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

朱雀 2022-09-18 11:45 251阅读 0赞

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：

<table> 
 <tbody> 
 <tr> 
 <td>14:22:02,783 ERROR SchemaExport:202 - schema export unsuccessful com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception:&nbsp; ** BEGIN NESTED EXCEPTION **&nbsp; 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.&lt;init&gt;(MysqlIO.java:284) at com.mysql.jdbc.Connection.createNewIO(Connection.java:2565) at com.mysql.jdbc.Connection.&lt;init&gt;(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.&lt;init&gt;(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)</td> 
 </tr> 
 </tbody> 
</table>

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：

<property name="hibernate.connection.provider_class">org.hibernate.connection.C3P0ConnectionProvider</property> 
 <property name="c3p0.min_size">5</property> 
 <property name="c3p0.max_size">30</property> 
 <property name="c3p0.time_out">1800</property>  
 <property name="c3p0.max_statement">50</property>  
 <property name="c3p0.acquire_increment">1</property>  
 <property name="c3p0.idle_test_period">120</property>  
 <property name="c3p0.validate">true</property>

按照提示设置好后，又遇到这样一个问题：

<table> 
 <tbody> 
 <tr> 
 <td>14:44:44,298 &nbsp;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</td> 
 </tr> 
 </tbody> 
</table>

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。

![SouthEast][]

Hibernate.cfg.xml配置文件连接数据库设置如下：

<hibernate-configuration>
 	<session-factory>
 		<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
 		
 		<property name="hibernate.connection.url">jdbc:mysql://localhost:3309/hibernate_hql</property>
 		<property name="hibernate.connection.username">root</property>
 		<property name="hibernate.connection.password">123456</property>
 		<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
 		<property name="hibernate.show_sql">true</property>
 		
 		 
 		<mapping resource="com/tgbnode/hibernate/Student.hbm.xml"/>
 		<mapping resource="com/tgbnode/hibernate/Classes.hbm.xml"/>
 	</session-factory>
 </hibernate-configuration>

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。

[SouthEast]: /images/20220828/0bf8b31b5f65431e8fd695d4d939c230.png