采用JDBC连接池遇到的问题

[align=left]最近采用多线程使用JDBC连接数据库时，遇到如下问题，当连接次数达到一定次数后，就出现了如下错误提示[/align]
[align=left][/align]
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[align=left]17:18:14,569 DEBUG BasicResourcePool:1831 - An exception occurred while acquiring a poolable resource. Will retry.[/align]
[align=left]com.mysql.jdbc.exceptions.MySQLNonTransientConnectionException: Too many connections[/align]
[align=left] at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:921)[/align]
[align=left] at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2822)[/align]
[align=left] at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:777)[/align]
[align=left] at com.mysql.jdbc.MysqlIO.secureAuth411(MysqlIO.java:3216)[/align]
[align=left] at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1147)[/align]
[align=left] at com.mysql.jdbc.Connection.createNewIO(Connection.java:2595)[/align]
[align=left] at com.mysql.jdbc.Connection.<init>(Connection.java:1509)[/align]
[align=left] at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266)[/align]
[align=left] at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:135)[/align]
[align=left] at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)[/align]
[align=left] at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)[/align]
[align=left] at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)[/align]
[align=left] at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)[/align]
[align=left] at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)[/align]
[align=left] at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)[/align]
[align=left] at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)[/align]
[align=left]17:18:14,569 DEBUG BasicResourcePool:404 - incremented pending_acquires: 3[/align]
[align=left]17:18:14,584 DEBUG BasicResourcePool:1831 - An exception occurred while acquiring a poolable resource. Will retry.[/align]
[align=left]com.mysql.jdbc.exceptions.MySQLNonTransientConnectionException: Data source rejected establishment of connection, message from server: "Too many connections"[/align]
[align=left] at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:921)[/align]
[align=left] at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:962)[/align]
[align=left] at com.mysql.jdbc.Connection.createNewIO(Connection.java:2595)[/align]
[align=left] at com.mysql.jdbc.Connection.<init>(Connection.java:1509)[/align]
[align=left] at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266)[/align]
[align=left] at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:135)[/align]
[align=left] at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)[/align]
[align=left] at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)[/align]
[align=left] at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)[/align]
[align=left] at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)[/align]
[align=left] at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)[/align]
[align=left] at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)[/align]
[align=left] at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)[/align]
这个是已经达到了数据库得最大连接个数，解决的方法可以设置数据库得max_connections的个数。另外一种就是可以采取连接池得方式，从连接池中获取Connection对象，由连接池来管理和数据库得连接。

c3p0-0.9.1.1就是提供了一个连接池得管理，c3p0 - JDBC3 Connection and Statement Pooling。

下栽网址 http://sourceforge.net/projects/c3p0/

使用得时候，把解压后lib/c3p0-0.9.1.1.jar 加入到ClASSPATH中中就行了。在里面提供了几个Example

线程和非线程的

DataSource ds_unpooled = DataSources.unpooledDataSource("jdbc:postgresql://localhost/testdb", "swaldman", "test-password");

DataSource ds_pooled = DataSources.pooledDataSource( ds_unpooled ); 你还可以自己设置数据源的一些属性

Map overrides = new HashMap();

overrides.put("maxStatements", "200");

DataSource ds_pooled = DataSources.pooledDataSource( ds_unpooled, overrides );

获取Connection对象

Connection conn = ds_pooled.getConnection();