c3p0--常见异常

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1. 获取资源timeout：

   ---

   异常信息如下：
   ?

   1

   Caused by: java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.<br>[Cause: com.mchange.v2.resourcepool.TimeoutException: <br>A client timed out while waiting to acquire a resource from com.mchange.v2.resourcepool.BasicResourcePool@4d5a39b7 -- timeout at awaitAvailable()

   

   查找发现，获取connection资源timeout的地方，在basicResourcePool类的awaitAvailable方法里，代码很简单：
   

    1 private void awaitAvailable(long timeout) throws InterruptedException, TimeoutException, ResourcePoolException
    2     {
    3         assert Thread.holdsLock( this );
    4 
    5         if (force_kill_acquires)
    6             throw new ResourcePoolException("A ResourcePool cannot acquire a new resource -- the factory or source appears to be down.");
    7 
    8         Thread t = Thread.currentThread();
    9         try
   10         {
   11             acquireWaiters.add( t );
   12 
   13             int avail;
   14             long start = ( timeout > 0 ? System.currentTimeMillis() : -1);
   15             if (Debug.DEBUG && Debug.TRACE == Debug.TRACE_MAX)
   16             {
   17                 if ( logger.isLoggable( MLevel.FINE ) )
   18                     logger.fine("awaitAvailable(): " + 
   19                                     (exampleResource != null ? 
   20                                                     exampleResource : 
   21                                     "[unknown]") );
   22                 trace();
   23             }
   24             while ((avail = unused.size()) == 0) 
   25             {
   26                 // the if case below can only occur when 1) a user attempts a
   27                 // checkout which would provoke an acquire; 2) this
   28                 // increments the pending acquires, so we go to the
   29                 // wait below without provoking postAcquireMore(); 3)
   30                 // the resources are acquired; 4) external management
   31                 // of the pool (via for instance unpoolResource() 
   32                 // depletes the newly acquired resources before we
   33                 // regain this‘ monitor; 5) we fall into wait() with
   34                 // no acquires being scheduled, and perhaps a managed.size()
   35                 // of zero, leading to deadlock. This could only occur in
   36                 // fairly pathological situations where the pool is being
   37                 // externally forced to a very low (even zero) size, but 
   38                 // since I‘ve seen it, I‘ve fixed it.
   39                 if (pending_acquires == 0 && managed.size() < max)
   40                     _recheckResizePool();
   41 
   42                 this.wait(timeout);
   43                 if (timeout > 0 && System.currentTimeMillis() - start > timeout)
   44                     throw new TimeoutException("A client timed out while waiting to acquire a resource from " + this + " -- timeout at awaitAvailable()");
   45                 if (force_kill_acquires)
   46                     throw new CannotAcquireResourceException("A ResourcePool could not acquire a resource from its primary factory or source.");
   47                 ensureNotBroken();
   48             }
   49         }
   50         finally
   51         {
   52             acquireWaiters.remove( t );
   53             if (acquireWaiters.size() == 0)
   54                 this.notifyAll();
   55         }
   56     }

   

   抛出异常的是第44行，而抛出异常的先决条件就是timeout了。表象来看是这样，再继续跟进代码的时候发现，进入等待资源的先决条件是：

   ?

   1 2 3

   1、available==0（unused==0） 2、managed。size>=max（池数量已达到最大） 3、获取到的资源异常：如果设置了checkedoutTest这一项时，进行test的时候失败，导致异常；

   所以更直接的原因是以下几种：

   ?

   1 2 3

   1、请求资源的队列太长，排队时间太长导致超时：线程数量应付不过来大量的请求数量，导致超时 2、testConnection的原因导致的异常：网络震荡 3、数据库服务器端无更多的链接可用

   那么相应的解决方案：

   ?

   1 2 3 4

   1、请求资源过长的，加大maxsize，如果请求量过大的，尽量将min和max设置成一个值，因此不存在shrink和expand的操作，这也是一种性能消耗 2、testConnection设置在checkin即可，checkout时无需test，否则卡在网络震荡就不好了 3、检查数据库端的最大链接数。 4、将c3p0的debug日志打开，检查它的managed、unused、available的分别size

2. deadlock　　

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c3p0--常见异常

标签：c   style   class   blog   code   java   

原文地址：http://www.cnblogs.com/leeying/p/3777294.html