[Java] socket 简介

今天药忘吃喽~ 2022-06-04 10:17 145阅读 0赞

## 一、简介 ##

内容来自: [What Is a Socket?][What Is a Socket]

Socket 是网络上两个程序间双向交流连接的一个端点（类似于打电话时的两台手机 [Java Socket对象原理的详细介绍][Java Socket]），**基于TCP/IP协议**，稳定有序。

> Definition:  
> A socket is one **endpoint** of a **two-way communication link** between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to.
> 
> An **endpoint** is a combination of an IP address and a port number. Every TCP connection can be uniquely identified by its two endpoints. That way you can have multiple connections between your host and the server.

socket 连接示意图：

![连接示意图][SouthEast]

工作模式:

> This client program is straightforward and simple because the echo server implements a simple protocol. The client sends text to the server, and the server echoes it back. When your client programs are talking to a more complicated server such as an HTTP server, your client program will also be more complicated. However, the basics are much the same as they are in this program:

*  Open a socket.
 *  Open an input stream and output stream to the socket.
 *  Read from and write to the stream according to the server's protocol.
 *  Close the streams.
 *  Close the socket. Only step 3 differs from client to client, depending on the server. The other steps remain largely the same.
 *  **socket 工作模式总结如下：**
    
    1.  server 端监听一个本地端口，client 端通过IP地址、端口号(endpoint)发起连接请求，客户端同时会提供自身的端口号给服务端以互相通信（通常由系统指定）；
    2.  一旦连接建立，accept() 方法会返回一个新的 socket 对象代表客户端，server 端则继续监听相同的本地端口号等待客户端连接；
    3.  客户端和服务端基于数据流读/写数据；
    4.  关闭流；
    5.  关闭socket。
    
    其中，**server 端和 client 端建立连接**官方文档说明如下: [Writing the Server Side of a Socket  
    ][Writing the Server Side of a Socket]
    
    > If the server successfully binds to its port, then the ServerSocket object is successfully created and the server continues to the next step—accepting a connection from a client (the next statement in the try-with-resources statement):
    > 
    > `clientSocket = serverSocket.accept();`
    
    **The accept method waits until a client starts up and requests a connection on the host and port of this server**. (Let’s assume that you ran the server program KnockKnockServer on the computer named [knockknockserver.example.com][].) In this example, the server is running on the port number specified by the first command-line argument. **When a connection is requested and successfully established, the accept method returns a new Socket object which is bound to the same local port and has its remote address and remote port set to that of the client**. The server can communicate with the client over this new Socket and **continue to listen for client connection requests on the original ServerSocket**.This particular version of the program doesn’t listen for more client connection requests. However, a modified version of the program is provided in Supporting Multiple Clients.
    
    ## 二、代码示例 ##
    
    代码来自：[Reading from and Writing to a Socket][]
    
    Socket 服务端代码:
    
        package com.kascend.test.socket;
        
        import java.io.BufferedReader;
        import java.io.IOException;
        import java.io.InputStreamReader;
        import java.io.PrintWriter;
        import java.net.ServerSocket;
        import java.net.Socket;
        
        /**
         * Socket 服务端类
         *
         * @author wengliemiao
         */
        public class EchoServer {
        
            public static void main(String[] args) {
                int portNumber = 8080;
                try (
                        ServerSocket serverSocket =
                                new ServerSocket(portNumber);
                        Socket clientSocket = serverSocket.accept();
                        PrintWriter out =
                                new PrintWriter(clientSocket.getOutputStream(), true);
                        BufferedReader in = new BufferedReader(
                                new InputStreamReader(clientSocket.getInputStream()));
                ) {
                    String inputLine;
                    while ((inputLine = in.readLine()) != null) {
                        System.out.println(inputLine);
                        out.println("from server: " + inputLine);
                    }
                } catch (IOException e) {
                    System.out.println("Exception caught when trying to listen on port "
                            + portNumber + " or listening for a connection");
                    System.out.println(e.getMessage());
                }
            }
        }
    
    Socket 客户端：
    
        package com.kascend.test.socket;
        
        import java.io.BufferedReader;
        import java.io.IOException;
        import java.io.InputStreamReader;
        import java.io.PrintWriter;
        import java.net.Socket;
        import java.net.UnknownHostException;
        
        /**
         * Socket 客户端类
         *
         * @author wengliemiao
         */
        public class EchoClient {
        
            public static void main(String[] args) {
                String hostName = "127.0.0.1";
                int portNumber = 8080;
        
                try (
                        Socket echoSocket = new Socket(hostName, portNumber);
                        PrintWriter out =
                                new PrintWriter(echoSocket.getOutputStream(), true);
                        BufferedReader in =
                                new BufferedReader(
                                        new InputStreamReader(echoSocket.getInputStream()));
                        BufferedReader stdIn =
                                new BufferedReader(
                                        new InputStreamReader(System.in))
                ) {
                    String userInput;
                    while ((userInput = stdIn.readLine()) != null) {
                        out.println(userInput);
                        System.out.println("echo: " + in.readLine());
                    }
                } catch (UnknownHostException e) {
                    System.err.println("Don't know about host " + hostName);
                    System.exit(1);
                } catch (IOException e) {
                    System.err.println("Couldn't get I/O for the connection to " +
                            hostName);
                    System.exit(1);
                }
            }
        }
    
    先运行服务端类，再运行客户端类。
    
    在客户端控制台输入:
    
        Hello, I'm wlmmm.
        666666
    
    服务端控制台输出为:  
    ![服务端控制台输出][SouthEast 1]
    
    客户端控制台输出为：  
    ![客户端控制台输出][SouthEast 2]
    
    注意: `Socket clientSocket = serverSocket.accept();` 这行代码会阻塞直到有客户端连接建立。
    
    ![accept方法][accept]
    
    ## 三、多线程处理 ##
    
    官方文档来源：[Writing the Server Side of a Socket][]
    
    > To keep the KnockKnockServer example simple, we designed it to listen for and handle a single connection request. However, **multiple client requests** can come into the same port and, consequently, **into the same ServerSocket**. **Client connection requests are queued at the port, so the server must accept the connections sequentially**. However, the server can service them simultaneously through the use of threads—one thread per each client connection.
    
    The basic flow of logic in such a server is this:
    
    while (true) \{  
    accept a connection;  
    create a thread to deal with the client;  
    \}
    
    The thread reads from and writes to the client connection as necessary.
    
    多线程实现方式如下:
    
        package com.wlm.socket;
        
        import java.io.BufferedReader;
        import java.io.IOException;
        import java.io.InputStreamReader;
        import java.io.PrintWriter;
        import java.net.ServerSocket;
        import java.net.Socket;
        
        /**
         * 服务端 socket 多线程方式
         *
         * @author wengliemiao
         */
        public class EchoServerMultiThread {
        
            public static void main(String[] args) {
                int portNumber = 8080;
                try {
                    ServerSocket serverSocket = new ServerSocket(portNumber);
                    while (true) {
                        new Thread(new ServerThread(serverSocket.accept())).start();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        
            private static class ServerThread implements Runnable {
        
                Socket socket = null;
        
                public ServerThread(Socket socket) {
                    this.socket = socket;
                }
        
                @Override
                public void run() {
                    try {
                        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
                        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                        String inputLine;
                        while ((inputLine = in.readLine()) != null) {
                            System.out.println(inputLine);
                            out.println("from server: " + inputLine);
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    
    启动`EchoServerMultiThread`，建立两个 socket 客户端 `EchoClient`, `EchoClient2` (实现和前面一致),同时向 server 端 socket 输入数据，结果为:  
    ![client1][]
    
    ![client2][]
    
    ![serverMultiThread][]

[What Is a Socket]: https://docs.oracle.com/javase/tutorial/networking/sockets/definition.html
[Java Socket]: http://blog.csdn.net/alen0217/article/details/42558085
[SouthEast]: /images/20220604/4218f2facd934fe2acd9df7a579fc9cb.png
[Writing the Server Side of a Socket]: https://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html
[knockknockserver.example.com]: http://knockknockserver.example.com
[Reading from and Writing to a Socket]: https://docs.oracle.com/javase/tutorial/networking/sockets/readingWriting.html
[SouthEast 1]: /images/20220604/44bebb66e2d24421abf667e1f635f8f0.png
[SouthEast 2]: /images/20220604/1bfc38f5104648d0926449e241af7201.png
[accept]: /images/20220604/c072993bad3348b9b9d7d4e920f2e000.png
[client1]: /images/20220604/034e97bdf49642a79721d319fd8141c0.png
[client2]: /images/20220604/9513a43b6ddc4950af8e10af9c3315d3.png
[serverMultiThread]: /images/20220604/c5f6c42b61834aa48c251a6b057abba1.png