走近SSH之Hibernate--com.mysql.jdbc.CommunicationsException: Communications link failure due to underlyi

    进行java的学习已有几个月的时间了，可还是感觉使用vs+sqlserver开发比较舒服，最近学习java框架，接触了很多新概念，遇到各式各样的问题真有些不习惯，提示都是英文字母+阿拉伯数字，不过通过一个一个词翻译也能理解出现的错误是什么意思。

   今天由于mysql的编码问题，我不小心重装了mysql，装完后在建立数据库表的时候，出现下面这样一个问题：

14:22:02,783 ERROR SchemaExport:202 - schema export unsuccessful com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception:  ** BEGIN NESTED EXCEPTION **  java.net.SocketException MESSAGE: java.net.ConnectException: Connection refused: connect STACKTRACE: java.net.SocketException: java.net.ConnectException: Connection refused: connect at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:156) at com.mysql.jdbc.MysqlIO.(MysqlIO.java:284) at com.mysql.jdbc.Connection.createNewIO(Connection.java:2565) at com.mysql.jdbc.Connection.(Connection.java:1485) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266) at java.sql.DriverManager.getConnection(DriverManager.java:582) at java.sql.DriverManager.getConnection(DriverManager.java:154) at org.hibernate.connection.DriverManagerConnectionProvider.getConnection(DriverManagerConnectionProvider.java:110) at org.hibernate.tool.hbm2ddl.ManagedProviderConnectionHelper.prepare(ManagedProviderConnectionHelper.java:28) at org.hibernate.tool.hbm2ddl.SchemaExport.execute(SchemaExport.java:180) at org.hibernate.tool.hbm2ddl.SchemaExport.create(SchemaExport.java:133) at com.tgbnode.hibernate.ExportDB.main(ExportDB.java:21) ** END NESTED EXCEPTION ** Last packet sent to the server was 20 ms ago. at com.mysql.jdbc.Connection.createNewIO(Connection.java:2631) at com.mysql.jdbc.Connection.(Connection.java:1485) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266) at java.sql.DriverManager.getConnection(DriverManager.java:582) at java.sql.DriverManager.getConnection(DriverManager.java:154) at org.hibernate.connection.DriverManagerConnectionProvider.getConnection(DriverManagerConnectionProvider.java:110) at org.hibernate.tool.hbm2ddl.ManagedProviderConnectionHelper.prepare(ManagedProviderConnectionHelper.java:28) at org.hibernate.tool.hbm2ddl.SchemaExport.execute(SchemaExport.java:180) at org.hibernate.tool.hbm2ddl.SchemaExport.create(SchemaExport.java:133) at com.tgbnode.hibernate.ExportDB.main(ExportDB.java:21)

    通过错误提示，可以大概看出这是数据库无法连接的问题。通过查找问题的解决方法，发现异常“com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception”，出现的原因是：Mysql服务器默认的“wait_timeout”是8小时，也就是说一个connection空闲超过8个小时，Mysql将自动断开该 connection。这就是问题的所在，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，将会造成上面的异常。

最后在网上找到一个办法。为hibernate配置连接池，推荐用c3p0，然后配置c3p0的反空闲设置idle_test_period，只要小于MySQL的wait timeout即可。
在hibernate.cfg.xml中增加下面几项：

 
org.hibernate.connection.C3P0ConnectionProvider 
5 
30 
1800  
50  
1  
120  
true 

按照提示设置好后，又遇到这样一个问题：

14:44:44,298  WARN BasicResourcePool:1222 - com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@6c69d02b -- Acquisition Attempt Failed!!! Clearing pending acquires. While trying to acquire a needed new resource, we failed to succeed more than the maximum number of allowed acquisition attempts (30). 14:44:44,311 ERROR SchemaExport:202 - schema export unsuccessful java.sql.SQLException: Connections could not be acquired from the underlying database! at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:104) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:236) at com.mchange.v2.c3p0.PoolBackedDataSource.getConnection(PoolBackedDataSource.java:94) at org.hibernate.connection.C3P0ConnectionProvider.getConnection(C3P0ConnectionProvider.java:35) at org.hibernate.tool.hbm2ddl.ManagedProviderConnectionHelper.prepare(ManagedProviderConnectionHelper.java:28) at org.hibernate.tool.hbm2ddl.SchemaExport.execute(SchemaExport.java:180) at org.hibernate.tool.hbm2ddl.SchemaExport.create(SchemaExport.java:133) at com.tgbnode.hibernate.ExportDB.main(ExportDB.java:21) 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.awaitAcquire(BasicResourcePool.java:970) at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:208) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:232) ... 6 more

这里提示的更清晰了，就是连接数据库不成功，这时候突然想起自己重装mysql时，端口号用的是3309，而我之前使用的端口号是3306，敲代码时使用的也是3306，将代码中的端口3306改为3309问题解决了。

Hibernate.cfg.xml配置文件连接数据库设置如下：

	
		com.mysql.jdbc.Driver
		
		jdbc:mysql://localhost:3309/hibernate_hql
		root
		123456
		org.hibernate.dialect.MySQLDialect
		true
		
		 
		
		
	

Hibernate连接MySql，Hibernate代码中设置的端口应该与MySql数据库中的对应才能正常连接，这看似不起眼的问题，其实恰恰是全局观的展现，也是知识网的一种展现。