Learn Google

Compute Engine is a terrific hosting platform for applications – not only HTTP-based applications, but for servers running all types of services. To run these services an application has to listen for incoming connections using a server socket. Here’s how to do that in Java.

First, bind a server socket to a port. Here we’re binding it to the SMTP-reserved port 25. The port binding will fail if there’s already a server bound to that port.

//The server socket that handles all incoming connections.
ServerSocket server_socket = null;
//Bind the server socket to port 25. Fail and exit 
//if we fail to bind.
try {
    server_socket = new ServerSocket(25);
    System.out.println("OK: Bound to port 25.");
} 
catch (IOException e) {
    System.err.println("ERROR: Unable to bind to port 25.");
    e.printStackTrace();
    System.exit(-1);
}

Now wait for an incoming connection to accept (you can put the following code into a while loop if you want to accept multiple connections):

//Now we need to wait for a client to connect to us.
//Accept() blocks until it receives a new connection.
try {
    //A new client has connected.
    Socket client_socket = server_socket.accept();
    System.out.println("Accepted!");
    //Hand off the socket for further handling.
    //Do something here with the socket.
} 
catch (IOException e) {
    System.err.println("Failed to accept incoming connection:" + e.getMessage());
    e.printStackTrace();
    System.exit(-1);
}

From here, handle the socket as needed.

Remember to import the following classes:

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;