NIO框架之MINA源代码解析：mina核心引擎

Posted 2020-09-26 lytwajue

NIO框架之MINA源代码解析（一）：背景

MINA的底层还是利用了jdk提供了nio功能，mina仅仅是对nio进行封装。包含MINA用的线程池都是jdk直接提供的。

MINA的server端主要有accept、processor、session三部分组成的。当中accept主要负责在指定的port监听。若有新连接则建立一个新的session。processor则负责处理session相应的发送数据和接收数据并调用上层处理；session则缓存当前连接数据。

MINA採用了线程懒启动的技术，即最少启动线程。在MINA server启动的时候，仅仅有一个线程-accept,而且accept线程仅仅有一个，在指定的port进行监听（能够同一时候监听多个port，mina能够绑定多port）。

1、acceptor

先看下acceptor的主要类图吧。

mina server的启动入口是在NiosocketAcceptor.bind（InetSocketAddress）或者NioSocketAcceptor.bind（SocketAddress...）方法， acceptor.bind(new InetSocketAddress(1234));  
然后会调用AbstractPollingIoAcceptor.bindInternal（List<? extends SocketAddress>）方法，在bindInternal方法里面会调用startupAcceptor()方法提交一个accept线程到线程池里面（仅仅提交一次），并初始化acceptor端的Selector。就这样一个acceptor线程启动了。

acceptor端业务相对简单，相当于在当前Selector里面监听acceptor事件，处理新连接并新建一个session放到相应的processor里面。

acceptor 代码。非常easy。

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

            int nHandles = 0;

            // Release the lock
            lock.release();

            while (selectable) {
                try {
                    // Detect if we have some keys ready to be processed
                    // The select() will be woke up if some new connection
                    // have occurred, or if the selector has been explicitly
                    // woke up
                    int selected = select();

                    // this actually sets the selector to OP_ACCEPT,
                    // and binds to the port on which this class will
                    // listen on  在通道里面注冊连接事件
                    nHandles += registerHandles();

                    // Now, if the number of registred handles is 0, we can
                    // quit the loop: we don‘t have any socket listening
                    // for incoming connection.
                    if (nHandles == 0) {
                        acceptorRef.set(null);

                        if (registerQueue.isEmpty() && cancelQueue.isEmpty()) {
                            assert (acceptorRef.get() != this);
                            break;
                        }

                        if (!acceptorRef.compareAndSet(null, this)) {
                            assert (acceptorRef.get() != this);
                            break;
                        }

                        assert (acceptorRef.get() == this);
                    }

                    if (selected > 0) {
                        // We have some connection request, let‘s process
                        // them here.处理连接
                        processHandles(selectedHandles());
                    }

                    // check to see if any cancellation request has been made.
                    nHandles -= unregisterHandles();
                } catch (ClosedSelectorException cse) {
                    // If the selector has been closed, we can exit the loop
                    break;
                } catch (Throwable e) {
                    ExceptionMonitor.getInstance().exceptionCaught(e);

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

            // Cleanup all the processors, and shutdown the acceptor.
            if (selectable && isDisposing()) {
                selectable = false;
                try {
                    if (createdProcessor) {
                        processor.dispose();
                    }
                } finally {
                    try {
                        synchronized (disposalLock) {
                            if (isDisposing()) {
                                destroy();
                            }
                        }
                    } catch (Exception e) {
                        ExceptionMonitor.getInstance().exceptionCaught(e);
                    } finally {
                        disposalFuture.setDone();
                    }
                }
            }
        }

2、processor

processor顾名思义，就是进行IO处理，处理当前session的数据读写，并进行业务处理。

在mina server初始化的时候，会初始化一个processor池，通过NioSocketAcceptor的构造器传入池的大小，默认是当前处理器的个数+1。

processor池里面有一个jdk提供的 线程池 - Executors.newCachedThreadPool()。各个processor线程会引用此线程池，即每一个processor线程都在这个线程池里面运行。

在mina server实际处理时，每一个processor相当于一个线程，轮流处理当前的session队列里面的数据（每一个processor里面的session相当于顺序处理，共享一个线程）。

每一个processor有一个Selector对象。

processor类图

processor端的处理逻辑相对有点复杂。看以下的流程图。

1、把新加入进来的session注冊到当前processor的Selector里面的read事件，并初始化session；
2、推断当前Selector是否有读写事件。
3、若第2步有读事件时。进入步骤4，若没有的话，直接到第6步。
4、处理当前读事件，并把处理后的数据放入到flush队列里面。
5、把第4步运行的结果flush到client；
6、处理session。比方session idle时间等。

7、又一次运行第1步，循环运行。

processor端代码。

 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  处理新加入进来的session，并注冊到当前processor的Selector读事件
                    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...处理读事件。并把结果放入flush队列里面
                        process();
                    }

                    // Write the pending requests  把flush队列里面的session的处理完的数据发送给client
                    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);
            }
        }
    }

3、session

session做为一个连接的详细对象，缓存当前连接用户的一些信息。

session类图

session对象是绑定在SelectableChannel的一个attach。

class  NioProcessor
 @Override
    protected void init(NioSession session) throws Exception {
        SelectableChannel ch = (SelectableChannel) session.getChannel();
        ch.configureBlocking(false);
        session.setSelectionKey(ch.register(selector, SelectionKey.OP_READ, session));//chanel 注冊读事件，并把session当做一个attach辅导SelectableChannel里面。
    }