c3p0數據庫連接池死鎖問題
項目進行壓力測試的時候，運行大概1小時候，後臺拋出以下異常：
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$PoolThread.run(ThreadPoolAsynchronousRunner.java:562)
目測該問題是c3p0連接池導致的線程死鎖。
這種數據庫連接池線程死鎖的問題發生的原因可能有很多，我將我的配置環境以及解決方法貼出來供大家參考一下：
使用環境，spring + hibernate +c3p0
配置如下：
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close"> <property name="driverClass" value="com.mysql.jdbc.Driver"/> <property name="jdbcUrl" value="${db.url}?useUnicode=true&characterEncoding=UTF-8"/> <property name="user" value="${db.usrname}"/> <property name="password" value="${db.password}"/> <!--連接池中保留的最小連接數。--> <property name="minPoolSize" value="5"/> <!--連接池中保留的最大連接數。Default: 15 --> <property name="maxPoolSize" value="500"/> <!--初始化時獲取的連接數，取值應在minPoolSize與maxPoolSize之間。Default: 3 --> <property name="initialPoolSize" value="10"/> <!--最大空閒時間,60秒內未使用則連接被丟棄。若爲0則永不丟棄。Default: 0 --> <property name="maxIdleTime" value="60"/> <!--當連接池中的連接耗盡的時候c3p0一次同時獲取的連接數。Default: 3 --> <property name="acquireIncrement" value="5"/> <!--JDBC的標準參數，用以控制數據源內加載的PreparedStatements數量。但由於預緩存的statements 屬於單個connection而不是整個連接池。所以設置這個參數需要考慮到多方面的因素。 如果maxStatements與maxStatementsPerConnection均爲0，則緩存被關閉。Default: 0--> <property name="maxStatements" value="0"/> <!--每60秒檢查所有連接池中的空閒連接。Default: 0 --> <property name="idleConnectionTestPeriod" value="60"/> <!--定義在從數據庫獲取新連接失敗後重復嘗試的次數。Default: 30 --> <property name="acquireRetryAttempts" value="30"/> <!--獲取連接失敗將會引起所有等待連接池來獲取連接的線程拋出異常。但是數據源仍有效 保留，並在下次調用getConnection()的時候繼續嘗試獲取連接。如果設爲true，那麼在嘗試 獲取連接失敗後該數據源將申明已斷開並永久關閉。Default: false--> <property name="breakAfterAcquireFailure" value="false"/> <!--因性能消耗大請只在需要的時候使用它。如果設爲true那麼在每個connection提交的 時候都將校驗其有效性。建議使用idleConnectionTestPeriod或automaticTestTable 等方法來提升連接測試的性能。Default: false --> <property name="testConnectionOnCheckout" value="false"/> </bean>
這個配置中有一些屬性可能和死鎖有關：maxStatements 和checkoutTimeout(我沒有使用該屬性)
有人說c3p0本身沒有問題，導致問題是spring的原因，c3p0作爲hibernate官方推薦的連接池，在性能和穩定性上都是中規中矩的。根據以前的使用經驗，我判斷也不是c3p0的問題。
我的修改如下：
<!--JDBC的標準參數，用以控制數據源內加載的PreparedStatements數量。但由於預緩存的statements 屬於單個connection而不是整個連接池。所以設置這個參數需要考慮到多方面的因素。 如果maxStatements與maxStatementsPerConnection均爲0，則緩存被關閉。Default: 0--> <property name="maxStatements" value="0"/> <!--連接池用完時客戶調用getConnection()後等待獲取連接的時間，單位：毫秒。超時後會拋出--> <!--SQLEXCEPTION，如果設置0，則無限等待。Default:0--> <property name="checkoutTimeout" value="100"/>
修改後，問題沒有重現。
