问题：
系统采用Spring MVC 2.5 + Spring 2.5 + Hibernate 3.2架构，其中数据源连接池采用的是Apache commons DBCP。问题是这样的，系统运行一段时间后（大致每隔8小时），访问系统会出现如下错误，再次访问恢复正常。
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.hibernate.exception.GenericJDBCException: Cannot release connection at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:583) at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:501) at javax.servlet.http.HttpServlet.service(HttpServlet.java:617) at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) Caused by: org.hibernate.exception.GenericJDBCException: Cannot release connection at org.hibernate.exception.SQLStateConverter.handledNonSpecificException(SQLStateConverter.java:103) at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:91) at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43) at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:29) at org.hibernate.jdbc.ConnectionManager.closeConnection(ConnectionManager.java:449) at org.hibernate.jdbc.ConnectionManager.cleanup(ConnectionManager.java:379) at org.hibernate.jdbc.ConnectionManager.manualDisconnect(ConnectionManager.java:333) at org.hibernate.impl.SessionImpl.disconnect(SessionImpl.java:375) at org.springframework.orm.hibernate3.HibernateTransactionManager.doCleanupAfterCompletion(HibernateTransactionManager.java:744) at org.springframework.transaction.support.AbstractPlatformTransactionManager.cleanupAfterCompletion(AbstractPlatformTransactionManager.java:989) at org.springframework.transaction.support.AbstractPlatformTransactionManager.processRollback(AbstractPlatformTransactionManager.java:855) at org.springframework.transaction.support.AbstractPlatformTransactionManager.rollback(AbstractPlatformTransactionManager.java:800) at org.springframework.transaction.interceptor.TransactionAspectSupport.completeTransactionAfterThrowing(TransactionAspectSupport.java:339) at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:110) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) at org.springframework.aop.framework.Cglib2AopProxy$DynamicAdvisedInterceptor.intercept(Cglib2AopProxy.java:635) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.doInvokeMethod(HandlerMethodInvoker.java:421) at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:136) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:326) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:313) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:875) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:807) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:571) 36 more Caused by: java.sql.SQLException: Already closed. at org.apache.commons.dbcp.PoolableConnection.close(PoolableConnection.java:77) at org.apache.commons.dbcp.PoolingDataSource$PoolGuardConnectionWrapper.close(PoolingDataSource.java:180) at org.springframework.orm.hibernate3.LocalDataSourceConnectionProvider.closeConnection(LocalDataSourceConnectionProvider.java:96) at org.hibernate.jdbc.ConnectionManager.closeConnection(ConnectionManager.java:445) 62 more
解决：
造成Cannot release connection的原因有很多，要具体问题具体分析。从异常分析，造成这个异常 org.hibernate.exception.GenericJDBCException: Cannot release connection 归根结底是Caused by: java.sql.SQLException: Already closed. 即连接已关闭。所以解决的办法就要从DBCP的参数配置入手，见下面的参数配置properties文件。
#### :: Apache DBCP :: #### jdbc.driverClassName=oracle.jdbc.driver.OracleDriver jdbc.url=jdbc:oracle:thin:@10.165.153.9:1521:PRDC jdbc.username=guser jdbc.password=guser #初始化连接 jdbc.initialSize=0 #连接池的最大活动个数 jdbc.maxActive=20 #没有人用连接的时候，最大闲置的连接个数。 jdbc.maxIdle=100 #没有人用连接的时候，最小闲置的连接个数。 jdbc.minIdle=0 #超时等待时间以毫秒为单位 jdbc.maxWait=10000 #是否自动回收超时连接 jdbc.removeAbandoned=true #设置被遗弃的连接的超时的时间（以秒数为单位），即当一个连接被遗弃的时间超过设置的时间，则它会自动转换成可利用的连接。默认的超时时间是300秒。 jdbc.removeAbandonedTimeout=60 #是否在自动回收超时连接的时候打印连接的超时错误 jdbc.logAbandoned = true #给出一条简单的sql语句进行验证 jdbc.validationQuery=select 1 from dual #在取出连接时进行有效验证 jdbc.testOnBorrow=true
其中标红的两个参数的作用是对池化连接进行验证，This will ensure that DBCP only hands out good connections to Hibernate. 故加上这两个参数后，这个异常就不会再出现了。在Spring中指定数据源如下：
<!-- 數據源定義 使用Apache DBCP連接池 -->
<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<property name="driverClassName" value="${jdbc.driverClassName}" />
<property name="url" value="${jdbc.url}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
<property name="maxActive" value="${jdbc.maxActive}" />
<property name="maxIdle" value="${jdbc.maxIdle}" />
<property name="maxWait" value="${jdbc.maxWait}" />
<property name="removeAbandoned" value="${jdbc.removeAbandoned}" />
<property name="removeAbandonedTimeout" value="${jdbc.removeAbandonedTimeout}" />
<property name="logAbandoned" value="${jdbc.logAbandoned}" />
<property name="validationQuery" value="${jdbc.validationQuery}" />
<property name="testOnBorrow" value="${jdbc.testOnBorrow}" />
</bean>
另附
DBCP参数详细说明（部分参考了 http://justin-ray.javaeye.com/blog/413591 ）：
在配置时，大部分参数一目了然，不再赘述，有不清楚的话，可以参考下面的详细说明。主要难以理解的参数主要有：removeAbandoned 、logAbandoned、removeAbandonedTimeout、maxWait这四个参数，设置了rmoveAbandoned=true那么在getNumActive()快要到getMaxActive()的时候，系统会进行无效的Connection的回收，回收的Connection为removeAbandonedTimeout(默认300秒)中设置的秒数后没有使用的Connection，激活回收机制大致为getNumActive()=getMaxActive()-3。logAbandoned=true的话，将会在回收事件后，在log中打印出回收Connection的错误信息，包括在哪个地方用了Connection却忘记关闭了，在调试的时候很有用。在这里个人建议maxWait的时间不要设得太长，maxWait如果设置太长那么客户端会等待很久才激发回收事件。
Parameters
Parameter Description
username The connection username to be passed to our JDBC driver to establish a connection.
password The connection password to be passed to our JDBC driver to establish a connection.
url The connection URL to be passed to our JDBC driver to establish a connection.
driverClassName The fully qualified Java class name of the JDBC driver to be used.
connectionProperties The connection properties that will be sent to our JDBC driver when establishing new connections.
Format of the string must be [propertyName=property;]*
NOTE - The "user" and "password" properties will be passed explicitly, so they do not need to be included here.
Parameter Default Description
defaultAutoCommit true The default auto-commit state of connections created by this pool.
defaultReadOnly driver default The default read-only state of connections created by this pool. If not set then the setReadOnly method will not be called. (Some drivers don't support read only mode, ex: Informix)
defaultTransactionIsolation driver default The default TransactionIsolation state of connections created by this pool. One of the following: (see javadoc)
NONE
READ_COMMITTED
READ_UNCOMMITTED
REPEATABLE_READ
SERIALIZABLE
defaultCatalog The default catalog of connections created by this pool.
Parameter Default Description
initialSize 0 The initial number of connections that are created when the pool is started.
Since: 1.2
maxActive 8 The maximum number of active connections that can be allocated from this pool at the same time, or negative for no limit.
maxIdle 8 The maximum number of connections that can remain idle in the pool, without extra ones being released, or negative for no limit.
minIdle 0 The minimum number of connections that can remain idle in the pool, without extra ones being created, or zero to create none.
maxWait indefinitely The maximum number of milliseconds that the pool will wait (when there are no available connections) for a connection to be returned before throwing an exception, or -1 to wait indefinitely.
NOTE: If maxIdle is set too low on heavily loaded systems it is possible you will see connections being closed and almost immediately new connections being opened. This is a result of the active threads momentarily closing connections faster than they are opening them, causing the number of idle connections to rise above maxIdle. The best value for maxIdle for heavily loaded system will vary but the default is a good starting point.
Parameter Default Description
validationQuery The SQL query that will be used to validate connections from this pool before returning them to the caller. If specified, this query MUST be an SQL SELECT statement that returns at least one row.
testOnBorrow true The indication of whether objects will be validated before being borrowed from the pool. If the object fails to validate, it will be dropped from the pool, and we will attempt to borrow another.
NOTE - for a true value to have any effect, the validationQuery parameter must be set to a non-null string.
testOnReturn false The indication of whether objects will be validated before being returned to the pool.
NOTE - for a true value to have any effect, the validationQuery parameter must be set to a non-null string.
testWhileIdle false The indication of whether objects will be validated by the idle object evictor (if any). If an object fails to validate, it will be dropped from the pool.
NOTE - for a true value to have any effect, the validationQuery parameter must be set to a non-null string.
timeBetweenEvictionRunsMillis -1 The number of milliseconds to sleep between runs of the idle object evictor thread. When non-positive, no idle object evictor thread will be run.
numTestsPerEvictionRun 3 The number of objects to examine during each run of the idle object evictor thread (if any).
minEvictableIdleTimeMillis 1000 * 60 * 30 The minimum amount of time an object may sit idle in the pool before it is eligable for eviction by the idle object evictor (if any).
connectionInitSqls null A Collection of SQL statements that will be used to initialize physical connections when they are first created. These statements are executed only once - when the configured connection factory creates the connection.
Parameter Default Description
poolPreparedStatements false Enable prepared statement pooling for this pool.
maxOpenPreparedStatements unlimited The maximum number of open statements that can be allocated from the statement pool at the same time, or zero for no limit.
This component has also the ability to pool PreparedStatements. When enabled a statement pool will be created for each Connection and PreparedStatements created by one of the following methods will be pooled:
public PreparedStatement prepareStatement(String sql)
public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency)
NOTE - Make sure your connection has some resources left for the other statements. Pooling PreparedStatements may keep their cursors open in the database, causing a connection to run out of cursors, especially if maxOpenPreparedStatements is left at the default (unlimited) and an application opens a large number of different PreparedStatements per connection. To avoid this problem, maxOpenPreparedStatements should be set to a value less than the maximum number of cursors that can be open on a Connection.
Parameter Default Description
accessToUnderlyingConnectionAllowed false Controls if the PoolGuard allows access to the underlying connection.
When allowed you can access the underlying connection using the following construct:
Connection conn = ds.getConnection();
Connection dconn = ((DelegatingConnection) conn).getInnermostDelegate();
...
conn.close()
Default is false, it is a potential dangerous operation and misbehaving programs can do harmfull things. (closing the underlying or continue using it when the guarded connection is already closed) Be carefull and only use when you need direct access to driver specific extentions.
NOTE: Do not close the underlying connection, only the original one.
Parameter Default Description
removeAbandoned false Flag to remove abandoned connections if they exceed the removeAbandonedTimout.
If set to true a connection is considered abandoned and eligible for removal if it has been idle longer than the removeAbandonedTimeout. Setting this to true can recover db connections from poorly written applications which fail to close a connection.
removeAbandonedTimeout 300 Timeout in seconds before an abandoned connection can be removed.
logAbandoned false Flag to log stack traces for application code which abandoned a Statement or Connection.
Logging of abandoned Statements and Connections adds overhead for every Connection open or new Statement because a stack trace has to be generated.
If you have enabled "removeAbandoned" then it is possible that a connection is reclaimed by the pool because it is considered to be abandoned. This mechanism is triggered when (getNumIdle() < 2) and (getNumActive() > getMaxActive() - 3)
For example maxActive=20 and 18 active connections and 1 idle connection would trigger the "removeAbandoned". But only the active connections that aren't used for more then "removeAbandonedTimeout" seconds are removed, default (300 sec). Traversing a resultset doesn't count as being used.