Client-Server Communication in Java

Introduction

Client-server communication is a model in which the client sends requests to a server, and the server responds with data. Java provides built-in support for client-server communication through the java.net package. In this tutorial, we will create a simple client-server application where the client sends a message to the server, and the server processes it and sends a response back.

The client communicates with the server using Socket for sending requests, and the server uses ServerSocket to listen for incoming connections.

Step 1: Create the Server

The server is responsible for listening for incoming connections from clients and responding to their requests. The server uses the ServerSocket class to listen on a specific port.

Example 1: Server Code

          import java.io.*;
          import java.net.*;
      
          public class Server {
              public static void main(String[] args) {
                  try {
                      // Create a ServerSocket to listen on port 1234
                      ServerSocket serverSocket = new ServerSocket(1234);
                      System.out.println("Server started, waiting for client...");
      
                      // Wait for a client to connect
                      Socket clientSocket = serverSocket.accept();
                      System.out.println("Client connected!");
      
                      // Set up input and output streams for communication
                      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("Received from client: " + message);
      
                      // Send response to client
                      out.println("Hello from server!");
      
                      // Close the streams and socket
                      in.close();
                      out.close();
                      clientSocket.close();
                      serverSocket.close();
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              }
          }
        

In this example:

• We create a ServerSocket to listen for incoming connections on port 1234.
• The server waits for a client to connect using accept() method.
• Once a client connects, the server reads the message from the client and sends a response.
• Finally, the server closes the input/output streams and the socket.

Step 2: Create the Client

The client is responsible for connecting to the server, sending a message, and receiving a response. The client uses the Socket class to establish a connection to the server.

Example 2: Client Code

          import java.io.*;
          import java.net.*;
      
          public class Client {
              public static void main(String[] args) {
                  try {
                      // Create a Socket to connect to the server at localhost on port 1234
                      Socket socket = new Socket("localhost", 1234);
      
                      // Set up input and output streams
                      BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                      PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
      
                      // Send a message to the server
                      out.println("Hello from client!");
      
                      // Read response from the server
                      String response = in.readLine();
                      System.out.println("Received from server: " + response);
      
                      // Close the streams and socket
                      in.close();
                      out.close();
                      socket.close();
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              }
          }
        

In this example:

• We create a Socket to connect to the server at localhost on port 1234.
• The client sends a message to the server using a PrintWriter.
• The client waits for the response from the server using a BufferedReader.
• Finally, the client closes the input/output streams and the socket.

Step 3: Running the Server and Client

To run the server and client applications:

1. First, run the Server class. The server will start and wait for client connections.
2. Next, run the Client class. The client will connect to the server, send a message, and print the response from the server.

Understanding the Communication Flow

The flow of communication between the client and the server in this example is as follows:

1. The client creates a socket and connects to the server at localhost on port 1234.
2. The server listens for incoming connections using ServerSocket.accept() method.
3. Once the client connects, the server reads the message from the client and sends a response.
4. The client receives the server's response and prints it to the console.

Exception Handling in Client-Server Communication

It's important to handle exceptions properly in client-server communication, as network operations can fail due to various reasons like timeouts or connectivity issues. Some common exceptions to handle include:

• IOException: This exception occurs if there's an error in input or output operations (e.g., reading from or writing to a socket).
• UnknownHostException: This exception occurs if the client cannot resolve the server's host name to an IP address.
• SocketException: This exception occurs if there's an error in socket communication (e.g., the connection is interrupted).

Always handle these exceptions to ensure that your application remains stable even when network issues occur.

Conclusion

In this tutorial, we learned how to set up client-server communication in Java. We created a simple server and client application using ServerSocket and Socket classes. The client sends a message to the server, and the server responds with a message.

• We covered how to create the server, listen for client connections, and send responses.
• We also saw how the client connects to the server, sends a request, and receives a response.