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