netty源码分析(六)Reactor模式透彻理解及其在Netty中的应用

前边讲了EventLoopGroup的一些知识,在netty的架构这块我们使用一种bossGroup加workerGroup的方式,bossGroup只负责请求的转发,workerGroup是具体的数据处理,其实netty整个框架使用的是Reactor(响应器)的设计模式。这方面知名的大佬就是Doug Lea,Java.util.current包的很多线程的API和工具都出自大佬之手。
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大佬的一片文章对这种模式做了非常细致的介绍,《Scalable IO in Java
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大多数的网络服务都是下面的流程:
读取请求
对请求进行解码
处理服务(业务逻辑)
编码相应
发送响应
经典的io模式是这样的:
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class Server implements Runnable {
public void run() {
try {
ServerSocket ss = new ServerSocket(PORT);
while (!Thread.interrupted())
new Thread(new Handler(ss.accept())).start();
// or, single-threaded, or a thread pool
} catch (IOException ex) { /* ... */ }
}
static class Handler implements Runnable {
final Socket socket;
Handler(Socket s) { socket = s; }
public void run() {
try {
byte[] input = new byte[MAX_INPUT];
socket.getInputStream().read(input);
byte[] output = process(input);
socket.getOutputStream().write(output);
} catch (IOException ex) { /* ... */ }
}
private byte[] process(byte[] cmd) { /* ... */ }
}
}

每一个请求开一个线程去处理。

这种方式不是一直能够好的做法,会有阻塞和瓶颈。接下来是Reactor Design的方式:
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class Reactor implements Runnable {
final Selector selector;
final ServerSocketChannel serverSocket;
Reactor(int port) throws IOException {
selector = Selector.open();
serverSocket = ServerSocketChannel.open();
serverSocket.socket().bind(new InetSocketAddress(port));
serverSocket.configureBlocking(false);
SelectionKey sk =serverSocket.register(selector,SelectionKey.OP_ACCEPT);
sk.attach(new Acceptor());
}
/*
Alternatively, use explicit SPI provider:
SelectorProvider p = SelectorProvider.provider();
selector = p.openSelector();
serverSocket = p.openServerSocketChannel();
*/
// class Reactor continued
public void run() { // normally in a newThread
try {
while (!Thread.interrupted()) {
selector.select();
Set selected = selector.selectedKeys();
Iterator it = selected.iterator();
while (it.hasNext())
dispatch((SelectionKey)(it.next());
selected.clear();
}
} catch (IOException ex) { /* ... */ }
}
}
void dispatch(SelectionKey k) {
Runnable r = (Runnable)(k.attachment());
if (r != null)
r.run();
}

Reactor 只负责请求的接受,和nio变成一样初始化注册的是OP_ACCEPT,然后绑定一个Acceptor(实现Runnable接口),主循环中,收到准备好的selectedKeys,并且遍历selectedKeys,将每一个keydispatch下去,在dispatch里边通过selectedKey得到绑定的Acceptor,看一下Acceptor的逻辑:

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// class Reactor continued
class Acceptor implements Runnable { // inner
public void run() {
try {
SocketChannel c = serverSocket.accept();
if (c != null)
new Handler(selector, c);
}catch(IOException ex) { /* ... */ }
}
}
}

final class Handler implements Runnable {
final SocketChannel socket;
final SelectionKey sk;
ByteBuffer input = ByteBuffer.allocate(MAXIN);
ByteBuffer output = ByteBuffer.allocate(MAXOUT);
static final int READING = 0, SENDING = 1;
int state = READING;
Handler(Selector sel, SocketChannel c)
throws IOException {
socket = c; c.configureBlocking(false);
// Optionally try first read now
sk = socket.register(sel, 0);
sk.attach(this);
sk.interestOps(SelectionKey.OP_READ);
sel.wakeup();//注册OP_READ兴趣之后,让select()方法返回,接受要读取的数据
}
boolean inputIsComplete() { /* ... */ }
boolean outputIsComplete() { /* ... */ }
void process() { /* ... */ }

// class Handler continued
public void run() {
try {
if (state == READING) read();
else if (state == SENDING) send();
} catch (IOException ex) { /* ... */ }
}
void read() throws IOException {
socket.read(input);
if (inputIsComplete()) {
process();
state = SENDING;
// Normally also do first write now
sk.interestOps(SelectionKey.OP_WRITE);//将状态变为SENDING之后,接下来就是往外写数据,对写感兴趣。
}
}
void send() throws IOException {
socket.write(output);
if (outputIsComplete()) sk.cancel();
}
}

Acceptor 获得SocketChannel接着进入到实际的处理类Handler里边,Handler有SocketChannel 和SelectionKey的引用,Handler的构造器将当前类(Handler)加入到绑定里边,并且对READ感兴趣,之后调sel.wakeup()意思是让select( )方法立刻返回,如果当前没有select()方法阻塞的话,那么下一次调用select()会立即返回,然后执行run()方法,是通过判断状态的方式来决定是写还是读 ,这个在Netty3中就是需要这样实现handler代码的,需要自己判断状态来决定业务逻辑。Netty4已经改成各种回调了,比如channelRead,channelActive等。
:’
文档接着介绍了 基于模式的设计,提前绑定合适的handler作为attachment:

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//A simple use of GoF State-Object pattern
//Rebind appropriate handler as attachment
class Handler { // ...
public void run() { // initial state is reader
socket.read(input);
if (inputIsComplete()) {
process();
sk.attach(new Sender());//绑定UI个发送者。
sk.interest(SelectionKey.OP_WRITE);//由于发送者是写操作,因此兴趣是OP_WRITE
sk.selector().wakeup();//让select方法立刻返回,执行写的逻辑。
}
}
class Sender implements Runnable {
public void run(){ // ...
socket.write(output);
if (outputIsComplete()) sk.cancel();
}
}
}

接着是handler基于线程池的实现:
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这个版本是对于handler的减压,接着多个selector的Reactor:

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mainReactor相当于bossGroup,subReactorx 相当于netty里边的workerGroup.整个过程下来其实就是netty的 框架内在的模式。