有时 Hibernate 无法通过 C3P0 获得连接

Question

我有一个项目使用Struts2+hibernate3(c3p0)，

以下是我的 C3P0 配置

    <property name="hibernate.connection.provider_class">org.hibernate.connection.C3P0ConnectionProvider</property>
    <property name="hibernate.c3p0.acquire_increment">2</property>
    <property name="hibernate.c3p0.idle_test_period">15</property>
    <property name="hibernate.c3p0.max_idle_time">10</property>
    <property name="hibernate.c3p0.max_size">50</property>
    <property name="hibernate.c3p0.max_statements">100</property>
    <property name="hibernate.c3p0.min_size">20</property>
    <property name="hibernate.c3p0.timeout">10</property>
    <property name="hibernate.c3p0.validate">true</property>
    <property name="hibernate.c3p0.preferredTestQuery">select 1 from dual</property>
    <property name="automaticTestTable">Test</property>
    <!-- <property name="hibernate.c3p0.testConnectionOnCheckout">true</property> -->

    <property name="hibernate.connection.autoReconnect">true</property>
    <property name="hibernate.connection.autoReconnectForPools">true</property>
    <property name="hibernate.connection.is-connection-validation-required">true</property>

我的数据库是 mysql，我无法配置它（在云端）

我的问题是我有一段时间收到以下消息

Caused by: org.hibernate.exception.GenericJDBCException: Cannot open connection
at org.hibernate.exception.SQLStateConverter.handledNonSpecificException(SQLStateConverter.java:126)
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:114)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:52)
at org.hibernate.jdbc.ConnectionManager.openConnection(ConnectionManager.java:449)
at org.hibernate.jdbc.ConnectionManager.getConnection(ConnectionManager.java:167)
at org.hibernate.jdbc.JDBCContext.connection(JDBCContext.java:142)
at org.hibernate.transaction.JDBCTransaction.begin(JDBCTransaction.java:85)
at org.hibernate.impl.SessionImpl.beginTransaction(SessionImpl.java:1354)
at com.rh.ws.getAllMessageAction.getAllMessage(getAllMessageAction.java:42)
... 86 more
Caused by: java.sql.SQLException: Connections could not be acquired from the underlying database!
    at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:118)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:690)
    at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:140)
    at org.hibernate.connection.C3P0ConnectionProvider.getConnection(C3P0ConnectionProvider.java:78)
    at org.hibernate.jdbc.ConnectionManager.openConnection(ConnectionManager.java:446)
    ... 91 more
Caused by: com.mchange.v2.resourcepool.CannotAcquireResourceException: A ResourcePool could not acquire a resource from its primary factory or source.
    at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1422)
    at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:606)
    at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:526)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutAndMarkConnectionInUse(C3P0PooledConnectionPool.java:756)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:683)

我的数据库（mysql）中的wait_timeout是30s，它不能改变。

我尝试将 max_idle_time 设置为 20，但我发现它不起作用。

我每次都需要关闭连接吗？还是直接关闭会话就好了？

Answer 1

当您关闭会话时，它会自动关闭底层连接。您还应该在成功时提交 Hibernate Transaction，并在失败时回滚。这将确保数据库事务被正确终止。

如果您不关闭 Hibernate Session，则不会释放 JDBC 连接。所以当所有hibernate.c3p0.max_size都在使用时，你很快就会得到连接获取连接异常。

另一个原因可能是查询运行时间过长。同时检查您的查询。

确保：

<prop key="hibernate.c3p0.checkoutTimeout">30</prop>

有足够的连接获取超时值。