass, as DriverManager may (in unusual cases) know of other driver classes which can handle the specified jdbcUrl. Setting this parameter to true causes c3p0 to ignore DriverManager and simply instantiate driverClass directly.
Does Not Support Per-User Overrides.
Default: 0
Default: 3
Default: null
Does Not Support Per-User Overrides.
Default: 0
Seconds before c3p0's thread pool will try to interrupt an apparently hung task. Rarely useful. Many of c3p0's functions are not performed by client threads, but asynchronously by an internal thread pool. c3p0's asynchrony enhances client performance directly, and minimizes the length of time that critical locks are held by ensuring that slow jdbc operations are performed in non-lock-holding threads. If, however, some of these tasks "hang", that is they neither succeed nor fail with an Exception for a prolonged period of time, c3p0's thread pool can become exhausted and administrative tasks backed up. If the tasks are simply slow, the best way to resolve the problem is to increase the number of threads, via numHelperThreads. But if tasks sometimes hang indefinitely, you can use this parameter to force a call to the task thread's interrupt() method if a task exceeds a set time limit. [c3p0 will eventually recover from hung tasks anyway by signalling an "APPARENT DEADLOCK" (you'll see it as a warning in the logs), replacing the thread pool task threads, and interrupt()ing the original threads. But letting the pool go into APPARENT DEADLOCK and then recover means that for some periods, c3p0's performance will be impaired. So if you're seeing these messages, increasing numHelperThreads and setting maxAdministrativeTaskTime might help. maxAdministrativeTaskTime should be large enough that any resonable attempt to acquire a Connection from the database, to test a Connection, or to destroy a Connection, would be expected to succeed or fail within the time set. Zero (the default) means tasks are never interrupted, which is the best and safest policy under most circumstances. If tasks are just slow, allocate more threads. If tasks are hanging forever, try to figure out why, and maybe setting maxAdministrativeTaskTime can help in the meantime.
Does Not Support Per-User Overrides.
Default: 0
Seconds, effectively a time to live. A Connection older than maxConnectionAge will be destroyed and purged from the pool. This differs from maxIdleTime in that it refers to absolute age. Even a Connection which has not been much idle will be purged from the pool if it exceeds maxConnectionAge. Zero means no maximum absolute age is enforced.
Default: 0
Default: 0
Number of seconds that Connections in excess of minPoolSize should be permitted to remain idle in the pool before being culled. Intended for applications that wish to aggressively minimize the number of open Connections, shrinking the pool back towards minPoolSize if, following a spike, the load level diminishes and Connections acquired are no longer needed. If maxIdleTime is set, maxIdleTimeExcessConnections should be smaller if the parameter is to have any effect. Zero means no enforcement, excess Connections are not idled out.
Default: 15
Default: 0
Default: 0
Default: 3
Default: 3
c3p0 is very asynchronous. Slow JDBC operations are generally performed by helper threads that don't hold contended locks. Spreading these operations over multiple threads can significantly improve performance by allowing multiple operations to be performed simultaneously.
Does Not Support Per-User Overrides.
Default: null
Does Not Support Per-User Overrides.
Default: null
Does Not Support Per-User Overrides.
Default: null
Does Not Support Per-User Overrides.
Default: null
Defines the query that will be executed for all connection tests, if the default ConnectionTester (or some other implementation of QueryConnectionTester, or better yet FullQueryConnectionTester) is being used. Defining a preferredTestQuery that will execute quickly in your database may dramatically speed up Connection tests. (If no preferredTestQuery is set, the default ConnectionTester executes a getTables() call on the Connection's DatabaseMetaData. Depending on your database, this may execute more slowly than a "normal" database query.) NOTE: The table against which your preferredTestQuery will be run must exist in the database schema prior to your initialization of your DataSource. If your application defines its own schema, try automaticTestTable instead. [See "Configuring Connection Testing"]
Default: false
If true, c3p0-spawned Threads will have the java.security.AccessControlContext associated with c3p0 library classes. By default, c3p0-spawned Threads (helper threads, java.util.Timer threads) inherit their AccessControlContext from the client Thread that provokes initialization of the pool. This can sometimes be a problem, especially in application servers that support hot redeployment of client apps. If c3p0's Threads hold a reference to an AccessControlContext from the first client that hits them, it may be impossible to garbage collect a ClassLoader associated with that client when it is undeployed in a running VM. Also, it is possible client Threads might lack sufficient permission to perform operations that c3p0 requires. Setting this to true can resolve these issues. [See "Configuring To Avoid Memory Leaks On Hot Redeploy Of Client"]
Does Not Support Per-User Overrides.
Default: 0
Default: 0
Default: false
Default: false
Default: 0
Default: null
Does Not Support Per-User Overrides.
Default: false
Deprecated. c3p0 originally used reflective dynamic proxies for implementations of Connections and other JDBC interfaces. As of c3p0-0.8.5, non-reflective, code-generated implementations are used instead. As this was a major change, and the old codebase had been extensively used and tested, this parameter was added to allow users to revert of they had problems. The new, non-reflexive implementation is faster, and has now been widely deployed and tested, so it is unlikely that this parameter will be useful. Both the old reflective and newer non-reflective codebases are being maintained, but support for the older codebase may (or may not) be dropped in the future.

Other Properties Go To Top

The following configuration properties affect the behavior of the c3p0 library as a whole. They may be set as system properties, in a c3p0.properties file, or in a HOCON (typesafe-config) file.

Locating Configuration Information

Normally, c3p0's configuration information is placed in a either a c3p0-config.xml or c3p0.properties file at the top-level of an application's CLASSPATH. However, if you wish to place configuration information elsewhere, you may place c3p0 configuration information (in the XML file format only!) anywhere you'd like in the filesystem visible to your application. Just set the following property to the full, absolute path of the XML config file:

  • com.mchange.v2.c3p0.cfg.xml

If you set this property to a value beginning with "classloader:", c3p0 will search for an XML config file as a ClassLoader resource, that is, in any location you specify under your classpath, including jar-file META-INF directories.

Logging-related properties

The following properties affect c3p0's logging behavior. Please see Configuring Logging above for specific information.

  • com.mchange.v2.log.MLog
  • com.mchange.v2.log.jdk14logging.suppressStackWalk
  • com.mchange.v2.log.NameTransformer
  • com.mchange.v2.log.FallbackMLog.DEFAULT_CUTOFF_LEVEL

Configuring JMX

The following properties affect c3p0's JMX management interface. Plese see Configuring and Managing c3p0 via JMX above for more information.

  • com.mchange.v2.c3p0.management.ExcludeIdentityToken
  • com.mchange.v2.c3p0.management.RegistryName
  • com.mchange.v2.c3p0.management.ManagementCoordinator

Configuring the VMID

Is it better to be beautiful or correct? Beginning with c3p0-0.9.1, c3p0 opts somewhat reluctantly for correctness.

Here's the deal. Every c3p0 DataSource is allocated a unique "identity token", which is used to ensure that multiple JNDI lookups of the same PooledDataSource always return the same instance, even if the JNDI name-server stores a Serialized or Referenced instance. Previously, c3p0 was happy for generated IDs to be unique within a single VM (and it didn't even get that quite right, before c3p0-0.9.1). But in theory, one VM might look up two different DataSources, generated by two different VMs, that by unlikely coincidence have the same "identity token", leading to errors as one of the two DataSources sneakily substitutes for the second. Though this hypothetical issue has never been reported in practice, c3p0 resolves it by prepending a VMID to its identity tokens. This makes them long and ugly, but correct.

If you don't like the long and ugly VMID, you can set your own, or you can turn off this solution to a hypothetical non-problem entirely with the following property:

  • com.mchange.v2.c3p0.VMID

Set it to NONE to turn off the VMID, set it to AUTO to let c3p0 generate a VMID, or provide any other String to set the VMID that will be used directly. The default is AUTO.

Configuring DefaultConnectionTester.isValidTimeout

Under circumstances when the JDBC 4+ isValid(...) test will be used by c3p0's built in DefaultConnectionTester (see below), by default the test will never time out. If you would the test to timeout and fail, set the following key

  • com.mchange.v2.c3p0.impl.DefaultConnectionTester.isValidTimeout

to the desired timeout, in seconds.

Configuring DefaultConnectionTester.QuerylessTestRunner

c3p0's built-in DefaultConnectionTester does the right and obvious thing when you've provided a preferredTestQuery or automaticTestTable parameter. But when it has no user-determined query to run to test a Connection, it's less clear what c3p0 should do. In the JDBC 3 API, there was no straightforward, reliable way to test a JDBC Connection. c3p0's DefaultConnectionTester adopted the very conservative technique, using the Connection to query DatabaseMetaData, since this represents a live query to the database that can be executed without any knowledge of a database's schema. Unfortunately, this technique is often very, very slow.

Fortunately, as of version 0.9.5, c3p0 supports the JDBC 4 API's testing using the new Connection.isValid() method. This is a fast, reliable test specified and implemented by the JDBC Driver provider.

Although it will very rarely be necessary, users can now specify exactly how DefaultConnectionTester will behave if no preferredTestQuery or automaticTestTable has been set via the following property:

  • com.mchange.v2.c3p0.impl.DefaultConnectionTester.querylessTestRunner

Possible values of this property include

This is c3p0's very slow, but very reliable, traditional default Connection test. It will work even with very old JDBC drivers.
This uses the new JDBC 4 API to perform a driver-defined Connection test. An AbstractMethodError will be provoked, however, if it is used with an old JDBC 3 driver.
This first attempts the new JDBC 4 Connection test (like IS_VALID), but catches any AbstractMethodError and falls back onto METADATA_TABLESEARCH if necessary.
This checks whether the new Connection.isValid() method is available for any implementation of Connection that it tests, and stores a ThreadLocal WeakHashMap to track which Connection implementations support the method. It then consults the map and runs the fast IS_VALID test or the universal METADATA_TABLESEARCH test as appropriate.

You can also provide the fully-qualified classname of an implementation of the DefaultConnectionTester.QuerylessTestRunner interface and define your own behavior, whatever you'd like to do. Your class should be public, have a public no argument constructor, and be Thread-safe and sharable. (A c3p0 pool typically uses just one ConnectionTester to test all of its Connections, often concurrently.) For examples, see the built-in implementations in c3p0's source code.

The default value is SWITCH, which should be fine almost always.

Really, you should almost never bother to set this property. If you are using an old JDBC driver and want to eliminate the small overhead of trying the isValid() method and then catching an AbstractMethodError, you can do so by setting its value to METADATA_TABLESEARCH. But why bother, when a much, much faster approach is to set a preferredTestQuery, and avoid the queryless test entirely? If you want to do something totally different, you can implement your own DefaultConnectionTester.QuerylessTestRunner. Or you can just implement ConnectionTester directly, and set the parameter connectionTesterClassName.

Experimental properties

c3p0-0.9.1 included a new implementation of asynchronous Connection acquisition that should improve c3p0's performance and resource utilization in cases where database acquisition attempts, for whatever reason, occasionally fail. The new implementation should be significantly better than the "traditional" Connection acquisition strategy, but was added too late in the c3p0-0.9.1 development cycle to be fully tested and enabled by default. Users are encouraged to try the new implementation, both because it is better, and to help iron out any unanticipated problems.

For a full description of the new implementation and the resource bottleneck it is designed to overcome, please see the CHANGELOG entry for c3p0-0.9.1-pre11.

As of c3p0-0.9.2 this feature is enabled by default. To revert to the traditional Connection acquisition behavior, set the following parameter to false.

  • com.mchange.v2.resourcepool.experimental.useScatteredAcquireTask

Appendix B: Configuration Files, etc. Go To Top

c3p0 configuration parameters can be set directly in Java code, via a simple Java properties file, via a Typesafe "HOCON" file, via an XML configuration file, or via System properties. Any which way works (the the XML configuration is most powerful, though, as it supports multiple named configurations and per-user overrides. Choose whatever works best for you.

Overriding c3p0 defaults via c3p0.properties Go To Top

To override the library's built-in defaults, create a file called c3p0.properties and place it at the "root" of your classpath or classloader. For a typical standalone application, that means place the file in a directory named in your CLASSPATH environment variable. For a typical web-application, the file should be placed in WEB-INF/classes. In general, the file must be available as a classloader resource under the name /c3p0.properties, in the classloader that loaded c3p0's jar file. Review the API docs (especilly getResource... methods) of java.lang.Class, java.lang.ClassLoader, and java.util.ResourceBundle if this is unfamiliar.

The format of c3p0.properties should be a normal Java Properties file format, whose keys are c3p0 configurable properties. See Appendix A. for the specifics. An example c3p0.properties file is produced below:

# turn on statement pooling c3p0.maxStatements=150 # close pooled Connections that go unused for # more than half an hour c3p0.maxIdleTime=1800

Overriding c3p0 defaults with "HOCON" (typesafe-config) configuration files Go To Top

Typesafe has defined a very nice configuration file format called "HOCON".

c3p0 does not include Typesafe's library, but if you bundle it with your application, c3p0 will support configuration in this format. You may place c3p0 configuration in the standard /application.conf or /reference.conf files, or you may use a special /c3p0.conf file. (These files must be placed as top-level ClassLoader resources, see above.) /application.json, /application.properties, and c3p0.json are also supported, and substitutions from /c3p0.properties will be resolved. The Typesafe config library can be downloaded from the Maven Central Repository.

Remember, HOCON configuration in only supported if you explicitly bundle the Typesafe config library with your application. The library is not included with c3p0's binary distribution, nor is it downloaded with c3p0 as a transitive dependency!

Here are some examples of setting c3p0 configuration in HOCON:

c3p0 { # turn on statement pooling maxStatements=150 # close pooled Connections that go unused for # more than half an hour maxIdleTime=1800 }

Note that you must specify ordinary config params explicitly inside a c3p0 scope one way or another, even in a c3p0.conf file. "Dot notation" can be used equivalently to scopes:

# equivalent to the example above, and # identical to the properties file format. c3p0.maxStatements=150 c3p0.maxIdleTime=1800

Also, the following two specifications are equivalent:

# properties-file-ish specification com.mchange.v2.log.MLog=com.mchange.v2.log.log4j.Log4jMLog # scoped specification of the same com { mchange { v2 { log { MLog="com.mchange.v2.log.log4j.Log4jMLog" } } } }

Overriding c3p0 defaults with System properties Go To Top

c3p0 properties can also be defined as System properties, using the same "c3p0." prefix for properties specified in a c3p0.properties file.

swaldman% java -Dc3p0.maxStatements=150 -Dc3p0.maxIdleTime=1800 example.MyC3P0App

System properties override settings in c3p0.properties. Please see Precedence of Configuration Settings for more information.

Any key that would be legal in a c3p0.properties file is legal as a System property, including keys that might have defined named, per-user, or user-defined-extension parameters.

Overriding c3p0 defaults via c3p0-config.xml Go To Top

You can use the XML config file for all c3p0 configuration, including configuration of defaults, named configurations, per-user overrides, and configuration extensions.

By default, c3p0 will look for an XML configuration file in its classloader's resource path under the name "/c3p0-config.xml". That means the XML file should be placed in a directly or jar file directly named in your applications CLASSPATH, in WEB-INF/classes, or some similar location.

If you prefer not to bundle your configuration with your code, you can specify an ordinary filesystem location for c3p0's configuration file via the system property com.mchange.v2.c3p0.cfg.xml. (You can also use this property to specify an alternative location in the ClassLoader resource path, e.g. META-INF. See Locating Configuration Information.)

Here is an example c3p0-config.xml file:

<c3p0-config> <default-config> <property name="automaticTestTable">con_test</property> <property name="checkoutTimeout">30000</property> <property name="idleConnectionTestPeriod">30</property> <property name="initialPoolSize">10</property> <property name="maxIdleTime">30</property> <property name="maxPoolSize">100</property> <property name="minPoolSize">10</property> <property name="maxStatements">200</property> <user-overrides user="test-user"> <property name="maxPoolSize">10</property> <property name="minPoolSize">1</property> <property name="maxStatements">0</property> </user-overrides> </default-config> <!-- This app is massive! --> <named-config name="intergalactoApp"> <property name="acquireIncrement">50</property> <property name="initialPoolSize">100</property> <property name="minPoolSize">50</property> <property name="maxPoolSize">1000</property> <!-- intergalactoApp adopts a different approach to configuring statement caching --> <property name="maxStatements">0</property> <property name="maxStatementsPerConnection">5</property> <!-- he's important, but there's only one of him --> <user-overrides user="master-of-the-universe"> <property name="acquireIncrement">1</property> <property name="initialPoolSize">1</property> <property name="minPoolSize">1</property> <property name="maxPoolSize">5</property> <property name="maxStatementsPerConnection">50</property> </user-overrides> </named-config> </c3p0-config>

Precedence of Configuration Settings Go To Top

c3p0 now permits configuration parameters to be set in a number of different ways and places. Fortunately, there is a clear order of precedence that determines which configuration will "take" in the event of conflicting settings. Conceptually, c3p0 goes down this list from top to bottom, using the first setting it finds.

Most applications will never use per-user or named configurations. For these applications, we present a simplified precedence hierarchy:

  1. Configuration values programmatically set.
  2. Configuration values set in system properties.
  3. Configuration values taken from the default configuration of a c3p0-config.xml file.
  4. Configuration values specified in a c3p0.properties file
  5. Configuration values in a c3p0.json file, if and only if Typesafe config libraries are available.
  6. Configuration values in a c3p0.conf file, if and only if Typesafe config libraries are available.
  7. Configuration values in a application.properties file, if and only if Typesafe config libraries are available.
  8. Configuration values in a application.json file, if and only if Typesafe config libraries are available.
  9. Configuration values in a application.conf file, if and only if Typesafe config libraries are available.
  10. Configuration values in a reference.conf file, if and only if Typesafe config libraries are available.
  11. c3p0's hard-coded default values.

For applications that do use named and per-user configurations, here is the complete, normative precedence hierarchy:

  1. User-specific overrides programmatically set via: Note that programmatically setting user-specific overrides replaces all user-specific configuration taken from other sources. If you want to merge programmatic changes with preconfigured overrides, you'll have to use getUserOverridesAsString() and modify the original settings before replacing.
  2. User-specific overrides taken from a DataSource's named configuration (specified in c3p0-config.xml)
  3. User-specific overrides taken from the default configuration (specified in c3p0-config.xml)
  4. Non-user-specific values programmatically set.
  5. Non-user-specific values taken from a DataSource's named configuration (specified in c3p0-config.xml)
  6. System property setting of configuration value.
  7. Non-user-specific values taken from the default configuration (specified in c3p0-config.xml)
  8. Configuration values in a c3p0.conf file, if and only if Typesafe config libraries are available.
  9. Configuration values specified in a c3p0.properties file
  10. Configuration values in a application.conf file, if and only if Typesafe config libraries are available.
  11. Configuration values in a reference.conf file, if and only if Typesafe config libraries are available.
  12. c3p0's hard-coded default values.

Appendix C: Hibernate-specific notes Go To Top

Hibernate's C3P0ConnectionProvider renames 7 c3p0 configuration properties, which, if set in your hibernate configuration, will override any configuration you may have set in a c3p0.properties file:

c3p0-native property namehibernate configuration key
c3p0.initialPoolSizenot available -- uses minimum size
c3p0.testConnectionOnCheckout hibernate.c3p0.validate hibernate 2.x only!

You can set any c3p0 properties in your hibernate config using the prefix hibernate.c3p0. For example

hibernate.c3p0.unreturnedConnectionTimeout=30 hibernate.c3p0.debugUnreturnedConnectionStackTraces=true

might be set to help debug Connection leaks.

You can always set c3p0 config in a c3p0.properties or c3p0-config.xml file (see "Overriding c3p0 defaults via c3p0.properties", "Overriding c3p0 defaults via c3p0-config.xml"), but any configuration set in Hibernate config files will override c3p0-native configuation!

Appendix D: Configuring c3p0 DataSources in TomcatGo To Top

Please see "Configuring To Avoid Memory Leaks On Hot Redeploy Of Client" if you experience memory leaks on Tomcat.

TL; DR: Set privilegeSpawnedThreads to true and set contextClassLoaderSource to library.

You can easily configure Apache's Tomcat web application server to use c3p0 pooled DataSources. Below is a Tomcat 5.0 sample config to get you started. It's a fragment of Tomcat's conf/server.xml file, which should be modified to suit and placed inside a <Context> element.

<Resource name="jdbc/pooledDS" auth="Container" type="com.mchange.v2.c3p0.ComboPooledDataSource" /> <ResourceParams name="jdbc/pooledDS"> <parameter> <name>factory</name> <value>org.apache.naming.factory.BeanFactory</value> </parameter> <parameter> <name>driverClass</name> <value>org.postgresql.Driver</value> </parameter> <parameter> <name>jdbcUrl</name> <value>jdbc:postgresql://localhost/c3p0-test</value> </parameter> <parameter> <name>user</name> <value>swaldman</value> </parameter> <parameter> <name>password</name> <value>test</value> </parameter> <parameter> <name>minPoolSize</name> <value>5</value> </parameter> <parameter> <name>maxPoolSize</name> <value>15</value> </parameter> <parameter> <name>acquireIncrement</name> <value>5</value> </parameter> </ResourceParams>

For Tomcat 5.5, try something like the following (thanks to Carl F. Hall for the sample!):

<Resource auth="Container" description="DB Connection" driverClass="com.mysql.jdbc.Driver" maxPoolSize="4" minPoolSize="2" acquireIncrement="1" name="jdbc/TestDB" user="test" password="ready2go" factory="org.apache.naming.factory.BeanFactory" type="com.mchange.v2.c3p0.ComboPooledDataSource" jdbcUrl="jdbc:mysql://localhost:3306/test?autoReconnect=true" />

The rest is standard J2EE stuff: You'll need to declare your DataSource reference in your web.xml file:

<resource-ref> <res-ref-name>jdbc/pooledDS</res-ref-name> <res-type>javax.sql.DataSource</res-type> <res-auth>Container</res-auth> </resource-ref>

And you can access your DataSource from code within your web application like this:

InitialContext ic = new InitialContext(); DataSource ds = (DataSource) ic.lookup("java:comp/env/jdbc/pooledDS");

That's it!

Appendix E: JBoss-specific notesGo To Top

To use c3p0 with JBoss:

  1. Place c3p0's jar file in the lib directory of your jboss server instance (e.g. ${JBOSS_HOME}/server/default/lib)
  2. Modify the file below, and save it as c3p0-service.xml in the deploy directory of your jboss server (e.g. ${JBOSS_HOME}/server/default/deploy). Note that parameters must be capitalized in this file, but otherwise they are defined as described above.

Please note: As of c3p0-0.9.1, the class name of the jboss configuration mbean has changed to com.mchange.v2.c3p0.jboss.C3P0PooledDataSource (from com.mchange.v2.c3p0.mbean.C3P0PooledDataSource), in order to distinguish what is really jboss-specific functionality from c3p0's more general JMX support.

The old jboss config mbeans are deprecated, but will still work. However, support for new configuration parameters will only be added under the new name. Updating requires a one-word change to your c3p0-service.xml, change "mbean" to "jboss" where your old file says 'code="com.mchange.v2.c3p0.mbean.C3P0PooledDataSource"'. Just do it!

Hide box.

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE server> <server> <mbean code="com.mchange.v2.c3p0.jboss.C3P0PooledDataSource" name="jboss:service=C3P0PooledDataSource"> <attribute name="JndiName">java:PooledDS</attribute> <attribute name="JdbcUrl">jdbc:postgresql://localhost/c3p0-test</attribute> <attribute name="DriverClass">org.postgresql.Driver</attribute> <attribute name="User">swaldman</attribute> <attribute name="Password">test</attribute> <!-- Uncomment and set any of the optional parameters below --> <!-- See c3p0's docs for more info. --> <!-- <attribute name="AcquireIncrement">3</attribute> --> <!-- <attribute name="AcquireRetryAttempts">30</attribute> --> <!-- <attribute name="AcquireRetryDelay">1000</attribute> --> <!-- <attribute name="AutoCommitOnClose">false</attribute> --> <!-- <attribute name="AutomaticTestTable"></attribute> --> <!-- <attribute name="BreakAfterAcquireFailure">false</attribute> --> <!-- <attribute name="CheckoutTimeout">0</attribute> --> <!-- <attribute name="ConnectionCustomizerClassName"></attribute> --> <!-- <attribute name="ConnectionTesterClassName"></attribute> --> <!-- <attribute name="Description">A pooled c3p0 DataSource</attribute> --> <!-- <attribute name="DebugUnreturnedConnectionStackTraces">false</attribute> --> <!-- <attribute name="FactoryClassLocation"></attribute> --> <!-- <attribute name="ForceIgnoreUnresolvedTransactions">false</attribute> --> <!-- <attribute name="IdleConnectionTestPeriod">0</attribute> --> <!-- <attribute name="InitialPoolSize">3</attribute> --> <!-- <attribute name="MaxAdministrativeTaskTime">0</attribute> --> <!-- <attribute name="MaxConnectionAge">0</attribute> --> <!-- <attribute name="MaxIdleTime">0</attribute> --> <!-- <attribute name="MaxIdleTimeExcessConnections">0</attribute> --> <!-- <attribute name="MaxPoolSize">15</attribute> --> <!-- <attribute name="MaxStatements">0</attribute> --> <!-- <attribute name="MaxStatementsPerConnection">0</attribute> --> <!-- <attribute name="MinPoolSize">0</attribute> --> <!-- <attribute name="NumHelperThreads">3</attribute> --> <!-- <attribute name="PreferredTestQuery"></attribute> --> <!-- <attribute name="TestConnectionOnCheckin">false</attribute> --> <!-- <attribute name="TestConnectionOnCheckout">false</attribute> --> <!-- <attribute name="UnreturnedConnectionTimeout">0</attribute> --> <!-- <attribute name="UsesTraditionalReflectiveProxies">false</attribute> --> <depends>jboss:service=Naming</depends> </mbean> </server>

Appendix F: Oracle-specific API: createTemporaryBLOB() and createTemporaryCLOB()Go To Top

The Oracle thin JDBC driver provides a non-standard API for creating temporary BLOBs and CLOBs that requires users to call methods on the raw, Oracle-specific Connection implementation. Advanced users might use the raw connection operations described above to access this functionality, but a convenience class is available in a separate jar file (c3p0-oracle-thin-extras- for easier access to this functionality. Please see the API docs for com.mchange.v2.c3p0.dbms.OracleUtils for details.

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