如何使用 Java Socket 监听客户端？

Question

I'm working with "ServerSocket" and "Socket", the problem I'm going through is this: I create a server using serverSocket, I'm waiting for the client to connect, when it connects I'll receive some information, and here's my question, how do I keep listening to the client and receive instructions from it?

In the example below I am creating a server, when the client connects I save the connection within the "clientSocket ".

@GET
@Path("/createServer")
public String conect() throws IOException { 
    
    serverSocket = new ServerSocket(3242);      
    clientSocket = serverSocket.accept();
    ...
    ...
  }

From this point I need to always listen to this clientSocket, when the client send some information I need to capture to perform some actions, how to do that?

Answer 1

ServerSocket.accept() gives you a java.net.Socket . Doc here

From there on, you can read on that socket using its input stream ( Socket.getInputStream() ) or write to its output stream ( Socket.getOutputStream() )

Your sockets (client and server) will live until they're close d, or garbage collected, so remember to keep a strong reference to each one as long as you need them.

Sample programs (simple echo server. Type bye in client to exit):

Server.java

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.charset.StandardCharsets;
import java.util.Scanner;

public class Server {

  private static volatile boolean stopped = false;

  public static void main(String[] args) throws IOException, InterruptedException {

    try (ServerSocket server = new ServerSocket(3001)) {
      System.out.println("Server ready to accept connections on port " + server.getLocalPort());

      final Socket client = server.accept();
      System.out.println("Client connected using remote port " + client.getPort());

      final Thread t = new Thread(() -> {
        try {
          try (InputStream clientIn = client.getInputStream()) {
            try (OutputStream clientOut = client.getOutputStream()) {
              echo(clientIn, clientOut);
            }
          }
        } catch (IOException ioe) {
          ioe.printStackTrace();
          stopped = true;
        }
      });
      t.setDaemon(true);
      t.start();

      while (!stopped) {
        Thread.sleep(10);
      }
      System.out.println("Program exit");

    }

  }

  private static void echo(InputStream clientIn, OutputStream clientOut) throws IOException {
    
    try (Scanner clientScan = new Scanner(clientIn, StandardCharsets.UTF_8)) {
      try (OutputStreamWriter clientWriter = new OutputStreamWriter(clientOut, StandardCharsets.UTF_8)) {
        
        while (!stopped) {
          final String fromClient = clientScan.nextLine();
          System.out.println("In: " + fromClient);
          clientWriter.write(fromClient);
          clientWriter.write(System.lineSeparator());
          clientWriter.flush();
          if ("bye".equalsIgnoreCase(fromClient.trim())) {
            stopped = true;
          }
        }
        System.out.println("Loop done");
        
      }
    }
    
  }

}

Client.java

import java.io.IOException;
import java.io.OutputStreamWriter;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;

public class Client {
  private static volatile boolean stopped = false;

  public static void main(String[] args) throws UnknownHostException, IOException {
    System.out.println("Client launched");
    try (Socket client = new Socket("localhost", 3001)) {
      System.out.println("Connected on remote port " + client.getPort() + " from " + client.getLocalPort());

      try (Scanner console = new Scanner(System.in)) {
        try (OutputStreamWriter toServer = new OutputStreamWriter(client.getOutputStream())) {

          final Thread t = new Thread(() -> printEverything(client));
          t.setDaemon(true);
          t.start();

          while (!stopped) {
            final String fromConsole = console.nextLine();
            if (stopped)
              break;
            toServer.write(fromConsole);
            toServer.write(System.lineSeparator());
            toServer.flush();
          }

        }
      }

    }
    System.out.println("Program exit");

  }

  private static void printEverything(Socket client) {
    try (Scanner server = new Scanner(client.getInputStream())) {
      while (!stopped) {
        final String fromServer = server.nextLine();
        System.out.println("Server said: " + fromServer);
        if ("bye".equalsIgnoreCase(fromServer.trim())) {
          stopped = true;
        }
      }
      System.out.println("Loop done. Press enter to exit");
    } catch (IOException e) {
      e.printStackTrace();
      stopped = true;
    }
  }

}