Java Socket

Socket的系统调用

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1. 用户态和内核态

Linux操作系统的体系架构分为用户态和内核态（或者用户空间和内核）。内核从本质上看是一种软件——控制计算机的硬件资源，并提供上层应用程序运行的环境。用户态即上层应用程序的活动空间，应用程序的执行必须依托于内核提供的资源，包括CPU资源、存储资源、I/O资源等。为了使上层应用能够访问到这些资源，内核必须为上层应用提供访问的接口：即系统调用。

2. 访问内核态方式

• 系统调用
• 库函数
• Shell脚本

3. Socket的系统调用

socket、connect、accept、listen、bind、recvfrom、read、write

三次握手

1. 服务端和客户端通过socket函数指向一个文件描述符，然后通过这个文件描述符建立连接。服务器通过bind()绑定端口，这样客户端可以IP+端口唯一标识到服务器的进程，调用listen()监听客户端，客户端调用connect请求连接,服务器的accept函数一直阻塞，直到有连接。三次握手建立连接之后，就可以调用write和read进行读写。

BIO

服务端

public class Server {
    public static void main(String[] args) {
        try {
            byte[] bt = new byte[1024];
            ServerSocket serverSocket = new ServerSocket();
            serverSocket.bind(new InetSocketAddress(8080));
            //等待连接:阻塞状态
            System.out.println("server start...");
            Socket accept = serverSocket.accept();
            System.out.println("connect success");

            //等待返回通知：阻塞状态
            int read = accept.getInputStream().read(bt);
            if(read ！= -1){
                System.out.println("result:"+new String(bt));
            }
        } catch (Exception e){
            System.out.println(e);
        }
    }
}

客户端实现

public class Clients {

    public static void main(String[] args) {
        try {
            Socket socket = new Socket("127.0.0.1",8080);
            socket.getOutputStream().write("返回结果".getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

这是BIO单线程实现，其中accept和read都会被阻塞。如果被阻塞在accpet和read中，那么不能接受其他连接。

多线程实现

public class Server {
    public static void main(String[] args) {
        try {
            byte[] bt = new byte[1024];
            ServerSocket serverSocket = new ServerSocket();
            serverSocket.bind(new InetSocketAddress(8080));
            while (true) {
                //等待连接:阻塞状态
                System.out.println("server start...");
                Socket accept = serverSocket.accept();
                System.out.println("connect success");

                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            while (true) {
                                System.out.println("--------------------------");
                                int read = accept.getInputStream().read(bt);
                                if(read != -1){
                                    System.out.println("result:" + new String(bt));
                                }
                            }
                        } catch (Exception e) {
                            System.out.println(e);
                        }
                    }
                }).start();
            }
        } catch (Exception e){
            System.out.println(e);
        }
    }
}

每个连接都会开一个新的线程来处理连接，但是read依然是阻塞的，那么当客户端阻塞在read时，会造成资源的浪费。导致其他连接无法处理。也可以用线程池处理，但是依然会有阻塞的问题。

NIO

1. Buffer缓冲区
2. 多路复用的Selctor，也就是用一个系统调用来管理多个连接上的IO读写，减少了系统调用的次数，减少用户态到内核态的切换，降低开销，因为切换时需要用额外的寄存器传递参数
3. 通过Channel

服务端

 public static void main(String[] args) throws IOException{
        int port = 8080;
        if(args != null &&args.length >0){
            try{
                port = Integer.valueOf(args[0]);
            }catch (NumberFormatException ex){
                //采用默认值
            }
        }
        MultiplexerTimeServer timeServer = new MultiplexerTimeServer(port);
        new Thread(timeServer,"NIO-MultiplexerTimeServer-001").start();
    }
    

public class MultiplexerTimeServer implements Runnable {

    private Selector selector;
    private ServerSocketChannel serverChannel;
    private volatile  boolean stop;

    /**
     * 初始化多路复用器，绑定监听端口
     * @param port
     */
    public MultiplexerTimeServer(int port){
        try{
            selector = Selector.open();//创建多路复用器
            serverChannel = ServerSocketChannel.open();
            serverChannel.configureBlocking(false);//设置为异步非阻塞模式
            serverChannel.socket().bind(new InetSocketAddress(port),1024);//绑定端口
            serverChannel.register(selector,SelectionKey.OP_ACCEPT);//注册到Selector
            System.out.println("The time server is start in port:" + port);
        }catch (IOException e){
            e.printStackTrace();
            System.exit(1);
        }
    }

    public void stop(){
        this.stop = true;
    }

    public void run(){
        while(!stop){
            try{
               selector.select(1000);//selector每隔1s都被唤醒一次
               Set<SelectionKey> selectedKeys = selector.selectedKeys();
               Iterator<SelectionKey> it = selectedKeys.iterator();
               SelectionKey key = null;
               while(it.hasNext()){
                   key = it.next();
                   it.remove();
                   try{
                       handleInput(key);
                   }catch (Exception e){
                       if(key !=null){
                           key.cancel();
                           if(key.channel() !=null)
                               key.channel().close();
                       }
                   }
               }
            }catch (Throwable t){
                t.printStackTrace();
            }
        }
        //多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源
        if(selector != null){
            try{
                selector.close();
            }catch (IOException e){
                e.printStackTrace();
            }
        }
    }

    private void handleInput(SelectionKey key) throws IOException{
        if(key.isValid()){
            //处理新接入的请求消息
            if(key.isAcceptable()){
                //Accept the new connection
                ServerSocketChannel ssc = (ServerSocketChannel)key.channel();
                SocketChannel sc = ssc.accept();//接收客户端的连接请求，完成TCP三次握手
                sc.configureBlocking(false);//设置为异步非阻塞
                //Add the new connection to the selector
                sc.register(selector,SelectionKey.OP_READ);
            }
            if(key.isReadable()){
                //Read the data
                SocketChannel sc = (SocketChannel)key.channel();
                ByteBuffer readBuffer = ByteBuffer.allocate(1024);
                int readBytes = sc.read(readBuffer);
                if(readBytes > 0 ){
                    readBuffer.flip();//将缓冲区当前的limit设置为position,position设置为0
                    byte[] bytes = new byte[readBuffer.remaining()];
                    readBuffer.get(bytes);
                    String body = new String(bytes,"UTF-8");
                    System.out.println("The time server receive order :" + body);
                    String currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body)?new java.util.Date(System.currentTimeMillis()).toString():"BAD ORDER";
                    doWrite(sc,currentTime);
                }else if(readBytes <0){
                    //对端链路关闭
                    key.cancel();
                    sc.close();
                }else{
                    //读到0字节，忽略
                }
            }
        }
    }

    /**
     * 将应答消息异步发送给客户端
     * @param channel
     * @param response
     * @throws IOException
     */
    private void doWrite(SocketChannel channel,String response) throws IOException{
        if(response !=null && response.trim().length() >0){
            byte[] bytes = response.getBytes();
            ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
            writeBuffer.put(bytes);
            writeBuffer.flip();
            channel.write(writeBuffer);
        }
    }
}

客户端代码

 public static void main(String[] args) throws IOException{
        int port = 8080;
        if(args != null &&args.length >0){
            try{
                port = Integer.valueOf(args[0]);
            }catch (NumberFormatException ex){
                //采用默认值
            }
        }
        new Thread(new TimeClientHandle("127.0.0.1",port),"TimeClient-001").start();
    }

public class TimeClientHandle implements Runnable{
    private String host;
    private int port;
    private Selector selector;
    private SocketChannel socketChannel;
    private volatile boolean stop;

    public TimeClientHandle(String host,int port){
        this.host = host == null?"127.0.0.1":host;
        this.port = port;
        try{
            selector = Selector.open();
            socketChannel = SocketChannel.open();
            socketChannel.configureBlocking(false);
        }catch (IOException e){
            e.printStackTrace();
            System.exit(1);
        }
    }

    public void run(){
        try{
            doConnect();
        }catch (IOException e){
            e.printStackTrace();
            System.exit(1);
        }
        while(!stop){
            try{
                selector.select(1000);
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                Iterator<SelectionKey> it = selectionKeys.iterator();
                SelectionKey key = null;
                while(it.hasNext()){
                    key = it.next();
                    it.remove();
                    try{
                        handleInput(key);
                    }catch (Exception e){
                        if(key != null){
                            key.cancel();
                            if(key.channel() !=null)
                                key.channel().close();
                        }
                    }
                }
            }catch (Exception e){
                e.printStackTrace();
                System.exit(1);
            }
        }
        if(selector !=null){
            try{
                selector.close();
            }catch (IOException e){
                e.printStackTrace();
            }
        }
    }

    private void handleInput(SelectionKey key) throws IOException{
        if(key.isValid()){
            //判断是否连接成功
            SocketChannel sc = (SocketChannel)key.channel();
            if(key.isConnectable()){
                if(sc.finishConnect()){
                    sc.register(selector,SelectionKey.OP_READ);
                    doWrite(sc);
                }else{
                    System.exit(1);//连接失败，进程退出
                }
            }
            if(key.isReadable()) {
                ByteBuffer readBuffer = ByteBuffer.allocate(1024);
                int readBytes = sc.read(readBuffer);
                if (readBytes > 0) {
                    readBuffer.flip();//将缓冲区当前的limit设置为position,position设置为0
                    byte[] bytes = new byte[readBuffer.remaining()];
                    readBuffer.get(bytes);
                    String body = new String(bytes, "UTF-8");
                    System.out.println("The time server receive order :" + body);
                    this.stop = true;
                } else if (readBytes < 0) {
                    //对端链路关闭
                    key.cancel();
                    sc.close();
                } else {
                    //读到0字节，忽略
                }
            }
        }
    }

    private void doConnect() throws IOException{
        if(socketChannel.connect(new InetSocketAddress(host,port))){
            socketChannel.register(selector,SelectionKey.OP_READ);
            doWrite(socketChannel);
        }else{
            socketChannel.register(selector,SelectionKey.OP_CONNECT);
        }
    }

    private void doWrite(SocketChannel sc) throws IOException {
        byte[] bytes = "QUERY TIME ORDER".getBytes();
        ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
        writeBuffer.put(bytes);
        writeBuffer.flip();
        sc.write(writeBuffer);
        if (!writeBuffer.hasRemaining())
            System.out.println("Send order 2 server succeed.");
    }
}

由于NIO比较难用，所以Socket编程更多地使用Netty框架

Reactor模型

code
Reactor模型中定义三种角色：Reactor将IO事件分发给Handler;Acceptor处理客户端连接，请求派发到处理器；Handler执行读写任务。

1. 单Reactor单线程模型

Reactor线程，负责多路分离套接字，有新连接到来触发connect 事件之后，交由Acceptor进行处理，有IO读写事件之后交给hanlder 处理

2. 单Reactor多线程模型

在处理业务逻辑，也就是获取到IO的读写事件之后，交由线程池来处理，这样可以减小主reactor的性能开销

3. 多Reactor多线程模型

这种模型下，将处理连接和处理业务分成分成主Reactor和从Reactor。此种模型中，每个模块的工作更加专一，耦合度更低，性能和稳定性也大量的提升，支持的可并发客户端数量可达到上百万级别，Netty中NIO就是基于这种模型。