∟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) 2002 by Dr. Herong Yang
*/
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 socker address = "
+s.getInetAddress().toString());
System.out.println("Server socker 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 applicaton 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:
Server socker address = 0.0.0.0/0.0.0.0
Server socker 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 communucation request and talk to ReverseEchoer. Open another
command window and type in the following command:
telnet 127.0.0.1 8888
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
.
Note that the telnet program is not displaying text you typed in.
It only displays the text received from ReverseEchoer program.
