일요일, 12월 22
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#024 Java Networking

by nobaksan

Lab Exercises

 

Exercise 1: Writing Client and Server

In this exercise, you are going to build and run a simple Hello server and client using Networking API.

  1. Build and run the server side code
  2. Build and run the client side code

(1.1) Build and run the server side code

0. Start NetBeans IDE if you have not done so yet.
1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in NetworkingServer as project name.
  • For Create Main Class field, type in NetworkingServer.  (Figure-1.10 below)
  • Click Finish.


Figure-1.10: Create a new project

  • Observe that NetworkingServer project appears and IDE generated NetworkingServer.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated NetworkingServer.java as shown in Code-1.11 below.  Study the code by paying special attention to the bold fonted parts.

/* SERVER – may enhance to work for multiple clients */
import java.net.*;
import java.io.*;
import java.util.*;

public class NetworkingServer {

public static void main(String [] args) {

ServerSocket server = null;
Socket client;

// Default port number we are going to use
int portnumber = 1234;
if (args.length >= 1){
portnumber = Integer.parseInt(args[0]);
}

// Create Server side socket
try {
server = new ServerSocket(portnumber);
} catch (IOException ie) {
System.out.println(“Cannot open socket.” + ie);
System.exit(1);
}
System.out.println(“ServerSocket is created ” + server);

// Wait for the data from the client and reply
while(true) {

try {

// Listens for a connection to be made to
// this socket and accepts it. The method blocks until
// a connection is made
System.out.println(“Waiting for connect request…”);
client = server.accept();

System.out.println(“Connect request is accepted…”);
String clientHost = client.getInetAddress().getHostAddress();
int clientPort = client.getPort();
System.out.println(“Client host = ” + clientHost + ” Client port = ” + clientPort);

// Read data from the client
InputStream clientIn = client.getInputStream();
BufferedReader br = new BufferedReader(new
InputStreamReader(clientIn));
String msgFromClient = br.readLine();
System.out.println(“Message received from client = ” + msgFromClient);

// Send response to the client
if (msgFromClient != null && !msgFromClient.equalsIgnoreCase(“bye”)) {
OutputStream clientOut = client.getOutputStream();
PrintWriter pw = new PrintWriter(clientOut, true);
String ansMsg = “Hello, ” + msgFromClient;
pw.println(ansMsg);
}

// Close sockets
if (msgFromClient != null && msgFromClient.equalsIgnoreCase(“bye”)) {
server.close();
client.close();
break;
}

} catch (IOException ie) {
}
}
}
}

Code-1.11: NetworkingServer.java

3. Build and run the project

  • Right click NetworkingServer project and select Run.
  • Observe that the server is waiting for a connection request from a client in the Output window. (Figure-1.13 below)
ServerSocket is created ServerSocket[addr=0.0.0.0/0.0.0.0,port=0,localport=1234]
Waiting for connect request…

Figure-1.13: Result of running NetworkingSever application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javanetworking/samples/NetworkingServer.  You can just open it and run it.

(1.2) Build and run the client side code

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in NetworkingClient as project name.
  • For Create Main Class field, type in NetworkingClient.
  • Click Finish.

  • Observe that NetworkingClient project appears and IDE generated NetworkingClient.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated NetworkingClient.java as shown in Code-1.21 below.  Study the code by paying special attention to the bold fonted parts.

/* CLIENT */
import java.io.*;
import java.net.*;

public class NetworkingClient {

public static void main(String args[]) {

Socket client = null;

// Default port number we are going to use
int portnumber = 1234;
if (args.length >= 1){
portnumber = Integer.parseInt(args[0]);
}

for (int i=0; i <10; i++) {
try {
String msg = “”;

// Create a client socket
client = new Socket(InetAddress.getLocalHost(), portnumber);
System.out.println(“Client socket is created ” + client);

// Create an output stream of the client socket
OutputStream clientOut = client.getOutputStream();
PrintWriter pw = new PrintWriter(clientOut, true);

// Create an input stream of the client socket
InputStream clientIn = client.getInputStream();
BufferedReader br = new BufferedReader(new
InputStreamReader(clientIn));

// Create BufferedReader for a standard input
BufferedReader stdIn = new BufferedReader(new
InputStreamReader(System.in));

System.out.println(“Enter your name. Type Bye to exit. “);

// Read data from standard input device and write it
// to the output stream of the client socket.
msg = stdIn.readLine().trim();
pw.println(msg);

// Read data from the input stream of the client socket.
System.out.println(“Message returned from the server = ” + br.readLine());

pw.close();
br.close();
client.close();

// Stop the operation
if (msg.equalsIgnoreCase(“Bye”)) {
break;
}

} catch (IOException ie) {
System.out.println(“I/O error ” + ie);
}
}
}
}

Code-1.21: NetworkingClient.java

3. Build and run the project

  • Right click NetworkingClient project and select Run.
  • Observe the client is prompting you to enter your name. (Figure-1.23 below)
Client socket is created Socket[addr=Passion2/192.168.2.4,port=1234,localport=1775]
Enter your name. Type Bye to exit.

Figure-1.23: Waiting for the user to enter name

Trouble-shooting:  If you see the following exception, it is highly likely that either you have not started the server or if you started the server, the firewall on your system blocks the incoming connection request.

I/O error java.net.ConnectException: Connection refused: connect
I/O error java.net.ConnectException: Connection refused: connect
I/O error java.net.ConnectException: Connection refused: connect
I/O error java.net.ConnectException: Connection refused: connect

Figure-1.24: Error condition

Solution: Make sure the server is run first.  Also make sure the firewall on your system is turned off.

  • Enter your name into the Input field, like Sang Shin in this example, and press Enter key. (Figure-1.25 below)
  • Observe that the server responds back with “Hello, Sang Shin”.


Figure-1.25: Enter your name

  • Enter a few more names and observe that the server kept sending back responses.
  • Type Bye.

4. Observe the server side.

  • Click NetworkingServer (run) tab to see the Output window of the server side.
  • Observe that the server received a message, Sang Shin in this example.  (Figure-1.27 and Figure-1.28 below.)
ServerSocket is created ServerSocket[addr=0.0.0.0/0.0.0.0,port=0,localport=1234]
Waiting for connect request…
Connect request is accepted…
Client host = 192.168.2.4 Client port = 1775
Message received from client = Sang Shin
Waiting for connect request…
Connect request is accepted…
Client host = 192.168.2.4 Client port = 1777

Figure-1.27: Client connection request is accepted and message is received


Figure-1.28: Client connection request is accepted and message is received

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javanetworking/samples/NeworkingClient.  You can just open it and run it.

Summary

In this exercise, you learned how to build and run a simple Hello server and client using Networking API.

Exercise 2: Writing Multicast Client and Server

In this exercise, you are going to build and run a simple Hello server and client using Multicast.

  1. Build and run the multicast server side code
  2. Build and run the multicast client side code

(2.1) Build and run the server side code

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in MulticastChatServer as project name.
  • For Create Main Class field, type in MulticastChatServer.

  • Observe that MulticastChatServer project appears and IDE generated MulticastChatServer.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated MulticastChatServer.java as shown in Code-2.11 below.  Study the code by paying special attention to the bold fonted parts.

import java.net.*;

public class MulticastChatServer {

public static void main(String args[])
throws Exception {

// Default port number we are going to use
int portnumber = 5000;
if (args.length >= 1) {
portnumber = Integer.parseInt(args[0]);
}

// Create a MulticastSocket
MulticastSocket serverMulticastSocket =
new MulticastSocket(portnumber);
System.out.println(“MulticastSocket is created at port ” + portnumber);

// Determine the IP address of a host, given the host name
InetAddress group =
InetAddress.getByName(“225.4.5.6”);

// getByName- returns IP address of given host
serverMulticastSocket.joinGroup(group);
System.out.println(“joinGroup method is called…”);
boolean infinite = true;

// Continually receives data and prints them
while (infinite) {
byte buf[] = new byte[1024];
DatagramPacket data =
new DatagramPacket(buf, buf.length);
serverMulticastSocket.receive(data);
String msg =
new String(data.getData()).trim();
System.out.println(“Message received from client = ” + msg);
}
serverMulticastSocket.close();
}
}

Code-2.11: MulticastChatServer.java

3. Build and run the project

  • Right click MulticastChatServer project and select Run.
  • Observe that the server is waiting for a datagram message from a client in the Output window. (Figure-2.12 below)
MulticastSocket is created at port 5000
joinGroup method is called…

Figure-2.12: Waiting for a message from a client

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javanetworking/samples/MulticastChatServer.  You can just open it and run it.

(2.2) Build and run the client side code

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in MulticastChatClient as project name.
  • For Create Main Class field, type in MulticastChatClient.
  • Click Finish.

  • Observe that MulticastChatClient project appears and IDE generated MulticastChatClient.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated MulticastChatClient.java as shown in Code-2.21 below.  Study the code by paying special attention to the bold fonted parts.

import java.net.*;
import java.io.*;

public class MulticastChatClient {

public static void main(String args[])
throws Exception {

// Default port number we are going to use
int portnumber = 5000;
if (args.length >= 1) {
portnumber = Integer.parseInt(args[0]);
}

// Create a MulticastSocket
MulticastSocket chatMulticastSocket = new MulticastSocket(portnumber);

// Determine the IP address of a host, given the host name
InetAddress group =
InetAddress.getByName(“225.4.5.6”);

// Joins a multicast group
chatMulticastSocket.joinGroup(group);

// Prompt a user to enter a message
String msg = “”;
System.out.println(“Type a message for the server:”);
BufferedReader br =
new BufferedReader(new InputStreamReader(System.in));
msg = br.readLine();

// Send the message to Multicast address
DatagramPacket data = new DatagramPacket(msg.getBytes(), 0,
msg.length(), group, portnumber);
chatMulticastSocket.send(data);

// Close the socket
chatMulticastSocket.close();
}
}

Code-2.21: MulticastChatClient.java

3. Build and run the project

  • Right click MulticastChatClient project and select Run.
  • Observe the client is prompting you to enter a message. (Figure-2.23 below)
Type a message for the server:

Figure-2.23: Waiting for the user to enter a message

  • Enter a message, like Hello Server in this example, and press Enter key. (Figure-2.24 below)


Figure-2.24: Enter a message

4. Observe the server side.

  • Click MulticastChatServer (run) tab to see the Output window of the server side.
  • Observe that the server received a multicast message, Hello, World! in this example.  (Figure-2.25 below)
MulticastSocket is created at port 5000
joinGroup method is called…
Message received from client = Hello Server

Figure-2.25: Message is received from a client

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javanetworking/samples/MulticastChatClient.  You can just open it and run it.

Summary

In this exercise, you learned how to build and run a simple Hello server and client using Multicast.

Homework exercise (for people who are taking Sang Shin’s “Java Programming online course”)

 

There is no homework for this topic. (Nov. 9th, 2007)

 

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