C3p0使用时出现的连接异常

所属分类：[数据库层] | 创建时间：2017-01-10 | 文章属性：原创 | 文章来源：http://windfly.cn | 作者：windfly

## 环境
> 多个程序公用一个数据库  
长时间运行

## 异常
```
  1. Caused by: java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.  
  2. at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:106)  
  3. at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:65)  
  4. at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:527)  
  5. at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:128)  
  6. at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:113)  
  7. at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:79)  
  8. ... 5 more  
  9. Caused by: com.mchange.v2.resourcepool.TimeoutException: A client timed out while waiting to acquire a resource from -- timeout at awaitAvailable()  
  10. at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1317)  
  11. at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:557)  
  12. at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:477)  
  13. at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:525)  
  14. ... 8 more  
```

## 解决
> 从网络查询分析，很多说是C3P0的bug问题。c3p0在同时关闭statement和connection的时候，或者关闭他们之间的时间很短的时候，有时候connection并没有被关闭，因为有些preparedstatement还在被cached住。这样就会有很多connection并没有真正的被关闭，连接池的连接都给耗尽了，就会产生上面的异常。解决的方案就是把缓存关闭也就是把c3p0.max_statements 设置成0，这样就不会有缓存的preparedstatement，而设置的c3p0.idle_test_period又小于c3p0.timeout，这样的设置应该没有什么问题了。  
***
> 经查，项目里每个程序的max_statements 都达到100到500，修改为10后，运行几天无异常