Mysql服务器默认的“wait_timeout”是8小时，Mysql将自动断开该connection。在C3P0 pools中的connections如果空闲超过8小时，mysql将其断开，而C3P0并不知道该connection已经失效，如果这时有 Client请求connection，C3P0将该失效的Connection提供给Client，将会造成异常问题。
mysql抛出异常Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost，具体如下：
The last packet successfully received from the server was 2,032,862 milliseconds ago. The last packet sent successfully to the server was 1 milliseconds ago. at sun.reflect.GeneratedConstructorAccessor86.newInstance(Unknown Source) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:526) at com.mysql.jdbc.Util.handleNewInstance(Util.java:411) at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1117) at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3567) at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3456) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3997) at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2468) at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2629) at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2713) at com.mysql.jdbc.ConnectionImpl.setAutoCommit(ConnectionImpl.java:5060) at com.mchange.v2.c3p0.impl.NewProxyConnection.setAutoCommit(NewProxyConnection.java:881) at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.doBegin(JdbcTransaction.java:72) ... 71 more Caused by: java.io.EOFException: Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost. at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:3017) at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3467) ... 79 more
分析，出现这种异常的原因是：执行sql语句时没有得到响应，连接超时将会造成上面的异常。针对此问题的解决方案，c3p0连接池的配置文件中增加如下内容：
<property name="preferredTestQuery" value="SELECT 1"/> <property name="idleConnectionTestPeriod" value="18000"/> <property name="maxIdleTime" value="25000"/> <property name="testConnectionOnCheckout" value="true"/>
连接池配置文件各参数含义，参考地址：http://blog.yoodb.com/yoodb/article/detail/159
