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.b
logspot.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
class="java" name="code">
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);
}
}
}
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