2018-01-04 15:02:03,319 ---com.mchange.v2.async.ThreadPoolAsynchronousRunner: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@4d6c4ebb -- APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks!
先说说APPARENT DEADLOCK!!!什么情况下会抛出这样的异常?
通过看源码，源码是个好东西，有个检测死锁的DeadlockDetector，每隔一段时间就会检测一次，是一个TimerTask,里面是这样写的：
current = (LinkedList) pendingTasks.clone();
if ( current.equals( last ) ){
logger.warning(this + " -- APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks!");
}
解释一下current和last
代码里这两个对象分别是一个LinkedList
current用来记录当前等待获取连接的AcquireTask
last用来记录上次等待获取连接的AcquireTask
如果二者相等的话，C3P0认为是死锁，抛出警告
什么情况下会出现这样的异常？
1.数据库连接不上的话，会报这样的异常，检查数据源配置是否正确？
我在本地测试，没有配置checkoutTime（获取连接的超时时间）这个参数，20根线程并发发起连接，检测死锁的线程的线程，试想，在某次检测过程中发现18个等待连接的任务，那么在下一次检测的时候，这18个连接任务，仍旧没有连上数据库，可能是数据库连接配置错误，检查数据源配置信息，满足出这个错误的条件，当然就会报APPARENT DEADLOCK!!!
2.在缓存中存在Statement，来说说这个
网上有人回答这个问题，说是设置c3p0.maxStatements=0，为啥设置一下这个就OK了呢？
这里说说我个人思考，因为在数据库里的Statement的缓存都是在连接基础上了，存在缓存了，这里我理解就是就是占用了数据库的连接数，而申请新的连接的过程中，在达到连接数最大时，检测死锁的线程又可能会检测出2次等待连接是AcquireTask相等，就会抛出异常。
My workaround:
In hibernate.cfg.xml:
In c3p0.properties:
c3p0.maxStatementsPerConnection=100
For such setting my multithreaded applications runs with no APPARENT DEADLOCK error (finally and ... hopefully that this problem is really solved).
My app running quite fast so I guess (and I believe) that Statement caching is working. Statements caching is crucial for me because of performance. In other words I couldn't just turn Statement caching off (*).
As far as I can see turning Statement caching off (this global statement caching) is one (or maybe the only one) solution to get rid of APPARENT DEADLOCK errors.
Damn, I really would like to have time to have a look at c3p0 source code and figure out where is the reason of those fu!@#$% errors (Oracle JDBC or Hibernate or c3p0).
总结一下：就是用户申请新的连接的时候，旧的连接又没有及时释放，那么就会抛出APPARENT DEADLOCK!!!的异常了。