Java Socket programming is the basis of network communication, and data exchange between clients and servers is realized through Socket. 1. Socket in Java is divided into the Socket class used by the client and the ServerSocket class used by the server; 2. When writing a Socket program, you must first start the server listening port, and then initiate the connection by the client; 3. The communication process includes connection establishment, data reading and writing, and stream closure; 4. Notes include avoiding port conflicts, correctly configuring IP addresses, reasonably closing resources, and supporting multiple clients. Mastering these can realize basic network communication functions.

In Java network programming, Socket is one of the most basic and important parts. It allows communication between two programs over the network, both local LAN and the Internet. This article mainly talks about the basic concepts of Socket programming in Java and a simple example to help you get started quickly.

What is Java Socket?

In Java, Socket is a mechanism used to establish connections and exchange data between different devices in the network. Java provides java.net package to support network communication, two core classes are:

• Socket : used for client, indicating a connection to the server.
• ServerSocket : used to listen to client requests and establish connections.

Simply put, Socket is like a telephone, the client dials (connects), the server answers (accepts connections), and then both parties can talk (transmits data).

How to write a simple Java Socket program?

Let’s use a classic example of “client send messages, server reply” to illustrate the entire process.

Step 1: Start the server

The server needs to be started first and wait for the client to connect. You can use ServerSocket to listen for a port.

 import java.io.*;
import java.net.*;

public class Server {
    public static void main(String[] args) {
        try (ServerSocket serverSocket = new ServerSocket(12345)) {
            System.out.println("The server has started, waiting for connection...");
            Socket socket = serverSocket.accept(); // Block until a client connects System.out.println("Client connected");

            BufferedReader in = new BufferedReader(
                new InputStreamReader(socket.getInputStream()));
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

            String clientMessage = in.readLine();
            System.out.println("Received:" clientMessage);
            out.println("The server receives your message: "clientMessage);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Step 2: Run the client

The client creates a Socket instance and connects to the server IP and port.

 import java.io.*;
import java.net.*;

public class Client {
    public static void main(String[] args) {
        try (Socket socket = new Socket("localhost", 12345)) {
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(
                new InputStreamReader(socket.getInputStream()));

            out.println("Hello, server!");
            String response = in.readLine();
            System.out.println("Response:" response);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Although this example is simple, it covers the core steps of Socket programming: connecting, reading and writing data.

Frequently Asked Questions and Precautions

• Port occupied : If the specified port number has been occupied by other programs, the server will not be started. You can try changing the port number, such as between 1024 and 65535.
• IP address configuration error : Make sure the client is connected to the correct IP address. If it is a native test, use localhost or 127.0.0.1 .
• Stream closing order : Be sure to remember to close the input and output streams and Socket connections, otherwise resource leakage may occur.
• Multithreading process multiple clients : The above example can only handle one client. If you want to handle multiple connections at the same time, you need to use multi-threading or NIO.
• Protocol design : In actual development, it is recommended to define your own communication protocol, such as transmitting structured data in JSON format.

Let's summarize

Java Socket programming is not difficult, the key is to understand how the client/server model works. After mastering the basic connection and data reading and writing processes, more complex functions can be expanded on this basis, such as file transfer, real-time chat, etc. Basically all that is it, just try it out.

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