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Writing custom applications on Compute Engine requires the use of sockets to communicate with clients. Here’s a code example demonstrating how to read and write to sockets in Java.

Assume that client_socket is the socket communicating with the client:

/**
 * Contains the socket we're communicating with.
 */
Socket client_socket;

Create a socket by using a ServerSocket (represented by server_socket ) to listen to and accept incoming connections:

Socket client_socket = server_socket.accept();

Then extract a PrintWriter and a BufferedReader from the socket. The PrintWriter out object sends data to the client, while the BufferedReader in object lets the application read in data sent by the client:

/**
 * Handles sending communications to the client.
 */
PrintWriter out;
/**
 * Handles receiving communications from the client.
 */
BufferedReader in;
try {
    //We can send information to the client by writing to out.
    out = new PrintWriter(client_socket.getOutputStream(), true);
    //We receive information from the client by reading in.
    in = new BufferedReader(new InputStreamReader(client_socket.getInputStream()));
    /**
     * Start talking to the other server.
     */
    //Read in data sent to us.
    String in_line = in.readLine();
    //Send back information to the client.
    out.println(send_info);
}//end try
catch (IOException e) {
    //A general problem was encountered while handling client communications.
}

A line of text sent by the client can be read in by calling in.readLine() demonstrated by the in_line string. To send a line of text to the client, call out.println(send_info) where send_info represents a string. If an error occurs during communication, an IOException will be thrown and caught by the above catchstatement.

Remember to add the following imports:

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.io.BufferedReader;