Download 13. Java Networking (D&D, Chapter 21)

Client/Server Distributed Systems
240-322, Semester 1, 2005-2006
11. Java Networking
 Objectives
– describe basic networking in Java
– Web page retrieval and sockets programming
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1
Contents
1. Networking Restrictions in Java
2. Basic Networking Classes
3. Sockets (Java Style)
4. Retrieving a Web Page
– 4 different approaches
5. A Stream Socket Client/Server
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continued
2
6.
7.
8.
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A Datagram Socket Client/Server
Networked Tic-Tac-Toe
More Information
3
1. Networking Restrictions in Java
 Many
browsers place networking restrictions
on applets
– usually communication is limited to being
between your machine and the one where the
applet came from
 There
are no restrictions on Java applications
or when using appletviewer
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Relaxing Security
 Applets
can be signed with trusted
certificates
– a browser can be configured to relax security
depending on an applet’s signature
– an advanced topic
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2. Basic Networking Classes
 The
J2SE documentation for java.net lists
over 30 classes. A few of the key ones:

InetAddress
– the class that represents IP addresses and
contains operations for manipulating them
 see
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section 2.1
continued
6

URL
– used to retrieve the Web page at the given URL

URLConnection
– also used to retrieve a Web page
– allows extra parameters to be sent to the URL
 e.g
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HTTP request headers
continued
7

Socket
– the client-side socket class for TCP
 ServerSocket
– the server-side socket class for TCP

DatagramSocket
– allows a client or server to send/receive UDP
packets
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2.1. Finding an IP Address
 Java’s InetAddress
class makes the
mapping between hostnames and IP
addresses much easier than in UNIX.
 For
details, look at the the documentation
for java.net.InetAddress
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WhatIP.java
import java.io.*;
import java.net.*;
public class WhatIP
{
public static void main(String args[])
throws IOException
{ InetAddress addr =
InetAddress.getByName(args[0]);
System.out.println("Inet address is "+ addr);
}
}
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Use
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Carried out from a DOS
prompt on my Windows machine.
11
3. Sockets (Java Style)
 1.
Stream Sockets
– the client/server connection exists for the entire
duration of the request and answer
 similar
to a telephone call
– a connection-oriented service
corresponds to TCP
– Java has separate classes for client and
server stream sockets (see section 5)
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 2.
Datagram Sockets
– the client/server send messages (packets,
datagrams) to each other
 similar
to the postal service
– a connectionless service
corresponds to UDP
– Java has classes for datagram sockets and
packets (see section 6).
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3.1. Pinging

ping is a DOS command, not Java.
uses the ICMP protocol's ECHO_REQUEST
datagram. The host’s answer is an ICMP
ECHO_RESPONSE.
ping
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 ICMP packets
can only be created via a
socket of type SOCK_RAW.
– but Java only supports SOCK_STREAM (TCP)
and SOCK_DGRAM (UDP) sockets
 Our
solution: use a TCP connection to the
daytime service to pretend to ping
– the daytime server listens at port 13
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3.2. DayPing.java
JJ2, p.677
import java.io.*;
import java.net.*;
public class DayPing {
public static void main(String args[])
throws Exception
{
if (args.length != 1) {
System.out.println(
"Usage: java DayPing <machine name>");
System.exit(0);
}
:
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String machine = args[0];
Socket so = new Socket(machine, 13);
// daytime server listens at port 13
BufferedReader br =
new BufferedReader(
new InputStreamReader(
so.getInputStream() ) );
System.out.println( machine +
" is alive at " + br.readLine());
so.close();
} // end of main()
} // end of DayPing class
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Notes

converts the socket
connection into a BufferedReader
DayPing.java
– this allows readLine() to be used for socket
input
 Using
layered streams to add I/O
functionality on top of sockets is a powerful
feature of Java.
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Use
All the examples were tested on
my Windows machine

C> javac DayPing.java

C> java DayPing calvin
calvin is alive at Sat May

7 13:46:06 2005
C> java DayPing fourdots
Exception in thread "main"
java.net.ConnectException: Connection refused:
no further information
at java.net.PlainSocketImpl.socketConnect
(Native Method)
at java.net.PlainSocketImpl.doConnect...
:
at ping.main(DayPing.java:23)
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continued
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

After a wait
C> java DayPing takasilla
Exception in thread "main"
java.net.UnknownHostException: takasilla
at java.net.PlainSocketImpl.
connect(Unknown Source)
:
at ping.main(DayPing.java:23)
After a long wait
C> java DayPing www.amazon.com
Exception in thread "main"
java.net.NoRouteToHostException: Operation
timed out: connect
at java.net.PlainSocketImpl.
socketConnect(Native Method)
:
at ping.main(DayPing.java:23)
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3.3. Why Doesn't DayPing Work?
 The
examples that don't work show three
different errors:
– ConnectException
– UnknownHostException
– NoRouteToHostException
 The
"unknown host" exception is caused by
an incorrect IP address.
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 The
"connection" exception probably means
that the server is unavailable (switched off)
– this can be confirmed for a TCP service by
trying to connect to it with telnet:
$ telnet fourdots 13
Trying 172.30.255.4...
telnet: Unable to connect to remote host:
Connection refused
$
executed from a different
machine (fivedots)
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 The
"no route to host" exception usually
means that there is a firewall preventing the
connection from being made.

telnet
response:
A long wait,
and then...
$ telnet www.amazon.com 13
Trying 207.171.163.90...
telnet: Unable to connect to remote host:
Connection timed out
$
tried on (fivedots)
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3.4. Firewalls at PSU
(simplified)
University
Departmental firewall
(no external socket creation;
URLs allowed)
calvin
takasila
ratree
Univ. firewall
(no external socket creation;
URLs only through
the ‘cache’ proxy)
fivedots
CoE Department
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The Internet
8080
cache
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 Ordinary
users (i.e. students) cannot write
socket-based programs that communicate
outside PSU
– this is true in any language (Java, C, etc.)
 But
programs can retrieve Web pages (and
other things) using the Java URL class
– the URL request must go through PSU's cache
machine
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URLs and Sockets
 From
your data communications course,
you may recall that the Web protocol,
HTTP, is built on top of TCP/IP.
 Java's
URL class uses stream sockets
(TCP/IP) for its implementation
– so why does it get past cache?
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continued
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
cache accepts socket links to valid Web
servers, but also checks that the HTTP GET
(or POST) message includes additional
valid HTTP headers.
– all of this HTTP stuff is carried out by the URL
class for us
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4. Retrieving a Web Page
 Five
ways of obtaining a Web page:
– 1. use a socket, and send a GET message to the
server
 see
GetSocketPage.java
– 2. use a URL object, and read the page via a
stream
 see
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GetURLPage.java
continued
28
– 3. use a URL object, and display the page in a
browser
 see
showPage.java
– 4. display a URL in an JEditorPane
 see
ShowURLPage.java
– 5. use a HTTP URL connection, and send a
GET message
 see
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Java’s networking tutorial
29
4.1. Sockets and GET

GetSocketPage.java
retrieves the page:
http://<host name>/index.html
e.g.
http://fivedots.coe.psu.ac.th/index.html
 It
prints the text of the page to stdout.
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 It
opens a socket at port 80 for the host,
which is the usually place where the Web
server is listening.
 It
sends the HTTP GET message:
GET /index.html
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Diagram
GET /index.html
80
GetSocketPage
client
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Web page
(as text)
Web
server
host
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GetSocketPage.java
import java.io.*;
import java.net.*;
public class GetSocketPage
{
public static void main(String args[])
throws IOException
{
Socket sock = new Socket(args[0],80);
:
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BufferedReader dis =
new BufferedReader(
new InputStreamReader(
sock.getInputStream() ));
PrintStream ps =
new PrintStream(
sock.getOutputStream() );
ps.println("GET /index.html");
String line;
while ((line = dis.readLine()) != null)
System.out.println(line);
sock.close();
}
} // end of GetSocketPage.java
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Notes

converts the socket
connection into a BufferedReader for input,
and a PrintStream for output
GetSocketPage.java
– uses readLine()for socket input
– uses println() for socket output
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Use
C> javac GetSocketPage.java
text of Web page
printed to stdout
C> java GetSocketPage fivedots
<html>
<head>
<title>Un title page</title>
</head>
<meta http-equiv="Content-Type"
content="text/html; charset=windows-874">
<style type="text/css">
:
C>
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But...
C> java GetSocketPage www.amazon.com
Exception in thread "main"
java.net.ConnectException:
Connection timed out: connect
at java.net.PlainSocketImpl.
socketConnect(Native Method)
:
at GetSocketPage.main(GetSocketPage.java:18)
The firewall around PSU
prevents Web server
access by sockets.
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 cache
rejected the GET message to the
external site since the message didn't
include additional HTTP headers.
 These
can be supplied by us, but it's easier
to use the URL class.
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4.2. URL Object

avoids the firewall
problem with sockets by using a URL object.
GetURLPage.java
 A URL
object allows a Web page to be
retrieved as a stream of text
– our program prints the text to stdout.
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GetURLPage.java
import java.net.*;
import java.io.*;
public class GetURLPage {
public static void main(String args[])
{
try {
URL url = new URL(args[0]);
BufferedReader dis =
new BufferedReader(
new InputStreamReader(
url.openStream() ));
:
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String line;
while ( (line = dis.readLine()) != null )
System.out.println(line);
dis.close();
}
catch (Exception e)
{ System.out.println(e); }
}
} // end of GetURLPage.java
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Notes
 A stream
for the URL object is obtained
using openStream()
– after that the same input stream layering
technique is used as in GetSocketPage.java
– there is no need to send a GET message
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Use


C> javac GetURLPage.java
C> java GetURLPage http://www.amazon.co.uk
java.net.ConnectException:
Connection timed out: connect
after a
long wait
C> java -DproxySet=true
-DproxyHost=cache.psu.ac.th
-DproxyPort=8080
GetURLPage http://www.amazon.com/
<html>
<head>
<title>Amazon Page</title>
</head>
<body bgcolor=#ffffff test=#000000>
<H1>
:
C>jnet/11
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
typed on
one line
43
 Ordinary
users can access outside PSU by
using URLs, but they must route their
requests via PSU cache machine.
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Batch Files for Long Commands

GetURLPage.bat contains:
@echo off
echo Executing GetURLPage...
java -DproxySet=true
-DproxyHost=cache.psu.ac.th
-DproxyPort=8080 GetURLPage %1
 Use:
c> GetURLPage http://www.amazon.co.uk
: // same output as last slide
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Proxy Username/Password
 A further
level of firewall security is to
require the user to enter a username and
password
– called proxy authorization
 Java
has network support for authorization
– it allows a username and password to be sent by
a client program to the firewall
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 A good
tutorial:
– “Jump a Proxy/Firewall and Live to Tell About it”
http://www.devx.com/upload/free/features/
javapro/2000/03mar00/te0003/te0003.asp
 Slightly
modified code is in
GetThroughProxy.java at:
http://fivedots.coe.psu.ac.th/
Software.coe/Cliserv/
Code%20Examples/Java%20Code/
Basic%20Networking/
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4.3. Passing a URL to a Browser
 If
the Java code is in an applet, then the
downloaded URL can be displayed in the
browser.

displays a user-specified
Web page in the (Opera) browser, using the
ShowPage.java applet.
showPage.html
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Usage
The dialog box appears in front
of the browser window.
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Loaded
Page:
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showPage.html
<html><head>
<title>Web Page Loading Applet</title></head>
<body>
<h1>Web Page Loading Applet</h1>
<P>Applet is placed here...
<applet code="ShowPage.class"
width=300 height=50>
</applet>
</P>
</body></html>
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showPage.java
import
import
import
import
java.net.*;
javax.swing.*;
java.applet.AppletContext;
javax.swing.JOptionPane;
public class ShowPage extends JApplet
{
public void init()
{
try {
String urlString =
JOptionPane.showInputDialog(
"Enter a URL:");
URL url = new URL(urlString);
:
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AppletContext browser = getAppletContext();
browser.showDocument(url);
}
catch (Exception e)
{ System.out.println(e); }
} // end of init()
} // end of ShowPage
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Notes
 The
download only works because the
browser is set up to work through cache.
 There
are no Java security restrictions on a
applet passing a URL to a browser to be
displayed.
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continued
54
 A URL is
downloaded with showDocument(),
which must be called in the applet’s
environment (i.e. the browser):
– AppletContext browser =
getAppletContext();
browser.showDocument(url);
 The
URL is automatically loaded into the
browser, replacing showPage.html.
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55
 There
is a two-argument version of
showDocument() that can load URLs into
frames.
 D&D
contains a more complex example
(SiteSelector.java, ch. 21) which allows
the user to select a URL from a list.
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4.4. Displaying a URL with JEditorPane
 The
Swing GUI includes JEditorPane
which allows 'structured' text to be
displayed and edited.
– offers basic support for plain text, RTF, and
HTML
– see javax.swing.JEditorPane
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57
 A JEditorPane
can be filled by calling
setPage() with a URL string argument.

also has support for capturing
hyperlink clicks as events.
JEditorPane
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58
 With
these features, it is possible to write a
basic Web browser!

has a text field for the
user to enter a URL
ShowURLPage.java
– the resulting downloaded page is displayed in a
JEditorPane
– if a link is clicked inside the pane, then its page
will be loaded automatically
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continued
59
I clicked on
“School of Advanced...”
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ShowURLPage’s Event Model
X
GUI
enter
contents
^
action
events
hyperlink
events
Code
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hyperlinkUpdate() {...}
actionPerformed() {...}
61
ShowURLPage.java
import
import
import
import
import
import
java.awt.*;
java.awt.event.*;
java.net.*;
java.io.*;
javax.swing.*;
javax.swing.event.*;
public class ShowURLPage extends JFrame
{
private JTextField enter;
private JEditorPane contents;
:
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public ShowURLPage()
{
super("Simple Web Browser");
Container c = getContentPane();
enter = new JTextField("Enter file URL here");
enter.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e )
{ getPage(e.getActionCommand());}
});
c.add(enter, BorderLayout.NORTH);
:
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contents = new JEditorPane();
contents.setEditable(false);
contents.addHyperlinkListener(
new HyperlinkListener() {
public void hyperlinkUpdate(
HyperlinkEvent e)
{
if(e.getEventType() ==
HyperlinkEvent.EventType.ACTIVATED)
getPage(e.getURL().toString());
}
});
:
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c.add( new JScrollPane(contents),
BorderLayout.CENTER);
setDefaultCloseOperation(
JFrame.EXIT_ON_CLOSE);
setSize(400, 300);
setVisible(true);
} // end of ShowURLPage()
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private void getPage(String location)
{
setCursor(Cursor.getPredefinedCursor(
Cursor.WAIT_CURSOR));
try {
contents.setPage(location);
enter.setText(location);
}
catch (IOException io) {
JOptionPane.showMessageDialog(this,
"Error retrieving specified URL",
"Bad URL",
JOptionPane.ERROR_MESSAGE);
}
setCursor( Cursor.getPredefinedCursor(
Cursor.DEFAULT_CURSOR) );
} // end of getPage()
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public static void main(String args[])
{ new ShowURLPage(); }
} // end of ShowURLPage class
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Use
C> javac ShowURLPage.java
C> java ShowURLPage
// A request for http://www.cs.ait.ac.th/~ad
// causes a “Bad URL” dialog box to appear
C> java -DproxySet=true
-DproxyHost=cache.psu.ac.th
-DproxyPort=8080 ShowURLPage
Typed all on one line.
The window shown on the left of slide 60 appears.
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Notes
 The JEditorPane
must be set uneditable so
that hyperlinks events can be captured.
events include: ENTERED, EXITED,
and ACTIVATED.
 Hyperlink
– see javax.swing.event.HyperlinkEvent
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continued
69

getPage()
calls setPage() to display a URL
in the JEditorPane
– the call is surrounded by changes to the cursor
icon so things look busy

getPage()
is called in two possible ways:
– as a response to pressing enter in the textfield
– as a response to a link being clicked
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5. A Stream Socket Client/Server
AlienClient.java
AlienServer1.java
Initial Internet
4444 Server socket
connection
socket
input and
output
streams
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socket
for client
comms.
input and
output
streams
the new socket is
made with accept()
71
 The
server creates a new socket when a
client contacts it
– client/server communication via the new socket
– this frees up the main server socket to receive
connections from other clients
– AlienServer1.java is an iterative server
 it
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can only process one client at a time
continued
72
 Our

client and server both run on localhost.
AlienServer1
actions:
– wait for a client connection
– after a connection, wait for a message from the
client
– respond depending on the content of the
message
– close the client connection, and loop, waiting
for another client
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Diagram for Client Processing
AlienClient.java
AlienServer1.java
1. send message
4. send answer
2. analyse message
3. print message locally
5. close socket
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AlienServer1.java
import java.io.*;
import java.net.*;
public class AlienServer1 {
public static void main(String args[])
throws IOException
{
Socket sock;
BufferedReader in;
PrintStream out;
ServerSocket servsock =
new ServerSocket(4444);
:
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while (true) {
// wait for the next client connection
sock = servsock.accept();
// Get I/O streams from the socket
out = new PrintStream(
sock.getOutputStream() );
in = new BufferedReader(
new InputStreamReader(
sock.getInputStream() ) );
:
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// get client message and respond
String msg = in.readLine();
System.out.println(" Received: " + msg);
if (msg.indexOf("hello") > -1) {
System.out.println(" Friendly contact made");
out.println("Welcome friend");
}
else {
System.out.println(" Probably an alien");
out.println("ET go home");
}
out.flush();
// Close this connection,
// (not the overall server socket)
sock.close();
}
} // of main()
}// end of AlienServer1
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The client: AlienClient.java

AlienClient.java’s
–
–
–
–
actions:
open a connection with the server
send a message taken from the command line
receive and print the server’s response
close
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AlienClient.java
import java.io.*;
import java.net.*;
public class AlienClient {
public static void main(String args[])
throws IOException {
Socket sock = new Socket("localhost", 4444);
// Get I/O streams from the socket
BufferedReader br =
new BufferedReader(
new InputStreamReader(
sock.getInputStream()) );
PrintStream ps = new PrintStream(
sock.getOutputStream() );
:
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// Send a message from the command line
ps.println(args[0]);
ps.flush();
// Read server’s response
String line = br.readLine();
System.out.println(
"Got this from server:\n
" + line);
sock.close();
} // end of main()
} // end of AlienClient class
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Use
Client:
Server:
I used two separate DOS windows
on the same machine.
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Telnet Client (from fivedots)
I typed this
Server:
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A More Complex Example
 D&D
gives a larger stream socket
client/server example (ch. 21):
– object input and output streams are used
– the sequence of communication is more complex
 the
client maintains a long-lived link with the server
until it sends a “TERMINATE” string
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5.1. A Concurrent Server

uses threads to handle
multiple clients concurrently.
AlienServer2.java
 Three
main advantages:
– the server code is simpler since server
processing is in a separate class called Worker
– clients do not have to wait
– the server is more scaleable
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AlienServer2 Visually
AlienServer2
AlienClient clients
4444
Worker
threads
each Worker thread can
deal with multiple messages
from its client
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5.2. Java Threads (Briefly)
 There
are two ways to create a thread:
– extend the Thread class
– write a class to implement the Runnable
interface
use the first approach since Worker
does not need to extend any other classes.
 We
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 A class
extended from Thread must define
the run() method:
class Worker extends Thread {
public void run()
{ ... }
}
 A threaded
start():
object is started by calling
Worker w = new Worker(...):
w.start();
 The
thread will call run() itself from
within start().
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5.3. AlienServer2.java
import java.io.*;
import java.net.*;
public class Worker extends Thread
{ private Socket sock;
private int id;
public Worker(Socket s, int c)
{ sock = s;
id = c;
}
:
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public void run()
{
// Get I/O streams from the socket
try {
PrintStream out = new PrintStream(
sock.getOutputStream() );
BufferedReader in =
new BufferedReader(
new InputStreamReader(
sock.getInputStream() ));
:
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The worker contains much the same
client processing code as the iterative
server.
// get client messages and respond
String msg;
while ((msg = in.readLine()) != null) {
System.out.println(" Worker "+id+
" received: "+ msg);
if (msg.indexOf("hello") > -1)
out.println("Welcome friend")
else
out.println("ET go home");
out.flush();
}
:
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// Close this connection,
// (not the overall server socket)
System.out.println(" Worker " + id +
" finished");
sock.close();
}
catch(IOException ioe)
{System.out.println(ioe); }
} // end of run()
} // end of Worker class
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public class AlienServer2
{
public static void main(String args[])
throws IOException
{ Socket sock;
ServerSocket servsock =
new ServerSocket(4444, 6);
while (true) {
// wait for the next client connection
sock = servsock.accept();
new Worker(sock).start();
}
} // of main()
} // of AlienServer2 class
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Use
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Server:
93
Client on fivedots
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Two Local Clients (one closed)
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6. A Datagram Socket Client/Server
UDPClient
Datagram
socket
UDPServer
packets
any
port
msg
5000
Datagram
socket
echo
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6.1. Server Event Model
X
^
display
1) receive from
client
GUI
Code
waitForPackets() {...}
packet
packet
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2) echo back
to client
97
6.2. UDPServer.java
import
import
import
import
import
java.io.*;
java.net.*;
java.awt.*;
java.awt.event.*;
javax.swing.*;
public class UDPServer extends JFrame
{
private JTextArea display;
:
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public UDPServer()
{ super("UDP Server");
display = new JTextArea();
getContentPane().add(
new JScrollPane( display),
BorderLayout.CENTER );
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setSize(400, 300);
show();
DatagramSocket socket = null;
try {
socket = new DatagramSocket( 5000 );
}
catch(SocketException se) {
se.printStackTrace();
System.exit(1);
}
waitForPackets(socket);
} // end of UDPServer()
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private void waitForPackets(DatagramSocket socket)
{
DatagramPacket sendPacket, receivePacket;
while (true) {
try {
// set up packet data structure
byte data[] = new byte[100];
receivePacket =
new DatagramPacket(data, data.length);
// wait for packet from client
socket.receive(receivePacket);
:
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// process client packet
display.append("\nPacket received:" +
"\nFrom host: "+receivePacket.getAddress()+
"\nHost port: " + receivePacket.getPort() +
"\nLength: " + receivePacket.getLength() +
"\nContaining:\n\t" +
new String(receivePacket.getData(), 0,
receivePacket.getLength()));
:
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// echo packet info back to client
display.append(
"\n\nEcho data to client...");
sendPacket =
new DatagramPacket(
receivePacket.getData(),
receivePacket.getLength(),
receivePacket.getAddress(),
receivePacket.getPort() );
socket.send( sendPacket );
display.append("Packet sent\n");
display.setCaretPosition(
display.getText().length() );
}
:
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catch(IOException io) {
display.append(io.toString() + "\n");
io.printStackTrace();
}
}
} // end of waitForPackets{}
public static void main( String args[] )
{
new UDPServer(); }
} // end of UDPServer
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Server Notes
 The
server creates one datagram socket at
port 5000:
socket = new DatagramSocket(5000);
 Datagram
socket creation must be inside a
try/catch block.
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 The
server loops inside waitForPackets()
– creates an empty packet (100 bytes long)
– blocks inside receive()
 When receive()
returns, the packet
contains data and:
– the Internet address of the sender
– the port number of the sender
– the actual length of the data
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 The
packet information can be accessed via
methods. e.g.:
receivePacket.getPort();
receivePacket.getData();

receive()
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must be in a try/catch block.
continued
106
 The
server echoes the datagram back to the
client at the address and port obtained from
the client’s message:
sendPacket = new DatagramPacket(
receivePacket.getData(),
receivePacket.getLength(),
receivePacket.getAddress(),
receivePacket.getPort() );
socket.send(sendPacket);

send()
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must be in a try/catch block.
107
6.3. Client Event Model
X
GUI
enter
^
display
Code
1) send to server
actionPerformed() {...}
waitForPackets() {...}
2) receive echo
from sender
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action
events
packet
packet
108
6.4. UDPClient.java
import
import
import
import
import
java.io.*;
java.net.*;
java.awt.*;
java.awt.event.*;
javax.swing.*;
public class UDPClient extends JFrame
implements ActionListener
{
private JTextField enter;
private JTextArea display;
private DatagramSocket socket;
:
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public UDPClient()
{
super("UDP Client");
enter = new JTextField("Type message here");
enter.addActionListener(this);
getContentPane().add(enter,BorderLayout.NORTH);
display = new JTextArea();
getContentPane().add(new JScrollPane(display),
BorderLayout.CENTER);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setSize(400, 300);
setVisible(true);
:
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try {
socket = new DatagramSocket();
}
catch(SocketException se) {
se.printStackTrace();
System.exit(1);
}
waitForPackets();
} // end of UDPClient()
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private void waitForPackets()
{
DatagramPacket receivePacket;
while (true) {
try {
// set up packet data structure
byte data[] = new byte[100];
receivePacket =
new DatagramPacket(data, data.length);
// wait for packet from server
socket.receive(receivePacket);
:
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// process packet from server
display.append("\nPacket received:" +
"\nFrom host: "+receivePacket.getAddress()+
"\nHost port: " + receivePacket.getPort() +
"\nLength: " + receivePacket.getLength() +
"\nContaining:\n\t" +
new String( receivePacket.getData(), 0,
receivePacket.getLength()));
display.setCaretPosition(
display.getText().length() );
}
catch(IOException ex) {
display.append(ex.toString()+"\n" );
ex.printStackTrace();
}
}
} // end of waitForPackets()
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public void actionPerformed(ActionEvent e)
{
DatagramPacket sendPacket;
try {
display.append(
"\nSending packet containing: " +
e.getActionCommand() + "\n" );
String s = e.getActionCommand();
byte data[] = s.getBytes();
sendPacket = new DatagramPacket(
data, data.length,
InetAddress.getLocalHost(), 5000);
socket.send(sendPacket);
:
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display.append("Packet sent\n");
display.setCaretPosition(
display.getText().length() );
}
catch (IOException ex) {
display.append(ex.toString()+"\n");
ex.printStackTrace();
}
}
// end of actionPerformed()
public static void main( String args[] )
{
new UDPClient(); }
} // end of UDPClient
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Client Notes
 The
client gets the data for each packet
from the user entering a string into a
TextField.
 The
client creates a datagram socket with no
specified port number:
socket = new DatagramSocket();
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 In actionPerformed(),
the user’s input is
converted into a packet, and sent to the
server:
sendPacket = new DatagramPacket(
data, data.length(),
InetAddress.getLocalHost(), 5000);
socket.send(sendPacket);
 The
code assumes that the server is on the
same machine as the client, and attached to
port 5000.
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continued
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 The
client receives the echoed reply in
waitForPackets():
– the loop blocks inside receive() until a
packet arrives from the server
– the packet details are displayed in the
JTextArea
 The
blocked waitForPackets() does not
stop the user from entering more strings for
the client to send out
– the GUI always runs in a separate thread
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Tested by starting the client and server in
two separate DOS windows.
6.5. Use
1. message sent to server
3. Echoed message
received from the server
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2. Client message
received and echoed
continued
119
Another message sent and echoed back:
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7. Networked Tic-Tac-Toe
D&D Section 21.8, p.1031
Server
thread
thread
synchronized
validMove()
5000
data
Client:
Player ‘X’
Client:
Player ‘O’
thread
thread
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data
121
Output
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p.1041-1042
122
8. More Information
 Chapter 21, Deitel & Deitel (D&D)
http://java.coe.psu.ac.th/ForMember/
Books.html#Network
 Killer
Game Programming in Java
Chapter 29, Network Basics
http://fivedots.coe.psu.ac.th/~ad/jg/ch18/
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 Core
Java 2, Vol II - Advanced Features
Cay S. Horstmann & Gary Cornell
Sun Microsystems Press, 2001,
chapter 3
– multithreaded server, sending e-mail,
URLConnection, HTTP POST
– http://java.coe.psu.ac.th/ForMember/
Books.html#Network
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 Aj.
Somchai's Java Site
– Network and Distributed Programming Books
http://java.coe.psu.ac.th/ForMember/
Books.html#Network
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