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c3p0 - JDBC3 Connection and Statement Pooling
by Steve Waldman <{}>

� 2006 Machinery For Change, Inc.

This software is made available for use, modification, and redistribution, under the terms of the {Lesser GNU Public License (LGPL)}, which you should have received with this distribution.
* API docs for c3p0 are {here}.
* Download the latest version from {c3p0's site on SourceForge}
* Looking for the definition of {configuration properties}?
* Looking for advice in {using c3p0 with hibernate}?

1. {Contents}
2. {Quickstart}
3. {What is c3p0?}
4. {Prerequisites}
5. {Installation}
6. {Using c3p0}
1. {Using ComboPooledDataSource}
2. {Using the DataSouces factory class}
* {Box: Overriding authentication information (from non-c3p0 DataSources)}
3. {Querying Pool Status}
* {Box: Using C3P0Registry to find a reference to a DataSource}
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. {Other DataSource Configuration}
11. {Configuring and Managing c3p0 via JMX}
12. {Configuring Logging}
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 file}
2. {Overriding c3p0 defaults with a System properties}
3. {Named and Per-User configuration: Overriding c3p0 defaults via c3p0-config.xml}
4. {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})
Quickstart{[Go To Top]}
c3p0 was designed to be butt-simple to use. Just put the jar file [lib/c3p0-
] in your application's effective CLASSPATH
, and 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:
DataSources.destroy( cpds );

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. In particular, c3p0 provides several useful services:
* Classes which adapt traditional DriverManager-based JDBC drivers to the new javax.sql.DataSource
scheme for acquiring database Connections.
* Transparent pooling of Connection and PreparedStatements behind DataSources which can "wrap" around traditional drivers or arbitrary unpooled DataSources.
The library tries hard to get the details right:
* c3p0 DataSources are both Referenceable
and Serializable
, and are thus suitable for binding to a wide-variety of JNDI-based naming services.
* Statement and ResultSets are carefully cleaned up when pooled Connections and Statements are checked in, to prevent resource- exhaustion when clients use the lazy but common resource-management strategy of only cleaning up their Connections....
* The library adopts the approach defined by the JDBC 2 and 3 specification (even where these conflict with the library author's preferences). DataSources are written in the JavaBean style, offering all the required and most of the optional properties (as well as some non-standard ones), and no-arg constructors. All JDBC-defined internal interfaces are implemented (ConnectionPoolDataSource, PooledConnection, ConnectionEvent-generating Connections, etc.) You can mix c3p0 classes with compliant third-party implementations (although not all c3p0 features will work with external implementations).
c3p0 hopes to provide DataSource implementations more than suitable for use by high-volume "J2EE enterprise applications". Please provide feedback, bug-fixes, etc.!

Prerequisites{[Go To Top]}
c3p0 requires a level 1.3.x or above Java Runtime Environment, and the JDBC 2.x or above javax.sql libraries. c3p0 works fine under Java 1.4.x and 1.5.x as well.

Installation{[Go To Top]}
Put the file lib/c3p0-
somewhere in your CLASSPATH (or any other place where your application's classloader will find it). That's it!

Using c3p0{[Go To Top]}
From a users' perspective, c3p0 simply provides standard jdbc2 DataSource objects. When creating these DataSources, users can control pooling-related, naming-related, and other properties (See {Appendix A}). 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 by creating a file called
and storing it as a top-level resource in the same CLASSPATH
(or ClassLoader) that loads c3p0's jar file. As of c3p0-0.9.1, you can also supply a file called c3p0-config.xml
for more advanced configuration. 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 have not externally preloaded the old-style JDBC driver you'll use 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 defaults of any c3p0 DataSource are determined by configuration you supply, or else revert to hard-coded defaults [see {configuration properties}]. c3p0-0.9.1 and above supports {multiple, named configurations}. 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 ds_pooled = DataSources.pooledDataSource( ds_unpooled, "intergalactoAppConfig", overrides );

{Show deprecated PoolConfig approach...}
Deprecated! Programmatic configuration via PoolConfig

If you use the {DataSources} factory class, and you want to programmatically override default configuration parameters, make use of the {PoolConfig
} class:
DataSource ds_unpooled = DataSources.unpooledDataSource("jdbc:postgresql://localhost/testdb", "swaldman", "test-password"); PoolConfig pc = new PoolConfig(); pc.setMaxStatements(200); //turn on Statement pooling // pass our overriding PoolConfig to the DataSources.pooledDataSource() factory method. ds_pooled = DataSources.pooledDataSource( ds_unpooled, pc ); // 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. ...
{Hide 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

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:
* public int getNumConnectionsDefaultUser()
* public int getNumConnections(String username, String password)
* public int getNumConnectionsAllUsers()
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:
* public static Set getPooledDataSources()
* public static Set pooledDataSourcesByName( String dataSourceName )
* public static {PooledDataSource} pooledDataSourceByName( String dataSourceName )
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 {PooledDataSource
}s 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!"); }

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]}

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.

} 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);

Note: C3P0 now includes special support for some Oracle-specific methods. See {Appendix F}.

Configuration{[Go 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, via a {simple Java properties file}, via an {XML configuration file}, or via {System properties}.

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}. The most basic and important c3p0 configuration topics are discussed below.

Basic Pool Configuration {[Go To Top]}

c3p0 Connection pools are very easy to configure via the following basic parameters:
* {acquireIncrement}
* {initialPoolSize}
* {maxPoolSize}
* {maxIdleTime}
* {minPoolSize}
, 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.
* {maxConnectionAge}
* {maxIdleTime}
* {maxIdleTimeExcessConnections}
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.

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:
* {automaticTestTable}
* {connectionTesterClassName}
* {idleConnectionTestPeriod}
* {preferredTestQuery}
* {testConnectionOnCheckin}
* {testConnectionOnCheckout}
, 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. By default, Connections are tested by calling the getTables()
method on a Connection's associated DatabaseMetaData
object. This has the advantage of working with any database, and regardless of the database schema. However, empirically a DatabaseMetaData.getTables()
call is often much slower than a simple database query.

The most convenient way to speed up Connection testing is to define 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. Alternatively, if your database schema is fixed prior to your application's use of the database, you can simply 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 table prior to initialization of your DataSource.

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.

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 testConnectionsOnCheckIn
. Both the idle test and the check-in test are performed asynchronously, which leads to better performance, both perceived and actual.

Note that for many 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.

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}
* {maxStatementsPerConnection}
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).

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.

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:
* {acquireRetryAttempts}
* {acquireRetryDelay}
* {breakAfterAcquireFailure}
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 (testConnectionsOnCheckout
set to true
, or testConnectionsOnCheckin
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:
* {connectionCustomizerClassName}
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.

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 + "]"); } }

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:
* {autoCommitOnClose}
* {forceIgnoreUnresolvedTransactions}
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.
* {debugUnreturnedConnectionStackTraces}
* {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.

Other DataSource Configuration {[Go To Top]}

See {Appendix A} for information about the following configuration properties:
* {checkoutTimeout}
* {factoryClassLocation}
* {maxAdministrativeTaskTime}
* {numHelperThreads}
* {usesTraditionalReflectiveProxies}
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.)

limits how long a client will wait for a Connection, if all Connections are checked out and one cannot be supplied immediately. usesTraditionalReflectiveProxies
, which is of little practical use, 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 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.)

If you do not want c3p0 to register MBeans with your JMX environment, you can suppress this behavior with the following, set either as a System property or in

Configuring Logging {[Go To Top]}

c3p0 uses a custom logging library similar to jakarta commons-logging. Log messages can be directed to the popular log4j logging 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
file, in a file called
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 log4j is available, it will use that library, otherwise if jdk1.4 logging apis are available it will use those, and if neither are available, it will use a simple fallback that logs to System.err
. If you want to directly control which library is used, you may set this property to one of:
* com.mchange.v2.log.log4j.Log4jMLog
* com.mchange.v2.log.jdk14logging.Jdk14MLog
* com.mchange.v2.log.FallbackMLog
You may also set this property to a comma separated list of the above alternatives, to define an order of preference among logging libraries.
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
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:
This property defaults to INFO

Performance{[Go 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 Shortcomings{[Go To Top]}
* Connections and Statements are pooled on a per-authentication basis. So, if one pool-backed DataSource is used to acquire Connections both for [user
=alice, password
=secret1] and [user
=bob, password
=secret2], there will be two distinct pools, and the DataSource might in the worst case manage twice the number of Connections specified by the maxPoolSize

This fact is a natural consequence of the definition of the DataSource spec (which allows Connections to be acquired with multiple user authentications), and the requirement that all Connections in a single pool be functionally identical. This "issue" will not be changed or fixed. It's noted here just so you understand what's going on.
* The overhead of Statement pooling is too high. For drivers that do not perform significant preprocessing of PreparedStatements, the pooling overhead outweighs any savings. Statement pooling is thus turned off by default. If your driver does preprocess PreparedStatements
, especially if it does so via IPC with the RDBMS, you will probably see a significant performance gain by turning Statement pooling on. (Do this by setting the configuration property maxStatements
or maxStatementsPerConnection
to a value greater than zero.).
Feedback and Support{[Go To Top]}
Please provide any and all feedback to <{}>! Also, feel free to join and ask questions on the c3p0-users
mailing list. Sign up at {}

Thank you for using c3p0!!!

Appendix A: Configuration Properties{[Go 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 {
} file (with c3p0.
prepended to the property name), or in a {c3p0-config.xml
} file. See the section on {Configuration} above. Click on the property name for a full description.
Default: 3
Determines how many connections at a time c3p0 will try to acquire when the pool is exhausted. [See {"Basic Pool Configuration"}]
Default: 30
Defines how many times c3p0 will try to acquire a new Connection from the database before giving up. If this value is less than or equal to zero, c3p0 will keep trying to fetch a Connection indefinitely. [See {"Configuring Recovery From Database Outages"}]
Default: 1000
Milliseconds, time c3p0 will wait between acquire attempts. [See {"Configuring Recovery From Database Outages"}]
Default: false
The JDBC spec is unforgivably silent on what should happen to unresolved, pending transactions on Connection close. C3P0's default policy is to rollback any uncommitted, pending work. (I think this is absolutely, undeniably the right policy, but there is no consensus among JDBC driver vendors.) Setting autoCommitOnClose
to true causes uncommitted pending work to be committed, rather than rolled back on Connection close. [Note: Since the spec is absurdly unclear on this question, application authors who wish to avoid bugs and inconsistent behavior should ensure that all transactions are explicitly either committed or rolled-back before close is called.] [See {"Configuring Unresolved Transaction Handling"}]
Default: null
If provided, c3p0 will create an empty table of the specified name, and use queries against that table to test the Connection. If automaticTestTable
is provided, c3p0 will generate its own test query, therefore any preferredTestQuery
set will be ignored. You should not work with the named table after c3p0 creates it; it should be strictly for c3p0's use in testing your Connection. (If you define your own ConnectionTester, it must implement the {QueryConnectionTester} interface for this parameter to be useful.) [See {"Configuring Connection Testing"}]
Default: false
If true, a pooled DataSource will declare itself broken and be permanently closed if a Connection cannot be obtained from the database after making acquireRetryAttempts
to acquire one. If false, failure to obtain a Connection will cause all Threads waiting for the pool to acquire a Connection to throw an Exception, but the DataSource will remain valid, and will attempt to acquire again following a call to getConnection()
. [See {"Configuring Recovery From Database Outages"}]
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: false
If true, and if
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.
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
Strongly disrecommended. Setting this to true
may lead to subtle and bizarre bugs. This is a terrible setting, leave it alone unless absolutely necessary. It is here to workaround broken databases / JDBC drivers that do not properly support transactions, but that allow Connections' autoCommit
flags to go to false regardless. If you are using a database that supports transactions "partially" (this is oxymoronic, as the whole point of transactions is to perform operations reliably and completely, but nonetheless such databases are out there), if you feel comfortable ignoring the fact that Connections with autoCommit == false
may be in the middle of transactions and may hold locks and other resources, you may turn off c3p0's wise default behavior, which is to protect itself, as well as the usability and consistency of the database, by either rolling back (default) or committing (see c3p0.autoCommitOnClose
above) unresolved transactions. This should only be set to true when you are sure you are using a database that allows Connections' autoCommit flag to go to false, but offers no other meaningful support of transactions. Otherwise setting this to true is just a bad idea. [See {"Configuring Unresolved Transaction Handling"}]
Default: 0
If this is a number greater than 0, c3p0 will test all idle, pooled but unchecked-out connections, every this number of seconds. [See {"Configuring Connection Testing"}]
Default: 3
Number of Connections a pool will try to acquire upon startup. Should be between minPoolSize
and maxPoolSize
. [See {"Basic Pool Configuration"}]
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 two 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
Seconds a Connection can remain pooled but unused before being discarded. Zero means idle connections never expire. [See {"Basic Pool Configuration"}]
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
Maximum number of Connections a pool will maintain at any given time. [See {"Basic Pool Configuration"}]
Default: 0
The size of c3p0's global PreparedStatement cache. If both maxStatements
and maxStatementsPerConnection
are zero, statement caching will not be enabled. If maxStatements
is zero but maxStatementsPerConnection
is a non-zero value, statement caching will be enabled, but no global limit will be enforced, only the per-connection maximum. maxStatements
controls the total number of Statements cached, for all Connections. If set, it should be a fairly large number, as each pooled Connection requires its own, distinct flock of cached statements. As a guide, consider how many distinct PreparedStatements are used frequently in your application, and multiply that number by maxPoolSize
to arrive at an appropriate value. Though maxStatements
is the JDBC standard parameter for controlling statement caching, users may find c3p0's alternative maxStatementsPerConnection
more intuitive to use. [See {"Configuring Statement Pooling"}]
Default: 0
The number of PreparedStatements c3p0 will cache for a single pooled Connection. If both maxStatements
and maxStatementsPerConnection
are zero, statement caching will not be enabled. If maxStatementsPerConnection
is zero but maxStatements
is a non-zero value, statement caching will be enabled, and a global limit enforced, but otherwise no limit will be set on the number of cached statements for a single Connection. If set, maxStatementsPerConnection should be set to about the number distinct PreparedStatements that are used frequently in your application, plus two or three extra so infrequently statements don't force the more common cached statements to be culled. Though maxStatements
is the JDBC standard parameter for controlling statement caching, users may find maxStatementsPerConnection
more intuitive to use. [See {"Configuring Statement Pooling"}]
Default: 3
Minimum number of Connections a pool will maintain at any given time. [See {"Basic Pool Configuration"}]
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
Forces the username that should by PooledDataSources when a user calls the default getConnection() method. This is primarily useful when applications are pooling Connections from a non-c3p0 unpooled DataSource. Applications that use ComboPooledDataSource
, or that wrap any c3p0-implemented unpooled DataSource can use the simple {user} property.
Does Not Support Per-User Overrides.
Default: null
Forces the password that should by PooledDataSources when a user calls the default getConnection() method. This is primarily useful when applications are pooling Connections from a non-c3p0 unpooled DataSource. Applications that use ComboPooledDataSource
, or that wrap any c3p0-implemented unpooled DataSource can use the simple {password} property.
Does Not Support Per-User Overrides.
Default: null
For applications using ComboPooledDataSource
or any c3p0-implemented unpooled DataSources - DriverManagerDataSource
or the DataSource returned by DataSources.unpooledDataSource( ... )
- defines the password that will be used for the DataSource's default getConnection()
method. (See also {user}.)
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: 0
Maximum time in seconds before user configuration constraints are enforced. Determines how frequently maxConnectionAge
, maxIdleTime
, maxIdleTimeExcessConnections
, unreturnedConnectionTimeout
are enforced. c3p0 periodically checks the age of Connections to see whether they've timed out. This parameter determines the period. Zero means automatic: A suitable period will be determined by c3p0. [You can call getEffectivePropertyCycle...()
methods on a c3p0 {PooledDataSource} to find the period automatically chosen.]
Default: false
If true, an operation will be performed asynchronously at every connection checkin to verify that the connection is valid. Use in combination with idleConnectionTestPeriod
for quite reliable, always asynchronous Connection testing. Also, setting an automaticTestTable
or preferredTestQuery
will usually speed up all connection tests. [See {"Configuring Connection Testing"}]
Default: false
Use only if necessary. Expensive. If true, an operation will be performed at every connection checkout to verify that the connection is valid. Better choice: verify connections periodically using idleConnectionTestPeriod
. Also, setting an automaticTestTable
or preferredTestQuery
will usually speed up all connection tests. [See {"Configuring Connection Testing"}]
Default: 0
Seconds. If set, if an application checks out but then fails to check-in [i.e. close()] a Connection within the specified period of time, the pool will unceremoniously destroy() the Connection. This permits applications with occasional Connection leaks to survive, rather than eventually exhausting the Connection pool. And that's a shame. Zero means no timeout, applications are expected to close() their own Connections. Obviously, if a non-zero value is set, it should be to a value longer than any Connection should reasonably be checked-out. Otherwise, the pool will occasionally kill Connections in active use, which is bad. This is basically a bad idea, but it's a commonly requested feature. Fix your $%!@% applications so they don't leak Connections! Use this temporarily in combination with debugUnreturnedConnectionStackTraces
to figure out where Connections are being checked-out that don't make it back into the pool!
Default: null
For applications using ComboPooledDataSource
or any c3p0-implemented unpooled DataSources - DriverManagerDataSource
or the DataSource returned by DataSources.unpooledDataSource()
- defines the username that will be used for the DataSource's default getConnection()
method. (See also {password}.)
Does Not Support Per-User Overrides.
Default: false
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, or in a {
} file.
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.NameTransformer
* com.mchange.v2.log.FallbackMLog.DEFAULT_CUTOFF_LEVEL
Configuring JMX

The following property controls c3p0's JMX management interface. Plese see {Configuring and Managing c3p0 via JMX} above for more information.
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

Experimental properties

c3p0-0.9.1 includes 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
. To enable the new implementation, set the following parameter to "true
* com.mchange.v2.resourcepool.experimental.useScatteredAcquireTask
This feature is expected to be enabled by default in c3p0-0.9.2 and above.
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 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
{[Go To Top]}

To override the library's built-in defaults, create a file called
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 /
, 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
should be a normal Java Properties file format, whose keys are c3p0 configurable properties. See {Appendix A}. for the specifics. An example
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 a System properties {[Go To Top]}

c3p0 properties can also be defined as System properties, using the same "c3p0." prefix for properties specified in a
swaldman% java -Dc3p0.maxStatements=150 -Dc3p0.maxIdleTime=1800 example.MyC3P0App

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

Named and Per-User configuration: Overriding c3p0 defaults via c3p0-config.xml
{[Go To Top]}

As of c3p0-0.9.1, you can define multiple configurations in an XML configuration file, and specify in your code which configuration to use. For any configurations (including the unnamed default configuration), you can define overrides for a particular database user. For example, if several applications access your database under different authentication credentials, you might define maxPoolSize
to be 100 for user highVolumeApp
, but only 10 for user lowLoadApp
. (Recall that Connections associated with different authentication credentials are of necessity separated into separate pools, so it makes sense that these could be configured separately.)

You can use the XML config file for all c3p0 configuration, including configuration of defaults. However, for users who don't want or need the extra complexity, the file will continue to be supported.

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

Here is an example c3p0-config.xml
con_test 30000 30 10 30 100 10 200 10 1 0 50 100 50 1000 0 5 1 1 1 5 50

To use a named configuration, you specify the config name when creating your DataSource. For example, using {ComboPooledDataSource
ComboPooledDataSource cpds = new ComboPooledDataSource("intergalactoApp");

Or using the {DataSources
} factory class:
DataSource ds_pooled = DataSources.pooledDataSource( ds_unpooled, "intergalactoApp" );

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. System property setting of configuration value.
3. Configuration values taken from the default configuration of a c3p0-config.xml
4. Configuration values specified in a
5. 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:
* {ComboPooledDataSource.setUserOverridesAsString()
* {WrapperConnectionPoolDataSource.setUserOverridesAsString()
} 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. Values specified in a
9. c3p0's hard-coded default values.
Appendix C: Hibernate-specific notes {[Go To Top]}
Hibernate's C3P0ConnectionProvider explicitly sets 7 c3p0 configuration properties, based on your hibernate configuration, overriding any configuration you may have set in a
file. If you are using Hibernate's C3P0ConnectionProvider you must set the following properties in your hibernate configuration, using hibernate-specific configuration keys. All other properties must be defined as usual in a
file. This is confusing, and will hopefully be simplified some time in the future, but for now...

The following properties must be set in your hibernate configuration:
c3p0-native property name hibernate configuration key
c3p0.acquireIncrement hibernate.c3p0.acquire_increment
c3p0.idleConnectionTestPeriod hibernate.c3p0.idle_test_period
c3p0.initialPoolSize not available -- uses minimum size
c3p0.maxIdleTime hibernate.c3p0.timeout
c3p0.maxPoolSize hibernate.c3p0.max_size
c3p0.maxStatements hibernate.c3p0.max_statements
c3p0.minPoolSize hibernate.c3p0.min_size
c3p0.testConnectionsOnCheckout hibernate.c3p0.validate hibernate 2.x only!

Remember -- these, and only these, properties must be defined in your hibernate configuration, or else they will be set to hibernate-specified defaults. All other configuration properties that you wish to set should be defined in a
file. (See {"Overriding c3p0 defaults via"}.)
Appendix D: Configuring c3p0 DataSources in Tomcat{[Go To Top]}
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
factory org.apache.naming.factory.BeanFactory driverClass org.postgresql.Driver jdbcUrl jdbc:postgresql://localhost/c3p0-test user swaldman password test minPoolSize 5 maxPoolSize 15 acquireIncrement 5

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

The rest is standard J2EE stuff: You'll need to declare your DataSource reference in your web.xml
jdbc/pooledDS javax.sql.DataSource Container

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 notes{[Go 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.
Note: {Users of c3p0 jboss support prior to c3p0-0.9.1 please click here!}

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.}

java:PooledDS jdbc:postgresql://localhost/c3p0-test org.postgresql.Driver swaldman test jboss:service=Naming

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.
{Back to Contents}