最近的一个项目在Hibernate使用C3P0的连接池，数据库为Mysql。开发测试没有问题，在运行中每个一段长的空闲时间就出现异常:
org.hibernate.exception.JDBCConnectionException: could not execute query at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java: 74 ) at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java: 43 ) . Caused by: com.mysql.jdbc.exceptions.MySQLNonTransientConnectionException: No operations allowed after connection closed.Connection was implicitly closed due to underlying exception / error: ** BEGIN NESTED EXCEPTION ** com.mysql.jdbc.CommunicationsException MESSAGE: Communications link failure due to underlying exception: ** BEGIN NESTED EXCEPTION ** java.net.SocketException MESSAGE: Broken pipe STACKTRACE: java.net.SocketException: Broken pipe at java.net.SocketOutputStream.socketWrite0(Native Method) ** END NESTED EXCEPTION **
查看了Mysql的文档，以及Connector/J的文档以及在线说明发现，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。
解决的方法有3种：
- 增加wait_timeout的时间。
- 减少Connection pools中connection的lifetime。
- 测试Connection pools中connection的有效性。
当然最好的办法是同时综合使用上述3种方法，下面就DBCP和C3P0分别做一说明，假设wait_timeout为默认的8小时
DBCP增加以下配置信息:
// set to 'SELECT 1' validationQuery = " SELECT 1 " // set to 'true' testWhileIdle = " true " // some positive integer timeBetweenEvictionRunsMillis = 3600000 // set to something smaller than 'wait_timeout' minEvictableIdleTimeMillis = 18000000 // if you don't mind a hit for every getConnection(), set to "true" testOnBorrow = " true "
C3P0增加以下配置信息:
// 获取connnection时测试是否有效 testConnectionOnCheckin = true // 自动测试的table名称 automaticTestTable = C3P0TestTable // set to something much less than wait_timeout, prevents connections from going stale idleConnectionTestPeriod = 18000 // set to something slightly less than wait_timeout, preventing 'stale' connections from being handed out maxIdleTime = 25000 // if you can take the performance 'hit', set to "true" testConnectionOnCheckout = true
