MINA2的多线程模型问题探讨

  最近和一友人沟通，他在一个项目中使用MINA2，他反馈在启动了服务端之后，发现IO阻塞和堆内存一直上升；





JCONSOLE面板中的解释：
阻塞总数

Blocked count is the total number of times that the thread blocked to enter or reenter a monitor. I.e. the number of times a thread has been in the java.lang.Thread.State.BLOCKED state.
当线程试图获取一个内部的对象锁(不是java.util.concurrent库中的锁)，而锁被其它线程占有，则该线程进入阻塞状态。

等待总数
Waited count is the total number of times that the thread waited for notification. i.e. the number of times that a thread has been in the ava.lang.Thread.State.WAITING or java.lang.Thread.State.TIMED_WAITING state.
当线程等待另外一个线程通知调度器的一个条件的时候，它自己进入等待状态。在调用Object.wait()或Thread.join()方法，或者等待java.util.concurrent库中的Lock或Condition时，会出现等待状况。

后这两天我自己本地拿一个简单案例测试了下，
采用JAVA原生的newCachedThreadPool，以及MINA2自带的 OrderedThreadPoolExecutor 线程池，在性能上没有太大的区别；
本身他原来的代码中就是使用了一种线程池，将IO线程和工作线程（业务线程）区分开的，所以这个处理的堆内存升高和IO堵塞是正常现象，我本地代码实测，暂用的堆内存并不多，而且是快速回收了，并不影响后续业务；
下图中大量的thread就是由于使用了ExecutorFilter之后，MINA2服务器端产生的大量线程。


  在服务端启动的代码中有一段如下：

filterChainBuilder.addLast("threadPool", new ExecutorFilter());

仔细研究了下JCONSOLE中的阻塞总数，结合本地运行的经验，下图的结果：



  根据图里面的信息，发现在NioProcessor.java中
   @Override
    protected int select(long timeout) throws Exception {
        return selector.select(timeout);
    }
然后继续跟踪，在AbstractPollingIoProcessor.java类中

 private class Processor implements Runnable {
        public void run() {
            assert (processorRef.get() == this);

            int nSessions = 0;
            lastIdleCheckTime = System.currentTimeMillis();

            for (;;) {
                try {
                    // This select has a timeout so that we can manage
                    // idle session when we get out of the select every
                    // second. (note : this is a hack to avoid creating
                    // a dedicated thread).
                    long t0 = System.currentTimeMillis();
                    int selected = select(SELECT_TIMEOUT);
                    long t1 = System.currentTimeMillis();
                    long delta = (t1 - t0);

                    if ((selected == 0) && !wakeupCalled.get() && (delta < 100)) {
                        // Last chance : the select() may have been
                        // interrupted because we have had an closed channel.
                        if (isBrokenConnection()) {
                            LOG.warn("Broken connection");

                            // we can reselect immediately
                            // set back the flag to false
                            wakeupCalled.getAndSet(false);

                            continue;
                        } else {
                            LOG.warn("Create a new selector. Selected is 0, delta = "
                                            + (t1 - t0));
                            // Ok, we are hit by the nasty epoll
                            // spinning.
                            // Basically, there is a race condition
                            // which causes a closing file descriptor not to be
                            // considered as available as a selected channel, but
                            // it stopped the select. The next time we will
                            // call select(), it will exit immediately for the same
                            // reason, and do so forever, consuming 100%
                            // CPU.
                            // We have to destroy the selector, and
                            // register all the socket on a new one.
                            registerNewSelector();
                        }

                        // Set back the flag to false
                        wakeupCalled.getAndSet(false);

                        // and continue the loop
                        continue;
                    }

                    // Manage newly created session first
                    nSessions += handleNewSessions();

                    updateTrafficMask();

                    // Now, if we have had some incoming or outgoing events,
                    // deal with them
                    if (selected > 0) {
                        //LOG.debug("Processing ..."); // This log hurts one of the MDCFilter test...
                        process();
                    }

                    // Write the pending requests
                    long currentTime = System.currentTimeMillis();
                    flush(currentTime);

                    // And manage removed sessions
                    nSessions -= removeSessions();

                    // Last, not least, send Idle events to the idle sessions
                    notifyIdleSessions(currentTime);

                    // Get a chance to exit the infinite loop if there are no
                    // more sessions on this Processor
                    if (nSessions == 0) {
                        processorRef.set(null);
                        
                        if (newSessions.isEmpty() && isSelectorEmpty()) {
                            // newSessions.add() precedes startupProcessor
                            assert (processorRef.get() != this);
                            break;
                        }
                        
                        assert (processorRef.get() != this);
                        
                        if (!processorRef.compareAndSet(null, this)) {
                            // startupProcessor won race, so must exit processor
                            assert (processorRef.get() != this);
                            break;
                        }
                        
                        assert (processorRef.get() == this);
                    }

                    // Disconnect all sessions immediately if disposal has been
                    // requested so that we exit this loop eventually.
                    if (isDisposing()) {
                        for (Iterator<S> i = allSessions(); i.hasNext();) {
                            scheduleRemove(i.next());
                        }

                        wakeup();
                    }
                } catch (ClosedSelectorException cse) {
                    // If the selector has been closed, we can exit the loop
                    break;
                } catch (Throwable t) {
                    ExceptionMonitor.getInstance().exceptionCaught(t);

                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e1) {
                        ExceptionMonitor.getInstance().exceptionCaught(e1);
                    }
                }
            }

            try {
                synchronized (disposalLock) {
                    if (disposing) {
                        doDispose();
                    }
                }
            } catch (Throwable t) {
                ExceptionMonitor.getInstance().exceptionCaught(t);
            } finally {
                disposalFuture.setValue(true);
            }
        }
    }

上述行发生了阻塞行为，这个类是干嘛的呢？这个类是MINA2中去轮训SELECTOR中是否有空闲的，查看一下selector的源码：

    /**
   [color=red][b]  * Selects a set of keys whose corresponding channels are ready for I/O
     * operations.
     *
     * <p> This method performs a blocking <a href="#selop">selection
     * operation</a>.  It returns only after at least one channel is selected,
     * this selector's {@link #wakeup wakeup} method is invoked, the current
     * thread is interrupted, or the given timeout period expires, whichever
     * comes first.
     *
     * <p> This method does not offer real-time guarantees: It schedules the
     * timeout as if by invoking the {@link Object#wait(long)} method. </p>[/b][/color]
     *
     * @param  timeout  If positive, block for up to <tt>timeout</tt>
     *                  milliseconds, more or less, while waiting for a
     *                  channel to become ready; if zero, block indefinitely;
     *                  must not be negative
     *
     * @return  The number of keys, possibly zero,
     *          whose ready-operation sets were updated
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  ClosedSelectorException
     *          If this selector is closed
     *
     * @throws  IllegalArgumentException
     *          If the value of the timeout argument is negative
     */
    public abstract int select(long timeout)
	throws IOException;

仔细的看看注释：
Selects a set of keys whose corresponding channels are ready for I/O[color=red][/color]
原来是去轮训IO通道是否空闲，也就是说上述阻塞是发生在这里，也就是说IO通道被轮训发生的阻塞，所以说本文开头的阻塞数量是这样产生的：
当大量的并发消息过来，MINA2服务端需要轮训IO通道是否OK，OK之后就马上交给工作线程（如果有）去处理。所以从上面看来，这个是正常现象。在同时并发大量消息的时候，MINA2的IO通道还是会有堵塞。

另外：
  我们知道：MINA2采用的是责任链模式，在处理过程中插入不同的filter,来进行不同的处理。
当服务器端启动的时候，会启动默认的I0线程，也就是主线程，MINA2建议是CPU数量+1,如果代码中不增加上述一行，那么如果加入上述的ExecutorFilter之后，工作线程将会和IO线程分开，简单的例子，如果你要发一些消息给服务端，要服务端提交一个事务，如果不采用ExecutorFilter，那么IO线程就会一直等待这个事务结束，才会接着处理下一个任务，否则一直等待，如果采用了ExecutorFilter，那么IO线程会将任务交给ExecutorFilter的线程池，由线程池去处理事务，而IO线程就释放了，这样IO线程非常高效的能够同时处理很多的并发任务。
用new ExecutorFilter()，缺省使用OrderedThreadPoolExecutor线程池。需要注意的是这个线程池，对应同一个session而言，是顺序执行的.也就是说mina2里面的message处理都是顺序的，不会说出现在消息没有接受完毕之前关闭。
常见的也有这样的处理：修改成：

filterChainBuilder.addLast("threadPool", new ExecutorFilter(Executors.newCachedThreadPool

()); 

然