最近在IDEA中启动Tomcat经常会碰到这个错误，起初、一直没在意，现在碰到的次数多了，就去查看下这个问题，现描述如下：

=2018-01-08 14:27:30,216 WARN  [com.mchange.v2.async.ThreadPoolAsynchronousRunner] - com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@29165a67 -- APPARENT DEADLOCK!!! Complete Status:

    Managed Threads: 3

    Active Threads: 3

    Active Tasks:

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@368ea496 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0)

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@4ed78fd5 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1)

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@2a241163 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2)

    Pending Tasks:

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@25f19032

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@1631d614

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@21ddc66e

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@670e7120

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@652520bb

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@688808dd

        com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@3afe1ac8

Pool thread stack traces:

    Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0,5,RMI Runtime]

        java.net.PlainSocketImpl.socketConnect(Native Method)

        java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351)

        java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213)

        java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200)

        java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)

        java.net.Socket.connect(Socket.java:529)

        java.net.Socket.connect(Socket.java:478)

        java.net.Socket.<init>(Socket.java:375)

        java.net.Socket.<init>(Socket.java:189)

        oracle.net.nt.TcpNTAdapter.connect(Unknown Source)

        oracle.net.nt.ConnOption.connect(Unknown Source)

        oracle.net.nt.ConnStrategy.execute(Unknown Source)

        oracle.net.resolver.AddrResolution.resolveAndExecute(Unknown Source)

        oracle.net.ns.NSProtocol.establishConnection(Unknown Source)

        oracle.net.ns.NSProtocol.connect(Unknown Source)

        oracle.jdbc.driver.T4CConnection.connect(T4CConnection.java:844)

        oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:268)

        oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:420)

        oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:165)

        oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:35)

        oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:801)

        com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134)

        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,RMI Runtime]

        java.net.PlainSocketImpl.socketConnect(Native Method)

        java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351)

        java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213)

        java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200)

        java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)

        java.net.Socket.connect(Socket.java:529)

        java.net.Socket.connect(Socket.java:478)

        java.net.Socket.<init>(Socket.java:375)

        java.net.Socket.<init>(Socket.java:189)

        oracle.net.nt.TcpNTAdapter.connect(Unknown Source)

        oracle.net.nt.ConnOption.connect(Unknown Source)

        oracle.net.nt.ConnStrategy.execute(Unknown Source)

        oracle.net.resolver.AddrResolution.resolveAndExecute(Unknown Source)

        oracle.net.ns.NSProtocol.establishConnection(Unknown Source)

        oracle.net.ns.NSProtocol.connect(Unknown Source)

        oracle.jdbc.driver.T4CConnection.connect(T4CConnection.java:844)

        oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:268)

        oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:420)

        oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:165)

        oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:35)

        oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:801)

        com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134)

        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-#2,5,RMI Runtime]

        java.net.PlainSocketImpl.socketConnect(Native Method)

        java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351)

        java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213)

        java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200)

        java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)

        java.net.Socket.connect(Socket.java:529)

        java.net.Socket.connect(Socket.java:478)

        java.net.Socket.<init>(Socket.java:375)

        java.net.Socket.<init>(Socket.java:189)

        oracle.net.nt.TcpNTAdapter.connect(Unknown Source)

        oracle.net.nt.ConnOption.connect(Unknown Source)

        oracle.net.nt.ConnStrategy.execute(Unknown Source)

        oracle.net.resolver.AddrResolution.resolveAndExecute(Unknown Source)

        oracle.net.ns.NSProtocol.establishConnection(Unknown Source)

        oracle.net.ns.NSProtocol.connect(Unknown Source)

        oracle.jdbc.driver.T4CConnection.connect(T4CConnection.java:844)

        oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:268)

        oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:420)

        oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:165)

        oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:35)

        oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:801)

        com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134)

        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)

在一番求知的过程中，大概了解到这个问题出现的几种可能情况，随记录下来，以供后人鉴赏：

1.数据库服务没有启动

大千世界无奇不有，这种错也有人遇到的话，真不想说什么了；如果是故意测试的话呢，个人感觉还是没有必要的，毕竟不开启数据库服务就去连接数据库，你让人作何感想啊。

2.没有创建对应数据库

这个问题同上，访问不到数据库，都不是技术性的问题，在此、不多啰嗦。

3.就涉及到c3p0连接池的配置文件了

这个问题就比较偏重于技术层面了，相关解决方案介绍如下：

①maxStatements 和checkoutTimeout

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。

通过上面的描述，解决方案：

<property name="maxStatements" value="0"/> 

<property name="checkoutTimeout" value="100"/> 

②maxPoolSize和initialPoolSize

关于这方面，我大概能够了解到我个人的问题是由于这个原因导致的。由于数据库的连接数是有限的，每有应用启动C3p0都会占用数据库的连接来填充C3p0的连接池，而当数据库的资源被占光时就会因为无法获得共享资源而报死锁。

③更新c3p0的包

至于c3p0的包版本过旧造成的，这大概是历史遗留的问题。低版本c3p0包会导致获取连接的时间过长而在获取新的连接时造成死锁，具体请参考stackoverflow中别人提出的一个问题

The tasks that are dealocking are Connection acquisition tasks. That is, c3p0 is trying to acquire new Connections from your database, and those Connection acquisition attempts are taking a long time.

The first thing I would do is upgrade to 0.9.2.1, which has a much improved means of performing a round of Connection acquisitions in situations where acquisition attempts sometimes fail.

If that doesn't solve your problem, then you'll need to figure out why c3p0's attempts to acquire a Connection are hanging for long periods of time: neither succeeding nor failing with an Exception.

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