c3p0 deadlocks in Hibernate

I am new to hibernate and have started using C3P0 as the connection pooling manager with hibernate because without that I was having timeout issues after 8 hours of no contact with MySQL server.

After switching to C3P0 I recently encountered a deadlock and I am not sure how it happened. I although have all the stack traces and configuration to show.

Here is the configuration log

Jan 27, 2017 5:56:23 PM com.mchange.v2.log.MLog <clinit>
INFO: MLog clients using java 1.4+ standard logging.
Jan 27, 2017 5:56:23 PM com.mchange.v2.c3p0.C3P0Registry banner
INFO: Initializing c3p0-0.9.2.1 [built 20-March-2013 10:47:27 +0000; debug? true; trace: 10]
Jan 27, 2017 5:56:23 PM com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager
INFO: Initializing c3p0 pool... com.mchange.v2.c3p0.PoolBackedDataSource@28c81cce [ connectionPoolDataSource -> com.mchange.v2.c3p0.WrapperConnectionPoolDataSource@35af8b6c [ acquireIncrement -> 1, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, debugUnreturnedConnectionStackTraces -> false, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 1hge14z9l1qg67m9nxywg8|c2db68f, idleConnectionTestPeriod -> 300, initialPoolSize -> 1, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 120, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 25, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 1, nestedDataSource -> com.mchange.v2.c3p0.DriverManagerDataSource@2e47880e [ description -> null, driverClass -> null, factoryClassLocation -> null, identityToken -> 1hge14z9l1qg67m9nxywg8|1b765a2c, jdbcUrl -> jdbc:mysql://localhost/Pokemon_Blaze_Online_Dev, properties -> {user=******, password=******} ], preferredTestQuery -> select 1;, propertyCycle -> 0, statementCacheNumDeferredCloseThreads -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false; userOverrides: {} ], dataSourceName -> null, factoryClassLocation -> null, identityToken -> 1hge14z9l1qg67m9nxywg8|7e276594, numHelperThreads -> 3 ]

Here are the properties I set

<property name="hibernate.c3p0.acquire_increment">1</property>
<property name="hibernate.c3p0.idle_test_period">300</property>
<property name="hibernate.c3p0.timeout">120</property>
<property name="hibernate.c3p0.max_size">25</property>
<property name="hibernate.c3p0.min_size">1</property>
<property name="hibernate.c3p0.max_statement">0</property>
<property name="hibernate.c3p0.preferredTestQuery">select 1;</property>
<property name="hibernate.connection.provider_class">org.hibernate.connection.C3P0ConnectionProvider</property>

Here is the thread dump of one of the thread which got locked.

"Request Pool-0" #71 prio=5 os_prio=0 tid=0x000000002b2df800 nid=0x1d48 in Object.wait() [0x000000003113e000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1414)
    at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:606)
    - locked <0x00000006c1b3c088> (a com.mchange.v2.resourcepool.BasicResourcePool)
    at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:526)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutAndMarkConnectionInUse(C3P0PooledConnectionPool.java:755)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:682)
    at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:140)
    at org.hibernate.c3p0.internal.C3P0ConnectionProvider.getConnection(C3P0ConnectionProvider.java:90)
    at org.hibernate.internal.AbstractSessionImpl$NonContextualJdbcConnectionAccess.obtainConnection(AbstractSessionImpl.java:387)
    at org.hibernate.engine.jdbc.internal.LogicalConnectionImpl.obtainConnection(LogicalConnectionImpl.java:228)
    at org.hibernate.engine.jdbc.internal.LogicalConnectionImpl.getConnection(LogicalConnectionImpl.java:171)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.connection(StatementPreparerImpl.java:63)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$5.doPrepare(StatementPreparerImpl.java:162)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$StatementPreparationTemplate.prepareStatement(StatementPreparerImpl.java:186)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.prepareQueryStatement(StatementPreparerImpl.java:160)
    at org.hibernate.loader.Loader.prepareQueryStatement(Loader.java:1884)
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1861)
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1838)
    at org.hibernate.loader.Loader.doQuery(Loader.java:909)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:354)
    at org.hibernate.loader.Loader.doList(Loader.java:2553)
    at org.hibernate.loader.Loader.doList(Loader.java:2539)
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2369)
    at org.hibernate.loader.Loader.list(Loader.java:2364)
    at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:126)
    at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1682)
    at org.hibernate.internal.CriteriaImpl.list(CriteriaImpl.java:380)
    at org.hibernate.internal.CriteriaImpl.uniqueResult(CriteriaImpl.java:402)
    at com.game.server.persistence.services.player.PlayerDataServiceImpl.areValidCredentials(PlayerDataServiceImpl.java:28)
    at com.game.server.manager.request.service.DefaultLoginService.login(DefaultLoginService.java:78)
    at com.game.server.manager.request.DefaultRequestManager.lambda$login$2(DefaultRequestManager.java:103)
    at com.game.server.manager.request.DefaultRequestManager$$Lambda$63/869031346.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - <0x000000079eea0890> (a java.util.concurrent.ThreadPoolExecutor$Worker)

Since I am little new to this I could not identify the problem from log and from the dump. I have also noticed that C3P0 has created a lot of threads.

Your application almost certainly is leaking Connections. See c3p0's docs for how to debug the problem.

TL; DR: Set

<property name="hibernate.c3p0.unreturnedConnectionTimeout">30</property>
<property name="hibernate.c3p0.debugUnreturnedConnectionStackTraces">true</property>

Check your logs for stack traces that check out Connections or Sessions in your application but fail to reliably close() them. Fix that, by using something like try-with-resources to reliably close things. When you have fixed the problem, remove the above properties.

See also this