如何实现服务器和客户端数据交互？

1 Socket概述
根据百度百科的定义，Socket 译为套接字，它是对网络中不同主机上的应用进程之间进行双向通信的端点的抽象。一个Socket实例就是网络上进程通信的一端，提供了应用层进程利用网络协议交换数据的机制。Socket向上连接各种应用进程，向下连接各种网络协议，是应用程序通过网络协议进行通信的接口。其示意图如下下图所示：


从上图可知，套接字Socket在OSI七层模型中处于应用层和传输层之间，是一个重要的接口。一般来说，传输层协议有TCP协议和UDP协议。这两个协议底层都基于IP网络层协议。


2 Java Socket 实现
在Java SDK中，对于Socket原生提供了支持，它分为ServerSocket和Socket，其中ServerSocket发起一个服务端的Socket，其中需要提供一个端口号，如果给定0，则自动申请可用的端口。当然了，也可以指定具体的端口号（有一定的范围，不超过 65535 ）。不过这里需要注意，传入的端口不能被其他应用占用，否则启动服务失败。下面给出一个简单的ServerSocket示例，代码如下：
package com.example.demo.network;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class MyServer {
    //<= 65535
    private static final int PORT = 65535;


    public static void main(String[] args) throws Exception {


        try (ServerSocket listener = new ServerSocket(PORT)) {
            System.out.println("Server Started");
            //线程池大小，它根据客户端数来决定，当大于10时，则之前的10个线程仍然可以工作，
            // 而超过的线程则进入队列中，等待。
            //当之前的客户端释放量后，则在队列中的线程仍然可以工作。
            ExecutorService pool = Executors.newFixedThreadPool(10);
            while (true) {
                //多线程
                pool.execute(new MyServerDemo01(listener.accept()));
                System.out.println(pool);
            }
        }
    }
    //Runnable接口的实现对象可以被线程Thread调用
    private static class MyServerDemo01 implements Runnable {


        private Socket socket;


        MyServerDemo01(Socket socket) {
            this.socket = socket;
        }


        @Override
        public void run() {
            System.out.println("Client [" + socket.getRemoteSocketAddress().toString()+" ] Connected");
            try {
                //输入
                Scanner in = new Scanner(socket.getInputStream());
                //输出
                PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
                while (in.hasNextLine()) {
                    String msg = in.nextLine();
                    System.out.println("Client [" + socket.getRemoteSocketAddress().toString()+" ] : " + msg);
                    out.println(msg.toUpperCase());
                }
            } catch (Exception e) {
                System.out.println("Error:" + socket+ e.getMessage());
            } finally {
                try {
                    //关闭socket
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                System.out.println("Closed: " + socket);
            }
        }
    }
}


启动该Server端，并开始监听客户端的连接。当客户端没有连接时，服务器线程池pool并未启动单独的线程。下面给出客户端的Java Socket实现，具体的示例代码如下：
package com.example.demo.network;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;
public class MyClient {
    //<= 65535
    private static final int PORT = 65535;
    //服务器地址
    private static final String IP = "127.0.0.1";


    public static void main(String[] args) throws Exception {


        try (Socket socket = new Socket(IP, PORT)) {
            System.out.println("Client ["+socket.getRemoteSocketAddress().toString()+" ] Started");
            Scanner scanner = new Scanner(System.in);
            Scanner in = new Scanner(socket.getInputStream());
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            while (scanner.hasNextLine()) {
                out.println(scanner.nextLine());
                System.out.println("Server Response:"+ in.nextLine());
            }
        }catch (Exception ex){
            System.out.println("Error : "+ ex.getMessage());
        }
    }
}
从代码可知，try (Socket socket = new Socket(IP, PORT)) 是一种包含资源释放的try-with-resource机制，它会自动进行资源释放，而不需要手动进行释放。Socket对象要想和服务器通信，必须要明确服务器的IP地址和端口，否则不能正确通信，Socket启动时，也会在主机上占用自己的端口。我们首先启动Server端，然后可以同时启动10个以上的Client端，比如13个，那么超过Executors.newFixedThreadPool(10)限定的数量10后，将进入queued tasks队列中进行排队等待。通过打印pool对象，可以看出当前的状态，比如[Running, pool size = 4, active threads = 4, queued tasks = 0, completed tasks = 0]说明当前在运行状态，线程池大小为10，激活的线程为10，等待的任务线程queued tasks为0。


下面给出Server端相关输出示例：
Server Started
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 1, active threads = 1, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:64590 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:64597 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 3, active threads = 3, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:64603 ] Connected
Client [/127.0.0.1:64590 ] : hello
Client [/127.0.0.1:64590 ] : hello
Client [/127.0.0.1:64597 ] : world
Client [/127.0.0.1:64597 ] : world
Client [/127.0.0.1:64603 ] : python
Client [/127.0.0.1:64597 ] : python02
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 4, active threads = 4, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:57806 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 5, active threads = 5, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:57814 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 6, active threads = 6, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:57820 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 7, active threads = 7, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:57827 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 8, active threads = 8, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:57833 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 9, active threads = 9, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:57839 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 0, completed tasks = 0]
Client [/127.0.0.1:57845 ] Connected
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 1, completed tasks = 0]
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 2, completed tasks = 0]
java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 3, completed tasks = 0]
Closed: Socket[addr=/127.0.0.1,port=64590,localport=65535]
Client [/127.0.0.1:57854 ] Connected
Client [/127.0.0.1:57814 ] : t2
Client [/127.0.0.1:57814 ] : tw2


Client [/127.0.0.1:65535 ] Started
world
Server Response:WORLD
world
Server Response:WORLD
python02
Server Response:PYTHON02