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:


java 代码 

1. org.hibernate.exception.JDBCConnectionException: could not execute query
2. at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:74)
3. at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
4. .......
5. Caused by: com.mysql.jdbc.exceptions.MySQLNonTransientConnectionException: No operations allowed after connection closed.Connection was implicitly closed due to underlying exception/error:
6. 
7. 
8. ** BEGIN NESTED EXCEPTION **
9. 
10. com.mysql.jdbc.CommunicationsException
11. MESSAGE: Communications link failure due to underlying exception:
12. 
13. ** BEGIN NESTED EXCEPTION **
14. 
15. java.net.SocketException
16. MESSAGE: Broken pipe
17. 
18. STACKTRACE:
19. 
20. java.net.SocketException: Broken pipe
21. at java.net.SocketOutputStream.socketWrite0(Native Method)
22. ......
23. ** 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：

  1. 增加wait_timeout的时间。
  2. 减少Connection pools中connection的lifetime。
  3. 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:

1. //set to 'SELECT 1'
2. validationQuery = "SELECT 1"
3. //set to 'true'
4. testWhileIdle = "true"
5. //some positive integer
6. timeBetweenEvictionRunsMillis = 3600000
7. //set to something smaller than 'wait_timeout'
8. minEvictableIdleTimeMillis = 18000000
9. //if you don't mind a hit for every getConnection(), set to "true"
10. testOnBorrow = "true"
 C3P0增加以下配置信息:
1. //获取connnection时测试是否有效
2. testConnectionOnCheckin = true
3. //自动测试的table名称
4. automaticTestTable=C3P0TestTable
5. //set to something much less than wait_timeout, prevents connections from going stale
6. idleConnectionTestPeriod = 18000
7. //set to something slightly less than wait_timeout, preventing 'stale' connections from being handed out
8. maxIdleTime = 25000
9. //if you can take the performance 'hit', set to "true"
10. testConnectionOnCheckout = true

更多的配置信息大家可以查看C3P0文档，Connector/J文档，以及DBCP的文档。