Apache Mina 源码再读2 IoSession创建过程源代码剖析

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在调用bind()函数后,AcceptorOperationFuture 被注册到AbstractPollingIoAcceptor 类中的registerQueue 队列。在AbstractPollingIoAcceptor中存在IoProcessor 类。

/**
 * An internal interface to represent an 'I/O processor' that performs
 * actual I/O operations for {@link IoSession}s.  It abstracts existing
 * reactor frameworks such as Java NIO once again to simplify transport
 * implementations.
 *
 * @author <a href="http://mina.apache.org">Apache MINA Project</a>
 * 
 * @param <S> the type of the {@link IoSession} this processor can handle
 */
public interface IoProcessor<S extends IoSession> {}

IoProcessor 是执行IoSession 的I/O操作的内部接口类。  IoProcessor再次 抽象了在Java Nio 内部的reactor  frameworks 框架,并简化传输层的实现。

IoProcessor 实现类为SimpleIoProcessorPool 类。SImpleIoProcessorPool主要把IoSession分配到一个或者多个IoProcessor中。

An IoProcessor pool that distributes IoSessions into one or more IoProcessors. Most current transport implementations use this pool internally to perform better in a multi-core environment, and therefore, you won't need to use this pool directly unless you are running multiple IoServices in the same JVM.

If you are running multiple IoServices, you could want to share the pool among all services. To do so, you can create a new SimpleIoProcessorPool instance by yourself and provide the pool as a constructor parameter when you create the services.

This pool uses Java reflection API to create multiple IoProcessor instances. It tries to instantiate the processor in the following order:

1. A public constructor with one ExecutorService parameter.
2. A public constructor with one Executor parameter.
3. A public default constructor

The following is an example for the NIO socket transport:

// Create a shared pool.SimpleIoProcessorPool<NioSession> pool = new SimpleIoProcessorPool<NioSession>(NioProcessor.class, 16);

 // Create two services that share the same pool.SocketAcceptor acceptor = new NioSocketAcceptor(pool);SocketConnector connector = new NioSocketConnector(pool)

在Acceptor 线程中,processHandles 会创建一个新的IoSession.

  /*** This method will process new sessions for the Worker class.  All* keys that have had their status updates as per the Selector.selectedKeys()* method will be processed here.  Only keys that are ready to accept* connections are handled here.* <p/>* Session objects are created by making new instances of SocketSessionImpl* and passing the session object to the SocketIoProcessor class.*/@SuppressWarnings("unchecked")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 sessionS session = accept(processor, handle);//当有一个新socket链接时,返回一个IoSessionif (session == null) {continue;}initSession(session, null, null);// add the session to the SocketIoProcessor//当Acceptor发现新的socket,把socket和ioproces相关联。所有io事件由ioprocess处理session.getProcessor().add(session);}}}

  protected NioSession accept(IoProcessor<NioSession> processor, ServerSocketChannel handle) throws Exception {SelectionKey key = null;if (handle != null) {key = handle.keyFor(selector);}if ((key == null) || (!key.isValid()) || (!key.isAcceptable())) {return null;}// accept the connection from the clientSocketChannel ch = handle.accept();if (ch == null) {return null;}return new NioSocketSession(this, processor, ch);}

当accept事件触发时,会创建一个新NioSocketSession .

public abstract class NioSession extends AbstractIoSession {
    /** The NioSession processor */
    protected final IoProcessor<NioSession> processor;


    /** The communication channel */
    protected final Channel channel;


    /** The SelectionKey used for this session */
    protected SelectionKey key;


    /** The FilterChain created for this session */
    private final IoFilterChain filterChain;


    /**
     * 
     * Creates a new instance of NioSession, with its associated IoProcessor.
     * <br>
     * This method is only called by the inherited class.
     *
     * @param processor The associated IoProcessor
     */
    protected NioSession(IoProcessor<NioSession> processor, IoService service, Channel channel) {
        super(service);
        this.channel = channel;
        this.processor = processor;
        filterChain = new DefaultIoFilterChain(this);
    }

}

在抽象类NioSession中,每创建一个NioSession就会创建一个DefaultFilterChain对象。也就意味着每一个NioSession与自己独立的DefaultFilterChain相关联。

NioSession创建后，会通过initSession 方法来初始化NioSession. 

在初始化NioSession 之前,先看看一个IoSessionDataStructureFactory 工厂类。Provides data structures to a newly created session. 这个工厂类提供了创建NioSession的数据结构。NioSession中的数据结构主要包括存储自定义对象的IoSessionAttributeMap 类以及IoSession写入到缓冲区的写出队列WriteRequestQueue。

IoSessionDataStructFactory 接口源代码如下：

/*** Provides data structures to a newly created session.* * @author <a href="http://mina.apache.org">Apache MINA Project</a>*/
public interface IoSessionDataStructureFactory {/*** Returns an {@link IoSessionAttributeMap} which is going to be associated* with the specified <tt>session</tt>.  Please note that the returned* implementation must be thread-safe.*/IoSessionAttributeMap getAttributeMap(IoSession session) throws Exception;/*** Returns an {@link WriteRequest} which is going to be associated with* the specified <tt>session</tt>.  Please note that the returned* implementation must be thread-safe and robust enough to deal* with various messages types (even what you didn't expect at all),* especially when you are going to implement a priority queue which* involves {@link Comparator}.*/WriteRequestQueue getWriteRequestQueue(IoSession session) throws Exception;
}

在DefaultIoSessionDataStructureFactory 默认实现工厂类中实现了IoSessionAttributeMap 和WriteRequestQueue两个数据结构的实现。

IoSessionAttributeMap内部为一个并发安全的HashMap可以存储键值对。

 private static class DefaultIoSessionAttributeMap implements IoSessionAttributeMap {
        private final ConcurrentHashMap<Object, Object> attributes = new ConcurrentHashMap<Object, Object>(4);

}

WriteRequestQueue为一个并发安全的队列实现。

    private static class DefaultWriteRequestQueue implements WriteRequestQueue {
        /** A queue to store incoming write requests */
        private final Queue<WriteRequest> q = new ConcurrentLinkedQueue<WriteRequest>();

}

回头看一下，在initSession()初始化方法中,主要创建IoSessionAttributeMap和WriteRequestQueue队列两个结构。

初始化NioSession完成后,就需要把NioSession分配到指定的Processor线程中。

在分配SimpleIoProcessorPool中提供分配NioSession 与Processor线程相关联的策略。

    private IoProcessor<S> getProcessor(S session) {   //获取在IoSession中PROCESSOR 为键的对象IoProcessor<S> processor = (IoProcessor<S>) session.getAttribute(PROCESSOR);//如果该IoSession尚未与IoProcessor相关联，则把IoProccessor按照如下原则相关联if (processor == null) {if (disposed || disposing) {throw new IllegalStateException("A disposed processor cannot be accessed.");}//把processor和session相关联。根据session id和proceor pool 的大小来决定。 把IoSession的ID与线程池中顺序相关联processor = pool[Math.abs((int) session.getId()) % pool.length];if (processor == null) {throw new IllegalStateException("A disposed processor cannot be accessed.");}//设置PROCESSOR 的对象为IoProccessorsession.setAttributeIfAbsent(PROCESSOR, processor);}return processor;}

然后把NioSession 方法指定Processor线程的newSessions队列中,并启动Processor线程。

同时在内部存在IoServiceStatistics 类来统计IoSession相关事件。

此时,IoSession创建过程就完成了。

小结一下,NioSession创建过程。

1、Acceptor 线程,在select()中,轮询accept事件。

2、当accept事件触发时,创建NioSession对象,每一个NioSession都有一个DefaultFilterChain相关联。

3、当创建NioSession后,需要通过initSession()方法来初始化化NioSession.这里主要涉及IoSessionDataStructFactory工厂类创建IoSessionAttrubteMap 来存储用户自定义数据，创建WriteRequestQueue队列。 

4、当NioSession构建完成后,需要在SimpleIoProcessorPool来把NioSession 分配到指定的Processor线程中。

5、NioSession被添加到指定Processor线程中newSession队列中,然后启动start  Processor线程。

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