ReverseEchoer.java - A Simple Server Socket Application

JDK Tutorials - Herong's Tutorial Examples

∟ReverseEchoer.java - A Simple Server Socket Application

This section provides a tutorial example on how to write a network application, ReverseEchoer.java, that creates a server socket and listens for remote connection requests.

The following program called ReverseEchoer is a simple server socket application, which listens with a server socket for a connect request. Once connected, it reads lines of text from the remote application, reverses the text lines, and echoes back to the remote application:

/* ReverseEchoer.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.io.*;
import java.net.*;
public class ReverseEchoer {
   public static void main(String[] args) {
      try {
         ServerSocket s = new ServerSocket(8888);
         printServerSocketInfo(s);
         Socket c = s.accept();
         printSocketInfo(c);
         BufferedWriter w = new BufferedWriter(new OutputStreamWriter(
            c.getOutputStream()));
         BufferedReader r = new BufferedReader(new InputStreamReader(
            c.getInputStream()));
         String m = "Welcome to Reverse Echo Server."+
            " Please type in some words.";
         w.write(m,0,m.length());
         w.newLine();
         w.flush();
         while ((m=r.readLine())!= null) {
            if (m.equals(".")) break;
            char[] a = m.toCharArray();
            int n = a.length;
            for (int i=0; i<n/2; i++) {
               char t = a[i];
               a[i] = a[n-1-i];
               a[n-i-1] = t;
            }
            w.write(a,0,n);
            w.newLine();
            w.flush();
         }
         w.close();
         r.close();
         c.close();
         s.close();
      } catch (IOException e) {
         System.err.println(e.toString());
      }
   }
   private static void printSocketInfo(Socket s) {
      System.out.println("Remote address = "
         +s.getInetAddress().toString());
      System.out.println("Remote port = "
         +s.getPort());
      System.out.println("Local socket address = "
         +s.getLocalSocketAddress().toString());
      System.out.println("Local address = "
         +s.getLocalAddress().toString());
      System.out.println("Local port = "
         +s.getLocalPort());
   }
   private static void printServerSocketInfo(ServerSocket s) {
      System.out.println("Server socket address = "
         +s.getInetAddress().toString());
      System.out.println("Server socket port = "
         +s.getLocalPort());
   }
}

Note that:

The ServerSocket object, s, instantiated in this program is bound to the Internet address of the computer system on which it is running. The port number, 8888, in this binding is supplied as a parameter to constructor.
Socket.getInputStream() and Socket.getOutputStream() are used to create an input stream and output stream, so the application program can talk to the remote application through the socket object.
If a text line received only contains a single character '.', the communication link will be terminated by the Socket.close() method.

Run ReverseEchoer, you will get the following output on the console window:

herong> java ReverseEchoer.java

Server socket address = 0.0.0.0/0.0.0.0
Server socket port = 8888

This tells us that the program is listening at address 0.0.0.0 and port 8888.

To test ReverseEchoer, we can use the existing client program called "telnet" to initiate the communication request and talk to ReverseEchoer. Open another command window and type in the following command:

herong> telnet 127.0.0.1 8888

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Welcome to Reverse Echo Server. Please type in some words.

Immediately, you will see more output on the console window of ReverseEchoer:

Remote address = /127.0.0.1
Remote port = 1068
Local socket address = /127.0.0.1:8888
Local address = /127.0.0.1
Local port = 8888

This tells us that the server socket received a connection request, and a communication link has be established with the remote application, which the telnet program running on the same machine. The local address and remote address are the same, 127.0.0.1. The remote port is 1068, which is picked up by the telnet program.

In the telnet window, type in the following text:

Fish, I love you and respect you very much.
But I will kill you dead before this day ends.
.

The text will be reversed and returned back from the ReverseEchoer:

Welcome to Reverse Echo Server. Please type in some words.
.hcum yrev uoy tcepser dna uoy evol I ,hsiF
.sdne yad siht erofeb daed uoy llik lliw I tuB
.