c3p0连接错误 An attempt by a client to checkout a...

c3p0作为连接池，报：java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.
网上查了很久，有人说是checkoutTimeout太短的原因，（Default: 0）于是由原来的1000改为2000,问题还是未解决。
然后找了份没有问题的配置作了些对比，发现我的maxPoolSize＝100显得有些大，于是改为默认的15.再测试，居然没有问题了。后来再试了大一点的60,也没有问题。
下面是我的配置：

<bean id="dataSource"
        class="com.mchange.v2.c3p0.ComboPooledDataSource"
        destroy-method="close">
        <property name="driverClass" value="com.mysql.jdbc.Driver" />
        <property name="jdbcUrl"
            value="jdbc:mysql://localhost:3306/gjp" />
        <property name="user" value="root" />
        <property name="password" value="gjpgjp" />
        <property name="initialPoolSize" value="20" />
        <property name="minPoolSize" value="10" />
        <property name="maxPoolSize" value="60" />
        <property name="maxIdleTime" value="7200" />
        <property name="idleConnectionTestPeriod" value="360" />
        <property name="preferredTestQuery" value="select 1" />
        <property name="acquireIncrement" value="5" />
        <property name="acquireRetryAttempts" value="50" />
        <property name="acquireRetryDelay" value="1000" />
        <property name="breakAfterAcquireFailure" value="true" />
        <property name="checkoutTimeout" value="20000" />
        <property name="autoCommitOnClose" value="false" />
        <property name="forceIgnoreUnresolvedTransactions" value="false" />
        <property name="unreturnedConnectionTimeout" value="1000" />
        <property name="maxStatements" value="0" />
        <property name="maxStatementsPerConnection" value="0" />
        <property name="testConnectionOnCheckin" value="true" />
        <property name="testConnectionOnCheckout" value="false" />
        <property name="usesTraditionalReflectiveProxies" value="false" />
        <property name="numHelperThreads" value="5" />
    </bean>