c3p0--常见异常

1. 获取资源timeout：

   ---

   异常信息如下：

   Caused by: java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.
   [Cause: com.mchange.v2.resourcepool.TimeoutException: 
   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：

   private void awaitAvailable(long timeout) throws InterruptedException, TimeoutException, ResourcePoolException
       {
           assert Thread.holdsLock( this );
   
           if (force_kill_acquires)
               throw new ResourcePoolException("A ResourcePool cannot acquire a new resource -- the factory or source appears to be down.");
   
           Thread t = Thread.currentThread();
           try
           {
               acquireWaiters.add( t );
   
               int avail;
               long start = ( timeout > 0 ? System.currentTimeMillis() : -1);
               if (Debug.DEBUG && Debug.TRACE == Debug.TRACE_MAX)
               {
                   if ( logger.isLoggable( MLevel.FINE ) )
                       logger.fine("awaitAvailable(): " + 
                                       (exampleResource != null ? 
                                                       exampleResource : 
                                       "[unknown]") );
                   trace();
               }
               while ((avail = unused.size()) == 0) 
               {
                   // the if case below can only occur when 1) a user attempts a
                   // checkout which would provoke an acquire; 2) this
                   // increments the pending acquires, so we go to the
                   // wait below without provoking postAcquireMore(); 3)
                   // the resources are acquired; 4) external management
                   // of the pool (via for instance unpoolResource() 
                   // depletes the newly acquired resources before we
                   // regain this' monitor; 5) we fall into wait() with
                   // no acquires being scheduled, and perhaps a managed.size()
                   // of zero, leading to deadlock. This could only occur in
                   // fairly pathological situations where the pool is being
                   // externally forced to a very low (even zero) size, but 
                   // since I've seen it, I've fixed it.
                   if (pending_acquires == 0 && managed.size() < max)
                       _recheckResizePool();
   
                   this.wait(timeout);
                   if (timeout > 0 && System.currentTimeMillis() - start > timeout)
                       throw new TimeoutException("A client timed out while waiting to acquire a resource from " + this + " -- timeout at awaitAvailable()");
                   if (force_kill_acquires)
                       throw new CannotAcquireResourceException("A ResourcePool could not acquire a resource from its primary factory or source.");
                   ensureNotBroken();
               }
           }
           finally
           {
               acquireWaiters.remove( t );
               if (acquireWaiters.size() == 0)
                   this.notifyAll();
           }
       }

   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、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、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、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