在Hibernate 3.2中使用连接池（c3p0-0.9.1.2）时，获取exception和应用程序无法与MySqL数据库连接？
我正在使用连接池（c3p0-0.9.1.2）和Hibernate 3.2，如下所示：hibernate.cfg.xml
org.hibernate.dialect.MySQLDialect com.mysql.jdbc.Driver jdbc:mysql://localhost:3306/bspersonaltrainer root admin thread org.hibernate.cache.NoCacheProvider org.hibernate.connection.C3P0ConnectionProvider 2 400 50 600 300 false 但我经常得到这个例外。
Exception in thread "com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1" java.lang.NoClassDefFoundError: com/mysql/jdbc/SQLError at com.mysql.jdbc.Util.handleNewInstance(Util.java:430) at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282) at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137) at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014) at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32) at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810) at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547) Caused by: java.lang.ClassNotFoundException: com.mysql.jdbc.SQLError at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1680) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1526) ... 11 more Exception in thread "com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0" java.lang.NoClassDefFoundError: com/mysql/jdbc/SQLError at com.mysql.jdbc.Util.handleNewInstance(Util.java:430) at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282) at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137) at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014) at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32) at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810) at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547) 16 Jan, 2013 10:44:42 AM org.apache.catalina.loader.WebappClassLoader loadClass INFO: Illegal access: this web application instance has been stopped already. Could not load com.mysql.jdbc.SQLError. The eventual following stack trace is caused by an error thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access, and has no functional impact. java.lang.IllegalStateException at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1566) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1526) at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2103) at com.mysql.jdbc.ConnectionImpl.(ConnectionImpl.java:718) at com.mysql.jdbc.JDBC4Connection.(JDBC4Connection.java:46) at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:532) at com.mysql.jdbc.Util.handleNewInstance(Util.java:406) at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282) at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137) at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014) at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32) at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810) at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547) 16 Jan, 2013 10:44:42 AM org.apache.catalina.loader.WebappClassLoader loadClass INFO: Illegal access: this web application instance has been stopped already. Could not load com.mysql.jdbc.SQLError. The eventual following stack trace is caused by an error thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access, and has no functional impact. java.lang.IllegalStateException at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1566) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1526) at com.mysql.jdbc.Util.handleNewInstance(Util.java:430) at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282) at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137) at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014) at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32) at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810) at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
看起来你可能忘记在热重启web应用程序时关闭你的hibernateSessionFactory。 这个离开的c3p0的助手来自死实例的线程（在Tomcat中暗示一个现在已经损坏的ClassLoader）无用地运行并导致问题。
c3p0 DataSources应该在不再使用时立即关闭（），以释放所有池化的连接和帮助程序线程。 对于Web应用程序，通常应该通过ServletContextListener来完成。 如果您的池由hibernate管理，请立即关闭SessionFactory。