c3p0配置以及连接MySQL数据库timeout的问题

一、
经过N多试验，终于自己把这个问题搞定了。

网上关于C3P0在spring中的配置，几乎没有完全正确的（至少我还没发现）。查了c3p0的文档，又试验过N次。得出如下配置是正确的：


<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
      <property name="driverClass"><value>${jdbc.driverClassName}</value></property>
      <property name="jdbcUrl"><value>${jdbc.url}</value></property>            
      <property name="user"><value>${jdbc.username}</value></property>
      <property name="password"><value>${jdbc.password}</value></property>
     
      <property name="minPoolSize"><value>1</value></property>
      <property name="maxPoolSize"><value>20</value></property>
      <property name="maxIdleTime"><value>1800</value></property>
      <property name="acquireIncrement"><value>2</value></property>
      <property name="maxStatements"><value>0</value></property>
      <property name="initialPoolSize"><value>2</value></property>
      <property name="idleConnectionTestPeriod"><value>1800</value></property>
      <property name="acquireRetryAttempts"><value>30</value></property>
      <property name="breakAfterAcquireFailure"><value>true</value></property>
      <property name="testConnectionOnCheckout"><value>false</value></property>
     
      <!--
         <property name="properties">
      <props>            
             <prop key="c3p0.minPoolSize">1</prop>
             <prop key="c3p0.maxPoolSize">10</prop>
             <prop key="c3p0.maxIdleTime">1800</prop>            
             <prop key="c3p0.acquireIncrement">2</prop>
             <prop key="c3p0.maxStatements">0</prop>
                   <prop key="c3p0.initialPoolSize">2</prop>
             <prop key="c3p0.idleConnectionTestPeriod">1800</prop>
             <prop key="c3p0.acquireRetryAttempts">30</prop>
             <prop key="c3p0.breakAfterAcquireFailure">true</prop>
             <prop key="c3p0.testConnectionOnCheckout">true</prop>
             <prop key="user">root</prop>
             <prop key="password">999999</prop>
           
      </props>
      </property>
       -->   
</bean>

<!-- Hibernate SessionFactory -->
<bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
   <property name="dataSource" ref="dataSource"/>
   <property name="mappingDirectoryLocations">
   <list>
<value>classpath:/com/licaionline/domain/</value>
   </list>
   </property>
   <property name="hibernateProperties">
<props>
<prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>
    <prop key="hibernate.show_sql">true</prop>
<prop key="hibernate.generate_statistics">true</prop>
            <prop key="hibernate.connection.release_mode">auto</prop>                  
            <prop key="hibernate.autoReconnect">true</prop>
            <prop key="hibernate.cglib.use_reflection_optimizer">true</prop>
            <!--
<prop key="hibernate.useUnicode"></prop>
<prop key="hibernate.characterEncoding"></prop>
<prop key="hibernate.default-lazy-init"></prop>
            <prop key="hibernate.connection.provider_class">org.hibernate.connection.C3P0ConnectionProvider</prop>   
-->
                             
            <!--
            <prop key="hibernate.c3p0.acquire_increment">2</prop>
            <prop key="hibernate.c3p0.idle_test_period">1800</prop>
            <prop key="hibernate.c3p0.timeout">1800</prop>
            <prop key="hibernate.c3p0.max_size">30</prop>
            <prop key="hibernate.c3p0.min_size">2</prop>
            <prop key="hibernate.c3p0.max_statements">50</prop>
-->      
</props>
   </property>




注意：注释掉的那些，都是错误的。网上流行的，基本上跟注释掉的那些差不多。配错了，并无异常，还是能正常使用。但是所作的配置不起作用。


二、
起作用的，是datasource里面的这一句：
<property name="maxIdleTime"><value>1800</value></property>

这儿设置成每隔1800秒就扫描一次，检查一下空闲的链接。所以，用户基本上不会得到空闲的链接了。

如果再不放心，
<property name="testConnectionOnCheckout"><value>false</value></property>
这儿设置成true。每次连接之前，都要测一下。但是这样会影响效率。


三、

解决的方法有3种：

增加wait_timeout的时间。
减少Connection pools中connection的lifetime。
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:

//set to 'SELECT 1'
validati
//set to 'true'
testWhileIdle = "true"   
//some positive integer
timeBetweenEvictionRunsMillis = 3600000
//set to something smaller than 'wait_timeout'
minEvictableIdleTimeMillis = 18000000
//if you don't mind a hit for every getConnection(), set to "true"
test

C3P0增加以下配置信息:

//set to 'SELECT 1'   
preferredTestQuery = 'SELECT 1'   
//set to something much less than wait_timeout, prevents connections from going stale
idleConnectionTestPeriod = 18000   
//set to something slightly less than wait_timeout, preventing 'stale' connections from being handed out
maxIdleTime = 25000   
//if you can take the performance 'hit', set to "true"
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。