TCP can be defined as Transmission Control Protocol. This is the standard protocol for transmitting data over a network. It provides reliable, structured, and error-checked data delivery between applications running on hosts connected via an IP (Internet Protocol) network. In Java, we can create TCP client-server connections using the Socket and ServerSocket classes from the java.net package.

In this article, we will learn how to create a simple TCP client-server connection in Java.

TCP client-server connection

• TCP client-server communication consists of two important components. These two consist of a server and more than one client.
• The servers receive incoming connections from clients on a specific port number using a ServerSocket.
• The client then connects to the port using a Socket with the server’s IP address.
• Once the connection is created, the data can be exchanged between the server and the client using input and output streams.

Key Terminologies

• TCP: Transmission Control Protocol used to transmit data over a network and can provide network communications, point-to-point communication channels and can ensure data integrity and sequencing.
• Socket: It is the terminal connection between two devices on the network and the Socket class can represent the client-side socket and can provide a connection to the server

Step-by-Step implementation of Server-Side to Create Connection

• Create the Java class named Server in that class write the main method.
• Create the instance of the ServerSocket and assign the port as 9090.
• Accept the client connections using accept() method of the ServerSocket to the wait for incoming client connections.
• Now, we can setup the input and output streams of the client socket and read and process the client data.
• Send the response to the client and close the sockets.

Implementation:

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

public class Server {
    public static void main(String args[]) throws IOException 
    {
        // create a server socket on port number 9090
        ServerSocket serverSocket = new ServerSocket(9090);
        System.out.println("Server is running and waiting for client connection...");

        // Accept incoming client connection
        Socket clientSocket = serverSocket.accept();
        System.out.println("Client connected!");

        // Setup input and output streams for communication with the client
        BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);

        // Read message from client
        String message = in.readLine();
        System.out.println("Client says: " + message);

        // Send response to the client
        out.println("Message received by the server.");

        // Close the client socket
        clientSocket.close();
        // Close the server socket
        serverSocket.close();
    }
}

Output:

Before client connected to the server:

Server is running and waiting for client connection...

After client connected:

Step-by-Step implementation of Client-Side to Create Connection

• Create the Java class named as Client in that class write the main method of the program.
• Create the instance of the Socket and assign the port as 9090.
• Setup the input and output streams and send the data to the server.
• Setup the receive the response from the server.
• Close the socket.

Implementation:

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

public class Client {
    public static void main(String args[]) throws IOException 
    {
        // create a socket to connect to the server running on localhost at port number 9090
        Socket socket = new Socket("localhost", 9090);
        
        // Setup output stream to send data to the server
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        
        // Setup input stream to receive data from the server
        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

        // Send message to the server
        out.println("Hello from client!");

        // Receive response from the server
        String response = in.readLine();
        System.out.println("Server says: " + response);

        // Close the socket
        socket.close();
    }
}

Output:

Below we can see the output received by the client in console.

In the above example, we have created a TCP connection between client and server by sending and receiving data between client and server on port 9090.