Basic Sockets - TCP/IP Sockets in Java Practical Guide for Programmers - page 13

2. Communicate using the socket's I/O streams: A connected instance of Socket contains
an InputStream and OutputStream that can be used just like any other Java I/O stream (see
Chapter 3).

3. Close the connection using the close() method of Socket .

Our first TCP application, called TCPEchoClient.java , is a client that communicates with an
echo server using TCP. An echo server simply repeats whatever it receives back to the client.
The string to be echoed is provided as a command-line argument to our client. Many systems
include an echo server for debugging and testing purposes. To test if the standard echo server
is running, try telnetting to port 7 (the default echo port) on the server (e.g., at command line
“ telnet server.example.com 7” or use your basic telnet application).

TCPEchoClient.java

0 import java.net.*; // for Socket
1 import java.io.*;

// for IOException and Input/OutputStream

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3 public class TCPEchoClient {
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public static void main(String[] args) throws IOException {

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if ((args.length < 2) || (args.length > 3)) // Test for correct # of args

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throw new IllegalArgumentException("Parameter(s): <Server> <Word> [<Port>]");

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String server = args[0]; // Server name or IP address

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// Convert input String to bytes using the default character encoding

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byte[] byteBuffer = args[1].getBytes();

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int servPort = (args.length == 3) ? Integer.parseInt(args[2]) : 7;

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16 // Create socket that is connected to server on specified port

17 Socket socket = new Socket(server, servPort);

18 System.out.println("Connected to server...sending echo string");

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InputStream in = socket.getInputStream();

21 OutputStream out = socket.getOutputStream();

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23 out.write(byteBuffer); // Send the encoded string to the server

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// Receive the same string back from the server

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int totalBytesRcvd = 0; // Total bytes received so far

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int bytesRcvd; // Bytes received in last read

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while (totalBytesRcvd < byteBuffer.length) {

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if ((bytesRcvd = in.read(byteBuffer, totalBytesRcvd,

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byteBuffer.length − totalBytesRcvd)) == −1)

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throw new SocketException("Connection closed prematurely");