Netty源码学习-Java-NIO-Reactor

Netty里面采用了NIO-based Reactor Pattern
了解这个模式对学习Netty非常有帮助

参考以下两篇文章：
http://jeewanthad.blogspot.com/2013/02/reactor-pattern-explained-part-1.html
http://gee.cs.oswego.edu/dl/cpjslides/nio.pdf

本文所贴的代码来自第一篇文章，在注释部分加入了我自己的理解
完整代码可以到我的github上下载，仅供参考：
https://github.com/bylijinnan/nettyLearn/tree/master/ljn-netty3-learn/src/main/java/com/ljn/reactor

package com.ljn.reactor;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;

/*

单线程的实现
Server端用一个Selector利用一个线程（在main方法里面start）来响应所有请求
1.当ACCEPT事件就绪，Acceptor被选中，执行它的run方法：创建一个Handler（例如为handlerA），并将Handler的interestOps初始为READ
2.当READ事件就绪，handlerA被选中，执行它的run方法：它根据自身的当前状态，来执行读或写操作
因此，每一个Client连接过来，Server就创建一个Handler，但都所有操作都在一个线程里面

Selection Key   Channel                 Handler     Interested Operation
------------------------------------------------------------------------
SelectionKey 0  ServerSocketChannel     Acceptor    Accept
SelectionKey 1  SocketChannel 1         Handler 1   Read and Write
SelectionKey 2  SocketChannel 2         Handler 2   Read and Write
SelectionKey 3  SocketChannel 3         Handler 3   Read and Write

如果采用多个selector，那就是所谓的“Multiple Reactor Threads”，大体思路如下：

Selector[] selectors; // also create threads
int next = 0;
class Acceptor { // ... 
     public synchronized void run() { ...
         Socket connection = serverSocket.accept();
         if (connection != null)
             new Handler(selectors[next], connection);
         if (++next == selectors.length) next = 0;
     }
}

 */
public class Reactor implements Runnable {
 
    final Selector selector;
    final ServerSocketChannel serverSocketChannel;
    final boolean isWithThreadPool;
 
    /*Reactor的主要工作：
     * 1.给ServerSocketChannel设置一个Acceptor，接收请求
     * 2.给每一个一个SocketChannel（代表一个Client）关联一个Handler
     * 要注意其实Acceptor也是一个Handler（只是与它关联的channel是ServerSocketChannel而不是SocketChannel）
     */
    Reactor(int port, boolean isWithThreadPool) throws IOException {
 
        this.isWithThreadPool = isWithThreadPool;
        selector = Selector.open();
        serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.socket().bind(new InetSocketAddress(port));
        serverSocketChannel.configureBlocking(false);
        SelectionKey selectionKey0 = serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
        selectionKey0.attach(new Acceptor());
    }
 
 
    public void run() {
        System.out.println("Server listening to port: " + serverSocketChannel.socket().getLocalPort());
        try {
            while (!Thread.interrupted()) {
                int readySelectionKeyCount = selector.select();
                if (readySelectionKeyCount == 0) {
                    continue;
                }
                Set<SelectionKey> selected = selector.selectedKeys();
                Iterator<SelectionKey> it = selected.iterator();
                while (it.hasNext()) {
                    dispatch((SelectionKey) (it.next()));
                }
                
                //不会自动remove，因此要手动清；下次事件到来会自动添加
                selected.clear();
            }
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
    
    //从SelectionKey中取出Handler并执行Handler的run方法，没有创建新线程
    void dispatch(SelectionKey k) {
        Runnable r = (Runnable) (k.attachment());
        if (r != null) {
            r.run();
        } 
    }
    
    //主要工作是为每一个连接成功后返回的SocketChannel关联一个Handler，详见Handler的构造函数
    class Acceptor implements Runnable {
        public void run() {
            try {
                SocketChannel socketChannel = serverSocketChannel.accept();
                if (socketChannel != null) {
                    if (isWithThreadPool)
                        new HandlerWithThreadPool(selector, socketChannel);
                    else
                        new Handler(selector, socketChannel);
                }
                System.out.println("Connection Accepted by Reactor2");
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    }
    
    public static void main(String[] args) throws IOException{
        
        int port = 9900;
        boolean withThreadPool = false;
        Reactor reactor  = new Reactor(port, withThreadPool);
        new Thread(reactor).start();
    }
}

package com.ljn.reactor;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;

/*
 * 单线程版本的Handler
 */
public class Handler implements Runnable {
 
    final SocketChannel socketChannel;
    final SelectionKey selectionKey;
    ByteBuffer input = ByteBuffer.allocate(1024);
    static final int READING = 0, SENDING = 1;
    
    //初始状态
    int state = READING;
    String clientName = "";
 
    //在handler里面设置interestOps，而且这个interestOps是会随着事件的进行而改变的
    Handler(Selector selector, SocketChannel c) throws IOException {
        socketChannel = c;
        c.configureBlocking(false);
        selectionKey = socketChannel.register(selector, 0);
        
        /*
        handler作为SellectionKey的attachment。这样，handler就与SelectionKey也就是interestOps对应起来了
        反过来说，当interestOps发生、SelectionKey被选中时，就能从SelectionKey中取得handler
        */
        selectionKey.attach(this);
        selectionKey.interestOps(SelectionKey.OP_READ);
        selector.wakeup();
    }
 
    //在Reactor的dispatch方法里面被调用，但是直接的方法调用，没有创建新线程
    public void run() {
        try {
            if (state == READING) {
                read();
            } else if (state == SENDING) {
                send();
            }
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
 
    void read() throws IOException {
        int readCount = socketChannel.read(input);
        if (readCount > 0) {
            readProcess(readCount);
        }
        state = SENDING;
        // Interested in writing
        selectionKey.interestOps(SelectionKey.OP_WRITE);
    }
 
    /**
     * Processing of the read message. This only prints the message to stdOut.
     * 非IO操作（业务逻辑，实际应用中可能会非常耗时）：将Client发过来的信息（clientName）转成字符串形式
     * @param readCount
     */
    synchronized void readProcess(int readCount) {
        StringBuilder sb = new StringBuilder();
        input.flip();   //from writing mode to reading mode
        byte[] subStringBytes = new byte[readCount];
        byte[] array = input.array();
        System.arraycopy(array, 0, subStringBytes, 0, readCount);
        // Assuming ASCII (bad assumption but simplifies the example)
        sb.append(new String(subStringBytes));
        input.clear();
        clientName = sb.toString().trim();
    }
 
    void send() throws IOException {
        System.out.println("Saying hello to " + clientName);
        ByteBuffer output = ByteBuffer.wrap(("Hello " + clientName + "\n").getBytes());
        socketChannel.write(output);
        selectionKey.interestOps(SelectionKey.OP_READ);
        state = READING;
    }
}

	package com.ljn.reactor;

import java.io.IOException;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/*
 * 多线程版本的Handler
 * 思路就是把耗时的操作（非IO操作）放到其他线程里面跑，
 * 使得Handler只专注与Channel之间的IO操作；
 * Handler快速地从Channel中读或写，可以使Channel及时地、更快地响应其他请求
 * 耗时的操作完成后，产生一个事件（改变state），再“通知”（由Handler轮询这个状态是否有改变）
 * Handler执行Channel的读写操作
 */
public class HandlerWithThreadPool extends Handler {
 
    static ExecutorService pool = Executors.newFixedThreadPool(2);
    static final int PROCESSING = 2;
 
    public HandlerWithThreadPool(Selector sel, SocketChannel c) throws IOException {
        super(sel, c);
    }
 
    //Handler从SocketChannel中读到数据后，把“数据的处理”这个工作扔到线程池里面执行
    void read() throws IOException {
        int readCount = socketChannel.read(input);
        if (readCount > 0) {
            state = PROCESSING;
            
            //execute是非阻塞的，所以要新增一个state（PROCESSING），表示数据在处理当中，Handler还不能执行send操作
            pool.execute(new Processer(readCount)); 
        }
        //We are interested in writing back to the client soon after read processing is done.
        //这时候虽然设置了OP_WRITE，但下一次本Handler被选中时不会执行send()方法，因为state=PROCESSING
        //或者可以把这个设置放到Processer里面，等process完成后再设为OP_WRITE
        selectionKey.interestOps(SelectionKey.OP_WRITE);
    }
 
    //Start processing in a new Processer Thread and Hand off to the reactor thread.
    synchronized void processAndHandOff(int readCount) {
        readProcess(readCount);
        //Read processing done. Now the server is ready to send a message to the client.
        state = SENDING;
    }
 
    class Processer implements Runnable {
        int readCount;
        Processer(int readCount) {
            this.readCount =  readCount;
        }
        public void run() {
            processAndHandOff(readCount);
        }
    }
}