c3p0 com.mysql.jdbc.CommunicationsException异常

使用c3p0，偶尔会报异常：

2008-02-18 10:52:53 ERROR Thread-4 com.feedsky.dao.AbstractMutiDbDAO - com.mysql.jdbc.CommunicationsException: Communications 

link failure due to underlying exception: 

** BEGIN NESTED EXCEPTION ** 

java.io.EOFException

STACKTRACE:

java.io.EOFException
        at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1913)
        at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2304)
        at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2803)
        at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1573)
        at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1665)
        at com.mysql.jdbc.Connection.execSQL(Connection.java:3170)
        at com.mysql.jdbc.Connection.execSQL(Connection.java:3099)
        at com.mysql.jdbc.Statement.executeQuery(Statement.java:1169)
        at com.mchange.v2.c3p0.impl.NewProxyStatement.executeQuery(NewProxyStatement.java:35)
        at com.feedsky.dao.AbstractMutiDbDAO.findBySQL(AbstractMutiDbDAO.java:34)
        at com.feedsky.dao.impl.SrcItemDAOImpl.findBySQL(SrcItemDAOImpl.java:326)
        at com.feedsky.dao.impl.SrcItemDAOImpl.loadByItemUuid(SrcItemDAOImpl.java:255)
        at com.feedsky.service.impl.SrcItemServiceImpl.loadByItemUuid(SrcItemServiceImpl.java:37)
        at com.feedsky.controller.SrcFeedController.saveSrcFeed(SrcFeedController.java:97)
        at com.feedsky.work.SrcStoreThread.saveSrcFeed(SrcStoreThread.java:174)
        at com.feedsky.work.SrcStoreThread.run(SrcStoreThread.java:53)
        at EDU.oswego.cs.dl.util.concurrent.PooledExecutor$Worker.run(PooledExecutor.java:748)
        at java.lang.Thread.run(Thread.java:619)


** END NESTED EXCEPTION **



Last packet sent to the server was 176 ms ago.
2008-02-18 10:52:53 ERROR Thread-4 com.feedsky.work.SrcStoreThread - java.lang.NullPointerException
2008-02-18 10:52:53 INFO Thread-2 com.feedsky.dao.impl.SrcItemDAOImpl - com.mysql.jdbc.CommunicationsException: 

Communications link failure due to underlying exception: 

** BEGIN NESTED EXCEPTION ** 

java.io.EOFException

STACKTRACE:

java.io.EOFException
        at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1913)
        at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2304)
        at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2803)
        at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1573)
        at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1665)
        at com.mysql.jdbc.Connection.execSQL(Connection.java:3170)
        at com.mysql.jdbc.Statement.executeUpdate(Statement.java:1316)
        at com.mysql.jdbc.Statement.executeUpdate(Statement.java:1235)
        at com.mchange.v2.c3p0.impl.NewProxyStatement.executeUpdate(NewProxyStatement.java:64)
        at com.feedsky.dao.impl.SrcItemDAOImpl.insert(SrcItemDAOImpl.java:178)
        at com.feedsky.service.impl.SrcItemServiceImpl.save(SrcItemServiceImpl.java:29)
        at com.feedsky.controller.SrcFeedController.saveSrcFeed(SrcFeedController.java:111)
        at com.feedsky.work.SrcStoreThread.saveSrcFeed(SrcStoreThread.java:174)
        at com.feedsky.work.SrcStoreThread.run(SrcStoreThread.java:53)
        at EDU.oswego.cs.dl.util.concurrent.PooledExecutor$Worker.run(PooledExecutor.java:748)
        at java.lang.Thread.run(Thread.java:619)


** END NESTED EXCEPTION **

出现这种异常的原因是：

Mysql服务器默认的“wait_timeout”是8小时，也就是说一个connection空闲超过8个小时，Mysql将自动断开该connection。这就是问题的所在，在C3P0 pools中的connections如果空闲超过8小时，Mysql将其断开，而C3P0并不知道该connection已经失效，如果这时有Client请求connection，C3P0将该失效的Connection提供给Client，将会造成上面的异常。


解决办法：

public synchronized static DataSource createDataSource(String driver,String url,String username,String password) throws SQLException, ClassNotFoundException {
		
		Class.forName(driver);
		
		DataSource ds_unpooled = DataSources.unpooledDataSource(url,username,password); 
		
		Map overrides = new HashMap(); 
		//当连接池中的连接耗尽的时候c3p0一次同时获取的连接数。Default: 3 
		overrides.put("acquireIncrement", 5);
		overrides.put("minPoolSize", 5);
		overrides.put("maxPoolSize", 10);
		//overrides.put("initialPoolSize",cfg.getMaxPoolSize());
		overrides.put("maxStatements", 10000);
		//最大空闲时间,3600秒内未使用则连接被丢弃。若为0则永不丢弃。Default: 0
		overrides.put("maxIdleTime", 3600 );
		overrides.put("automaticTestTable", "C3P0TestTable");
		overrides.put("testConnectionOnCheckin", true);
//		每60秒检查所有连接池中的空闲连接。Default: 0
		overrides.put("idleConnectionTestPeriod",18000);
		overrides.put("testConnectionOnCheckout", true);
		//获取连接失败将会引起所有等待连接池来获取连接的线程抛出异常。但是数据源仍有效 
		//保留，并在下次调用getConnection()的时候继续尝试获取连接。如果设为true，那么在尝试 
		//获取连接失败后该数据源将申明已断开并永久关闭。Default: false 
		overrides.put("breakAfterAcquireFailure", true);
		
		//c3p0是异步操作的，缓慢的JDBC操作通过帮助进程完成。扩展这些操作可以有效的提升性能 
		//通过多线程实现多个操作同时被执行。Default: 3
		overrides.put("numHelperThreads", 10);
		
		// create the PooledDataSource using the default configuration and our overrides
		DataSource ds_pooled = DataSources.pooledDataSource( ds_unpooled, overrides );
		return ds_pooled; 
	}

注意参数:

//如果设为true那么在取得连接的同时将校验连接的有效性。Default: false
testConnectionOnCheckin = true
//自动测试的table名称,c3p0将建一张名为C3P0TestTable的空表，并使用其自带的查询语句进行测试。如果定义了这个参数那么
//属性preferredTestQuery将被忽略。你不能在这张Test表上进行任何操作，它将只供c3p0测试使用。Default: null
automaticTestTable=C3P0TestTable
//每1800秒检查所有连接池中的空闲连接。Default: 0
idleConnectionTestPeriod = 1800
//最大空闲时间,3600秒内未使用则连接被丢弃。若为0则永不丢弃。Default: 0
maxIdleTime = 3600
/**因性能消耗大请只在需要的时候使用它。如果设为true那么在每个connection提交的
时候都将校验其有效性。建议使用idleConnectionTestPeriod或automaticTestTable
等方法来提升连接测试的性能。Default: false */
testConnectionOnCheckout = true

另外:需要在JDBC URL上面加一个autoReconnect=true 参数

还有一个需要注意：mysql服务的wait_timeout和interactive_timeout的值要大于c3p0的maxIdleTime参数值