初次使用C3P0会如果配置错误，会引发以下异常，乍一看以为是会连接池超时问题。
其实是配置错误，连接池根本没有Connection对象，所以根本拿不到连接对象。
Exception in thread "main" java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.
at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:118)
at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:77)
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:690)
at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:140)
at datasource.C3p0Test.main(C3p0Test.java:13)
Caused by: com.mchange.v2.resourcepool.TimeoutException: A client timed out while waiting to acquire a resource from com.mchange.v2.resourcepool.BasicResourcePool@12bc6874 -- timeout at awaitAvailable()
at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1467)
at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:644)
at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:554)
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutAndMarkConnectionInUse(C3P0PooledConnectionPool.java:758)
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:685)
... 2 more
我的之前的配置文件如下所示，<property name="jdbcUrl">jdbc:postgresql//localhost:5432/shop</property>两个postgresql和//之间漏打了:，倒是参数配置异常报错，所以初学者最好仔细检查配置参数是否正确。
<c3p0-config>
<!-- 使用默认的配置读取连接池对象 -->
<default-config>
<!-- 连接参数 -->
<property name="driverClass">org.postgresql.Driver</property>
<property name="jdbcUrl">jdbc:postgresql//localhost:5432/shop</property>
<property name="user">postgres</property>
<property name="password">123456</property>
<!-- 连接池参数 -->
<property name="initialPoolSize">5</property>
<property name="maxPoolSize">10</property>
<property name="checkoutTimeout">3000</property>
</default-config>
</c3p0-config>
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