c3p0数据库连接池死锁问题
项目进行压力测试的时候，运行大概1小时候，后台抛出以下异常：
Nov 9, 2012 1:41:59 AM com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector run
WARNING: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@4b9cafa7 -- APPARENT DEADLOCK!!! Complete Status:
Managed Threads: 3
Active Threads: 3
Active Tasks:
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@19f67d43 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2)
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@599c706 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1)
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@32d95dc9 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0)
Pending Tasks:
com.mchange.v2.resourcepool.BasicResourcePool$1DestroyResourceTask@5c9c5133
com.mchange.v2.resourcepool.BasicResourcePool$1DestroyResourceTask@1a971ac5
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@365e8926
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@787e39ac
Pool thread stack traces:
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2,5,main]
java.lang.StringCoding.set(StringCoding.java:53)
java.lang.StringCoding.encode(StringCoding.java:270)
java.lang.String.getBytes(String.java:946)
com.mysql.jdbc.ConnectionImpl.configureCharsetProperties(ConnectionImpl.java:1578)
com.mysql.jdbc.ConnectionImpl.configureClientCharacterSet(ConnectionImpl.java:1628)
com.mysql.jdbc.ConnectionImpl.initializePropsFromServer(ConnectionImpl.java:3425)
com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2045)
com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:718)
com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:46)
sun.reflect.GeneratedConstructorAccessor35.newInstance(Unknown Source)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302)
com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282)
com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:135)
com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)
com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1,5,main]
com.mysql.jdbc.ResultSetImpl.getInstance(ResultSetImpl.java:344)
com.mysql.jdbc.MysqlIO.buildResultSetWithUpdates(MysqlIO.java:2607)
com.mysql.jdbc.MysqlIO.readResultsForQueryOrUpdate(MysqlIO.java:2490)
com.mysql.jdbc.MysqlIO.readAllResults(MysqlIO.java:1746)
com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2135)
com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2536)
com.mysql.jdbc.ConnectionImpl.setupServerForTruncationChecks(ConnectionImpl.java:5240)
com.mysql.jdbc.ConnectionImpl.initializePropsFromServer(ConnectionImpl.java:3508)
com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2045)
com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:718)
com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:46)
sun.reflect.GeneratedConstructorAccessor35.newInstance(Unknown Source)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302)
com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282)
com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:135)
com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)
com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0,5,main]
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:562)
目测该问题是c3p0连接池导致的线程死锁。
这种数据库连接池线程死锁的问题发生的原因可能有很多，我将我的配置环境以及解决方法贴出来供大家参考一下：
使用环境，spring + hibernate +c3p0
配置如下：
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource"
destroy-method="close">
<property name="driverClass" value="com.mysql.jdbc.Driver"/>
<property name="jdbcUrl" value="${db.url}?useUnicode=true&characterEncoding=UTF-8"/>
<property name="user" value="${db.usrname}"/>
<property name="password" value="${db.password}"/>
<!--连接池中保留的最小连接数。-->
<property name="minPoolSize" value="5"/>
<!--连接池中保留的最大连接数。Default: 15 -->
<property name="maxPoolSize" value="500"/>
<!--初始化时获取的连接数，取值应在minPoolSize与maxPoolSize之间。Default: 3 -->
<property name="initialPoolSize" value="10"/>
<!--最大空闲时间,60秒内未使用则连接被丢弃。若为0则永不丢弃。Default: 0 -->
<property name="maxIdleTime" value="60"/>
<!--当连接池中的连接耗尽的时候c3p0一次同时获取的连接数。Default: 3 -->
<property name="acquireIncrement" value="5"/>
<!--JDBC的标准参数，用以控制数据源内加载的PreparedStatements数量。但由于预缓存的statements
属于单个connection而不是整个连接池。所以设置这个参数需要考虑到多方面的因素。
如果maxStatements与maxStatementsPerConnection均为0，则缓存被关闭。Default: 0-->
<property name="maxStatements" value="0"/>
<!--每60秒检查所有连接池中的空闲连接。Default: 0 -->
<property name="idleConnectionTestPeriod" value="60"/>
<!--定义在从数据库获取新连接失败后重复尝试的次数。Default: 30 -->
<property name="acquireRetryAttempts" value="30"/>
<!--获取连接失败将会引起所有等待连接池来获取连接的线程抛出异常。但是数据源仍有效
保留，并在下次调用getConnection()的时候继续尝试获取连接。如果设为true，那么在尝试
获取连接失败后该数据源将申明已断开并永久关闭。Default: false-->
<property name="breakAfterAcquireFailure" value="false"/>
<!--因性能消耗大请只在需要的时候使用它。如果设为true那么在每个connection提交的
时候都将校验其有效性。建议使用idleConnectionTestPeriod或automaticTestTable
等方法来提升连接测试的性能。Default: false -->
<property name="testConnectionOnCheckout" value="false"/>
</bean>
这个配置中有一些属性可能和死锁有关：maxStatements 和checkoutTimeout(我没有使用该属性)
有人说c3p0本身没有问题，导致问题是spring的原因，c3p0作为hibernate官方推荐的连接池，在性能和稳定性上都是中规中矩的。根据以前的使用经验，我判断也不是c3p0的问题。
我的修改如下：
<!--JDBC的标准参数，用以控制数据源内加载的PreparedStatements数量。但由于预缓存的statements
属于单个connection而不是整个连接池。所以设置这个参数需要考虑到多方面的因素。
如果maxStatements与maxStatementsPerConnection均为0，则缓存被关闭。Default: 0-->
<property name="maxStatements" value="0"/>
<!--连接池用完时客户调用getConnection()后等待获取连接的时间，单位：毫秒。超时后会抛出-->
<!--SQLEXCEPTION，如果设置0，则无限等待。Default:0-->
<property name="checkoutTimeout" value="100"/>
修改后，问题没有重现。
