(转载)c3p0 - JDBC3 Connection and Statement Pooling

http://www.mchange.com/projects/c3p0/index.html

 

c3p0 - JDBC3 Connection and Statement Pooling
version 0.9.1.1

by Steve Waldman <[e-mail]>

© 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?
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Contents
Contents
Quickstart
What is c3p0?
Prerequisites
Installation
Using c3p0
Using ComboPooledDataSource
Using the DataSouces factory class
Box: Overriding authentication information (from non-c3p0 DataSources)
Querying Pool Status
Box: Using C3P0Registry to find a reference to a DataSource
Cleaning Up Pool Resources
Advanced: Building Your Own PoolBackedDataSource
Advanced: Raw Connection and Statement Operations
Configuration
Introduction
Basic Pool Configuration
Managing Pool Size and Connection Age
Configuring Connection Testing
Configuring Statement Pooling
Configuring Recovery From Database Outages
Managing Connection Lifecycles with Connection Customizers
Configuring Unresolved Transaction Handling
Configuring to Debug and Workaround Broken Client Applications
Other DataSource Configuration
Configuring and Managing c3p0 via JMX
Configuring Logging
Performance
Known shortcomings
Feedback and support
Appendix A: Configuration Properties
Appendix B: Configuation Files, etc.
Overriding c3p0 defaults with a c3p0.properties file
Overriding c3p0 defaults with a System properties
Named and Per-User configuration: Overriding c3p0 defaults via c3p0-config.xml
Precedence of Configuration Settings
Appendix C: Hibernate-specific notes
Appendix D: Configuring c3p0 pooled DataSources for Apache Tomcat
Appendix E: JBoss-specific notes
Appendix F: Oracle-specific API: createTemporaryBLOB() and createTemporaryCLOB()
(See also the API Documentation here)

Quickstart

c3p0 was designed to be butt-simple to use. Just put the jar file [lib/c3p0-0.9.1.1.jar] 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?

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

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

Put the file lib/c3p0-0.9.1.1.jar somewhere in your CLASSPATH (or any other place where your application's classloader will find it). That's it!

Using c3p0

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 c3p0.properties 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

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

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...
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

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

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!");
}
 

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

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

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);   
     

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

Configuration

Introduction

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

c3p0 Connection pools are very easy to configure via the following basic parameters:
acquireIncrement
initialPoolSize
maxPoolSize
maxIdleTime
minPoolSize

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

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.

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

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

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. 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

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

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.

Configuring Recovery From Database Outages

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

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

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

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

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

See Appendix A for information about the following configuration properties:
checkoutTimeout
factoryClassLocation
maxAdministrativeTaskTime
numHelperThreads
usesTraditionalReflectiveProxies

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.)

checkoutTimeout 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

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 c3p0.properties:

com.mchange.v2.c3p0.management.ManagementCoordinator=com.mchange.v2.c3p0.management.NullManagementCoordinator     
     

Configuring Logging

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 c3p0.properties file, 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.
com.mchange.v2.log.MLog
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.
com.mchange.v2.log.NameTransformer
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.
com.mchange.v2.log.FallbackMLog.DEFAULT_CUTOFF_LEVEL
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
SEVERE
WARNING
INFO
CONFIG
FINE
FINER
FINEST
ALL
This property defaults to INFO.

Performance

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

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 property.

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

Please provide any and all feedback to <[e-mail]>! 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 Properties

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

JavaBeans-style Properties

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), or in a c3p0-config.xml file. See the section on Configuration above. Click on the property name for a full descrip-tion. acquireIncrement
acquireRetryAttempts
acquireRetryDelay
autoCommitOnClose
automaticTestTable
breakAfterAcquireFailure
checkoutTimeout
connectionCustomizerClassName
connectionTesterClassName
debugUnreturnedConnectionStackTraces
factoryClassLocation forceIgnoreUnresolvedTransactions
idleConnectionTestPeriod
initialPoolSize
maxAdministrativeTaskTime
maxConnectionAge
maxIdleTime
maxIdleTimeExcessConnections
maxPoolSize
maxStatements
maxStatementsPerConnection
minPoolSize numHelperThreads
overrideDefaultUser
overrideDefaultPassword
password
preferredTestQuery
propertyCycle
testConnectionOnCheckin
testConnectionOnCheckout
unreturnedConnectionTimeout
user
usesTraditionalReflectiveProxies

acquireIncrement
Default: 3
Determines how many connections at a time c3p0 will try to acquire when the pool is exhausted. [See "Basic Pool Configuration"]
acquireRetryAttempts
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"]
acquireRetryDelay
Default: 1000
Milliseconds, time c3p0 will wait between acquire attempts. [See "Configuring Recovery From Database Outages"]
autoCommitOnClose
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"]
automaticTestTable
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"]
breakAfterAcquireFailure
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"]
checkoutTimeout
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.
connectionCustomizerClassName
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.
connectionTesterClassName
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"]
debugUnreturnedConnectionStackTraces
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.
Does Not Support Per-User Overrides.
factoryClassLocation
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.
forceIgnoreUnresolvedTransactions
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"]
idleConnectionTestPeriod
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"]
initialPoolSize
Default: 3
Number of Connections a pool will try to acquire upon startup. Should be between minPoolSize and maxPoolSize. [See "Basic Pool Configuration"]
maxAdministrativeTaskTime
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.
maxConnectionAge
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.
maxIdleTime
Default: 0
Seconds a Connection can remain pooled but unused before being discarded. Zero means idle connections never expire. [See "Basic Pool Configuration"]
maxIdleTimeExcessConnections
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.
maxPoolSize
Default: 15
Maximum number of Connections a pool will maintain at any given time. [See "Basic Pool Configuration"]
maxStatements
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"]
maxStatementsPerConnection
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"]
minPoolSize
Default: 3
Minimum number of Connections a pool will maintain at any given time. [See "Basic Pool Configuration"]
numHelperThreads
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.
overrideDefaultUser
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.
overrideDefaultPassword
Default: null
Forces the password that should by Po