使用套接字的基本客戶端和伺服器通訊

伺服器：啟動，並等待傳入​​連線

//Open a listening "ServerSocket" on port 1234.
ServerSocket serverSocket = new ServerSocket(1234); 

while (true) {
    // Wait for a client connection.
    // Once a client connected, we get a "Socket" object
    // that can be used to send and receive messages to/from the newly 
    // connected client
    Socket clientSocket = serverSocket.accept();            
    
    // Here we'll add the code to handle one specific client.
}

伺服器：處理客戶端

我們將在一個單獨的執行緒中處理每個客戶端，以便多個客戶端可以同時與伺服器進行互動。只要客戶端數量較少（<< 1000 個客戶端，具體取決於作業系統體系結構和每個執行緒的預期負載），此技術就可以正常工作。

new Thread(() -> {
    // Get the socket's InputStream, to read bytes from the socket
    InputStream in = clientSocket.getInputStream();
    // wrap the InputStream in a reader so you can read a String instead of bytes
    BufferedReader reader = new BufferedReader(
            new InputStreamReader(in, StandardCharsets.UTF_8));
    // Read text from the socket and print line by line
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
    }).start();

客戶端：連線到伺服器併傳送訊息

// 127.0.0.1 is the address of the server (this is the localhost address; i.e.
// the address of our own machine)
// 1234 is the port that the server will be listening on
Socket socket = new Socket("127.0.0.1", 1234);

// Write a string into the socket, and flush the buffer
OutputStream outStream = socket.getOutputStream();
PrintWriter writer = new PrintWriter(
        new OutputStreamWriter(outStream, StandardCharsets.UTF_8));
writer.println("Hello world!");
writer.flush();

關閉套接字和處理異常

上面的例子省略了一些使它們更容易閱讀的東西。

1. 就像檔案和其他外部資源一樣，我們在完成作業系統時告訴作業系統非常重要。當我們完成套接字時，請呼叫 socket.close() 以正確關閉它。

2. 套接字處理依賴於各種外部因素的 I / O（輸入/輸出）操作。例如，如果另一方突然斷開連線怎麼辦？如果有網路錯誤怎麼辦？這些事情是我們無法控制的。這就是許多套接字操作可能丟擲異常的原因，尤其是 IOException。

因此，客戶端的更完整程式碼將是這樣的：

 // "try-with-resources" will close the socket once we leave its scope
 try (Socket socket = new Socket("127.0.0.1", 1234)) {
     OutputStream outStream = socket.getOutputStream();
     PrintWriter writer = new PrintWriter(
             new OutputStreamWriter(outStream, StandardCharsets.UTF_8));
     writer.println("Hello world!");
     writer.flush();
 } catch (IOException e) {
     //Handle the error
 }

基本伺服器和客戶端 - 完整示例

伺服器：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.charset.StandardCharsets;

public class Server {
    public static void main(String args[]) {
        try (ServerSocket serverSocket = new ServerSocket(1234)) {
            while (true) {
                // Wait for a client connection.
                Socket clientSocket = serverSocket.accept();
                
                // Create and start a thread to handle the new client
                new Thread(() -> {
                    try {
                        // Get the socket's InputStream, to read bytes 
                        // from the socket
                        InputStream in = clientSocket.getInputStream();
                        // wrap the InputStream in a reader so you can 
                        // read a String instead of bytes
                        BufferedReader reader = new BufferedReader(
                             new InputStreamReader(in, StandardCharsets.UTF_8));
                        // Read from the socket and print line by line
                        String line;
                        while ((line = reader.readLine()) != null) {
                            System.out.println(line);
                        }
                    }
                    catch (IOException e) {
                        e.printStackTrace();
                    } finally {
                        // This finally block ensures the socket is closed.
                        // A try-with-resources block cannot be used because
                        // the socket is passed into a thread, so it isn't 
                        // created and closed in the same block
                        try {
                            clientSocket.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                }).start();
            }
        }
        catch (IOException e) {
            e.printStackTrace();
        }

    }
}

客戶：

import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.Socket;
import java.nio.charset.StandardCharsets;

public class Client {
    public static void main(String args[]) {
        try (Socket socket = new Socket("127.0.0.1", 1234)) {
            // We'll reach this code once we've connected to the server
            
            // Write a string into the socket, and flush the buffer
            OutputStream outStream = socket.getOutputStream();
            PrintWriter writer = new PrintWriter(
                    new OutputStreamWriter(outStream, StandardCharsets.UTF_8));
            writer.println("Hello world!");
            writer.flush();
        } catch (IOException e) {
            // Exception should be handled.
            e.printStackTrace();
        }
    }
}