我的服务器曾经在日志中看到APPARENT DEADLOCK。我有几台服务器在负载均衡器后面运行，有趣的是我看到DEADLOCK同时在所有服务器上发生（有人知道为什么它会影响所有服务器）吗？在这段时间内，通常需要200毫秒的MySQL查询需要60秒钟以上的时间。日志如下所示：
com.mchange.v2.async.ThreadPoolAsynchronousRunner: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@58780f76
-- APPARENT DEADLOCK!!! Complete Status:
Managed Threads: 3
Active Threads: 3
Active Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@25ff87d4 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@10ccf7ef (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3305ec37 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2)
Pending Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@39cc9e5a
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@60d46f90
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@17509fea
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@b28bd63
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@56cbdc12
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@15a091b4
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@61ce325
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@48119520
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@4032fb7c
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@518eefff
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@30ea3b20
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@74960088
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@23a8fc7d
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@5ff0ee0
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@642d0644
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@207bc809
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@44d4936f
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@39a10d1b
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3532334d
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@4bf79e62
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@2bd83398
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@1a202a2d
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3eacda7f
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@495f5746
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@23f1f906
因此，我来到了Stack Overflow，并且found this answer which suggested将
statementCacheNumDeferredCloseThreads设置为1。我这样做了，并且我看到DEADLOCK的频率降低了，只出现在负载均衡器后面的几个服务器上，而不是所有服务器上。现在，日志看起来有些不同，但是在DEADLOCK期间，查询仍然很长：
10 Oct 2018 06:33:32,037 [WARN] (Timer-0) com.mchange.v2.async.ThreadPoolAsynchronousRunner: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@4f39ad63 -- APPARENT DEADLOCK!!! Complete Status:
Managed Threads: 3
Active Threads: 3
Active Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@34dee200 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3727ee6b (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@4afb8b9 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0)
Pending Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@384a3b5b
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@7bc700b0
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@731bfd15
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@a88e9bf
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@63f18b56
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@20f0c518
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@caf7746
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@41a7a27d
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@2ee32a24
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@81df2e5
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@7f7fa1e7
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@337503f
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@34b2f877
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@53dfbede
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@512d5ddb
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@68a25969
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@4bf0754a
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@65770ba4
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@5e0f4154
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@249c22ed
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@6c8e5911
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@3179550f
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@15d8a795
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@50966489
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@4ecee95b
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@35640ca0
com.mchange.v2.resourcepool.BasicResourcePool$AsyncTestIdleResourceTask@6550f196
com.mchange.v2.resourcepool.BasicResourcePool$AsyncTestIdleResourceTask@6816399
com.mchange.v2.resourcepool.BasicResourcePool$AsyncTestIdleResourceTask@3fbcd623
Pool thread stack traces:
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2,5,main]
com.mysql.jdbc.PreparedStatement.realClose(PreparedStatement.java:2765)
com.mysql.jdbc.StatementImpl.close(StatementImpl.java:541)
com.mchange.v1.db.sql.StatementUtils.attemptClose(StatementUtils.java:41)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask.run(GooGooStatementCache.java:404)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1,5,main]
com.mysql.jdbc.PreparedStatement.realClose(PreparedStatement.java:2765)
com.mysql.jdbc.StatementImpl.close(StatementImpl.java:541)
com.mchange.v1.db.sql.StatementUtils.attemptClose(StatementUtils.java:41)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask.run(GooGooStatementCache.java:404)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0,5,main]
com.mysql.jdbc.PreparedStatement.realClose(PreparedStatement.java:2765)
com.mysql.jdbc.StatementImpl.close(StatementImpl.java:541)
com.mchange.v1.db.sql.StatementUtils.attemptClose(StatementUtils.java:41)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask.run(GooGooStatementCache.java:404)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
任何想法如何解决这个问题？我可以尝试完全禁用语句缓存，但是我担心总体性能受到影响。其他一些相关参数：
minPoolSize = 30
maxPoolSize = 30
maxStatements = 100
unreturnedConnectionTimeout = 500
idleConnectionTestPeriod = 60
acquireIncrements = 3
C3p0 version = 0.9.1.2
编辑：我忘了提一下，在此改进过程中，我看到的死锁更少，同时还增加了
maxStatements可以解释这种改进。但是现在我只看到https://github.com/swaldman/c3p0/issues/53，其中说version 0.9.2引入了这个新参数statementCacheNumDeferredCloseThreads。我的版本太旧了。我没有关于此参数不存在的警告/错误。 最佳答案
也许为时已晚，但是您是否尝试增加numHelperThreads的数量？
关于mysql - C3P0明显的死锁，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/52742651/