9.2.2 ServerSocket Class Explained

The ServerSocket class in Java SE 11 is a fundamental component for creating server-side applications that listen for and accept incoming client connections. Understanding the ServerSocket class is essential for building robust network applications.

Key Concepts

1. ServerSocket Creation

The ServerSocket class is used to create a server socket that listens on a specific port. The constructor of the ServerSocket class can take the port number as an argument.

Example

        ServerSocket serverSocket = new ServerSocket(8080);
    

2. Binding and Listening

Once a ServerSocket is created, it binds to the specified port and starts listening for incoming client connections. The accept() method is used to accept a connection request from a client.

Example

        Socket clientSocket = serverSocket.accept();
    

3. Handling Client Connections

After accepting a client connection, the server can communicate with the client using the Socket object returned by the accept() method. This allows the server to send and receive data with the client.

Example

        InputStream input = clientSocket.getInputStream();
        OutputStream output = clientSocket.getOutputStream();
    

4. Closing the ServerSocket

It is important to close the ServerSocket when it is no longer needed to free up system resources. The close() method is used to close the server socket.

Example

        serverSocket.close();
    

5. Handling Multiple Clients

To handle multiple clients concurrently, the server can use threads. Each time a client connection is accepted, a new thread can be created to handle the communication with that client.

Example

        while (true) {
            Socket clientSocket = serverSocket.accept();
            new Thread(() -> {
                // Handle client communication
            }).start();
        }
    

Examples and Analogies

Think of the ServerSocket as a receptionist at a busy office. The receptionist (server socket) sits at the front desk (listens on a port) and waits for visitors (client connections). When a visitor arrives, the receptionist greets them (accepts the connection) and assigns a staff member (thread) to assist the visitor. The receptionist continues to wait for more visitors until the office closes (server socket is closed).

By mastering the ServerSocket class, you can create powerful server-side applications in Java SE 11 that efficiently handle multiple client connections, ensuring smooth and reliable communication over a network.