c3p0存在严重bug “APPARENT DEADLOCK“的问题
开发时用了Spring JdbcTemplate + c3p0组合，做性能测试时出现严重的性能问题。40的并发访问，开始时候系统正常，几分钟后，应用就不能访问了。停止40并发的压力，1分钟左右又可以正常访问应用程序了，log日志：
2011-11-01 13:48:39,378 [com.mchange.v2.async.ThreadPoolAsynchronousRunner:435]
-[WARN] com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@4ec5571b
-- APPARENT DEADLOCK!!!
Complete Status: [num_managed_threads: 10, num_active: 10; activeTasks: com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@7dd9d603
(com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#6),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@68719f81 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#9),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@7b9f03b8 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#7),
把c3p0换为DBCP后，可以正常进行性能测试，访问正常，判定c3p0存在bug。
c3p0从连接池中获取和释放连接时，采用了异步的处理方式。在调用了从c3p0获得的连接的close方法时，不是立即放回池中的，而是放入一 个事件队列中等待c3p0内部的线程池顺序的处理。
c3p0中的2个跟内部的连接池有关参数：maxAdministrativeTaskTime、 numHelperThreads
官方描述在：http://www.mchange.com/projects/c3p0/index.html
maxAdministrativeTaskTime中说c3p0中的很多功能不是由 client threads完成的，是通过内部线程池完成的，如果某些内部功能“挂起”了线程池中的线程，那线程池中的线程将被耗尽，其最后结果就是c3p0停止响应了。
需要内部线程池处理的功能：
expandPool -- 在扩大池中连接的时候，如管理的连接从20扩大到40个
shrinkPool -- 在缩小连接池的时候，这个过程需要真正关闭一些连接
doCheckinManaged -- 把连接返回给连接池
checkIdleResources -- 定时检查池中的连接
“APPARENT DEADLOCK”警告就是由于c3p0内部线程池的所有线程都被挂起而造成的，因此官方文档说把numHelperThreads设置大一些可以有效避免这个警告。但是这种方法只能延缓问题的出现，并不能避免整个应用被挂起的问题。
查看log日志，会发现哪些任务的线程：
[com.mchange.v2.async.ThreadPoolAsynchronousRunner:435]-[WARN] com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@4ec5571b
-- APPARENT DEADLOCK!!! Complete Status: [num_managed_threads: 10, num_active: 10;
activeTasks:
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@7dd9d603 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#6),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@68719f81 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#9),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@7b9f03b8 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#7),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@4ad6470 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#4),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@139cf776 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@263a6e09 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@460e247a (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#3),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@4a7ce984 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@4ec6ff50 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#5),
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@6aa40597 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#8);
pendingTasks:
com.mchange.v2.resourcepool.BasicResourcePool$6@366b3333,
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@37ee752e,
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@84f3bb2,
com.mchange.v2.resourcepool.BasicResourcePool$6@9d82761,
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@4e015653,
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@39ca8b27,
com.mchange.v2.resourcepool.BasicResourcePool$6@2075cb15,
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@1567e059,
com.mchange.v2.resourcepool.BasicResourcePool$6@56b9fe09,
可以看到AcquireTask占用了内部线程池的所有线程，没有线程可以来执行BasicResourcePool$6对应的任务，而这个任务的作用就是把池外使用完的连接放回池内的，c3p0被挂起了。
发生 DEADLOCK 的前提条件：
- 连接池会进行expandPool操作，因为这样才能产生AcquireTask对象，这个对象会阻塞内部连接池，因此，如果把minPoolSize和maxPoolSize设置为一样大可以避免这个问题
- AcquireTask的执行持续很长时间，要大于10秒，在实际环境中是有可能放生的
- acquireIncrement要大于numHelperThreads，这样才有可能把所有内部线程都占用
- 连接池的访问压力要很大，压力线程数要大于连接池现有的连接数