Java套接字将发送但不接收响应

Question

I am currently learning Java and am playing around with socket programming.

My goal is to implement request-response communication like HTTP . I can get one way communication from the client to the server. But when I program the client to listen for a response it causes the server to not print out the request.

I've googled the issue and most posts mention using the flush() method, which I have tried. Anyone have any thoughts?

Here is the client code:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.Socket;

class Client {
    public static void main(String args[]) throws IOException {

        // Get the port number from the first command line argument

        int port_number;

        if(args.length == 0) {
            port_number = 80;
        } else {
            port_number = Integer.parseInt(args[0]);
        }

        try (

            // Create a socket
            Socket socket = new Socket("localhost", port_number);

            // Input reader
            BufferedReader in = new BufferedReader(
                new InputStreamReader(socket.getInputStream())
            );

            // Output writer
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

        ) {

            System.out.println("Sending request");
            out.print("Foo");
            out.flush();

            System.out.println("Sending Complete");

            // If this next section is commented out then the server prints the message

            StringBuilder message = new StringBuilder();
            int data;

            while((data = in.read()) != -1) {
                message.append((char) data);
            }

            System.out.print(message);

        }
    }
}

and here is my server code:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class Server {

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

        // Get the port number from the first command line argument
        int portNumber = Integer.parseInt(args[0]);

        System.out.println("Listening on port " + portNumber);
        ServerSocket serverSocket = new ServerSocket(portNumber);

        while (true) {

            try (
                Socket socket = serverSocket.accept();
                PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
                BufferedReader in = new BufferedReader(
                    new InputStreamReader(socket.getInputStream())
                );
            ){  

            StringBuilder message = new StringBuilder();
            int data;

            while((data = in.read()) != -1) {
                message.append((char) data);
            }

            System.out.println(message);

            out.print("Bar");

            }
        }
    }
}

Answer 1

This is because your sever currently loop until read() returns -1 which never happens in your case so he never reaches the part of the code where he is supposed to send Bar , you need a specific marker to indicate the end of your message so for example here you can send data line by line, for this proceed as next:

Client side:

System.out.println("Sending request");
// Send a line to the server
out.println("Foo");
out.flush();

System.out.println("Sending Complete");

// Read a line sent by the server
System.out.print(in.readLine());

Output:

Sending request
Sending Complete
Bar

Server side:

// Print the line sent by the client
System.out.println(in.readLine());
// Send a message to the client
out.println("Bar");

Output:

Listening on port 9999
Foo