Apache Mina IoAcceptor与IoConnector

简介

Apache mina是一个介于应用程序与网络之间的NIO框架，它使程序员从繁琐的网络操作中解脱出来，花更多的时间在业务处理上。

如下图所示,mina分为三层

1、IOService层：处理IO操作

2、IOFilter层：过滤器链，日志处理、字节变换、对象转换等操作

3、IOHandler层：真正的处理业务逻辑的地方

IOService层根据不同的角色又分为IOAcceptor和IOConnector，分别用于接受连接与请求连接操作。

IOAcceptor

上图是IOAcceptor的类图，IOAcceptor相当于是对ServerSocketChannel的封装，最重要的两个操作是绑定与接受连接，IOService接口中有多个重载的bind方法

public interface IoAcceptor extends IoService {            void bind() throws IOException;       void bind(SocketAddress localAddress) throws IOException;           void bind(SocketAddress firstLocalAddress, SocketAddress... addresses) throws IOException;       void bind(Iterable<? extends SocketAddress> localAddresses) throws IOException;}   

其方法的实现在抽象类AbstractIOAcceptor的bind方法中，这个方法在做了参数检查等操作后，将真正的绑定操作交给抽象方法bindInternal来完成。对于bindInternal有基于TCP/IP,UDP/IP,VMPipe三种实现，以TCP/IP为例来看绑定过程

   protected final Set<SocketAddress> bindInternal(            List<? extends SocketAddress> localAddresses) throws Exception {        // Create a bind request as a Future operation. When the selector        // have handled the registration, it will signal this future.        AcceptorOperationFuture request = new AcceptorOperationFuture(                localAddresses);        // adds the Registration request to the queue for the Workers        // to handle        registerQueue.add(request);        // creates the Acceptor instance and has the local        // executor kick it off.        startupAcceptor();                // As we just started the acceptor, we have to unblock the select()        // in order to process the bind request we just have added to the         // registerQueue.        wakeup();                // Now, we wait until this request is completed.        request.awaitUninterruptibly();        if (request.getException() != null) {            throw request.getException();        }        // Update the local addresses.        // setLocalAddresses() shouldn't be called from the worker thread        // because of deadlock.        Set<SocketAddress> newLocalAddresses = new HashSet<SocketAddress>();                for (H handle:boundHandles.values()) {            newLocalAddresses.add(localAddress(handle));        }        return newLocalAddresses;    }

主要干了以下几件事情：

1、将绑定请求放入registerQueue中

2、启动Acceptor，从Acceptor类的run方法可以看到，这一步会阻塞在Acceptor选择器的选择操作中

3、调用wakeup让选择器返回

4、等待请求处理完成，这一步会阻塞在ready变量中，当ready变量为true时才会返回，当接受连接后ready会被设置为true.

现在重点看一下Acceptor的run方法

public void run() {......    while (selectable) {        try {            int selected = select();            nHandles += registerHandles();            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) {                processHandles(selectedHandles());            }            // check to see if any cancellation request has been made.            nHandles -= unregisterHandles();        } catch (...) {       ...    }    // Cleanup all the processors, and shutdown the acceptor. set ready=true}

(1)、selector被wakeup唤醒后，调用registerHandles方法从registerQueue中取出请求依次调用open方法

 protected ServerSocketChannel open(SocketAddress localAddress)            throws Exception {        // Creates the listening ServerSocket        ServerSocketChannel channel = ServerSocketChannel.open();                boolean success = false;                try {            // This is a non blocking socket channel            channel.configureBlocking(false);                    // Configure the server socket,            ServerSocket socket = channel.socket();                        // Set the reuseAddress flag accordingly with the setting            socket.setReuseAddress(isReuseAddress());                        // and bind.            socket.bind(localAddress, getBacklog());                        // Register the channel within the selector for ACCEPT event            channel.register(selector, SelectionKey.OP_ACCEPT);            success = true;        } finally {            if (!success) {                close(channel);            }        }        return channel;    }

open方法完成了ServerSocket的绑定和注册
(2)、从(1)中可以知道selector上注册了ServerSocketChannel的OP_ACCEPT键，注册后nHandles==0,selected==0，进行下一次循环，同样是阻塞在select方法上

(3)、当连接到来时，select方法返回，selected>0,执行processHandles方法

private void processHandles(Iterator<H> handles) throws Exception {            while (handles.hasNext()) {                H handle = handles.next();                handles.remove();                // Associates a new created connection to a processor,                // and get back a session                S session = accept(processor, handle);                                if (session == null) {                    break;                }                initSession(session, null, null);                // add the session to the SocketIoProcessor                session.getProcessor().add(session);            }        }

该方法在完成真正的接受连接操作后，创建session并扔到processor中，后续的工作交给processor来完成。每个session中其实有一个SocketChannel，这个socketChannel实际上是被注册到了processor的selector上。注册代码在NioProcessor类中可以找到

    protected void init(NioSession session) throws Exception {        SelectableChannel ch = (SelectableChannel) session.getChannel();        ch.configureBlocking(false);        session.setSelectionKey(ch.register(selector, SelectionKey.OP_READ,                session));    }

整个Acceptor的实现就讲解完了，总结一下：Acceptor线程专门负责接受连接，在其上有一个selector，轮询是否有连接建立上来，当有连接建立上来，调用ServerSocketChannel.accept方法来接受连接，这个方法返回一个session对象，然后将这个session对象加入processor中，由processor遍历每个session来完成真正的IO操作。processor上也有一个selector与一个Processor线程,selector用于轮询session，Processor线程处理每个session的IO操作。

IOConnector

IOConnector的类图如下：

IOConnector的设计与IOAcceptor几乎完全一样，唯一不同的是与Acceptor线程对应的是Connector线程，在完成连接操作后也是扔了一个session对象到Processor中。

关于Processor与Session后续再分析......