Download EE2E1 Lecture 10

EE2E1. JAVA Programming
Lecture 10
Applets
Contents
Introduction
 Web-servers and clients
 A simple example “Hello World!” applet
 Converting an application to an applet
 Browsers and plug-ins
 Applet attributes
 Passing parameters to applets
 Applet security
 JAR files

Introduction



Java applets are programs written in Java which
are run by a web browser such as Netscape or
Internet Explorer
 The applet code is downloaded from its host
computer each time it is run
 Applets are used for adding extra
functionality to web pages
As we shall see, converting a normal Java
application program to an applet is easy
 The main problems stem from the browser
incompatibility and security issues
Java applets are not the be all and end all of
Java programming but they can be useful
Web-servers and clients

When we run a web browser to access the internet,
the web browser program runs on a client
computer


Usually the PC from which we are working
The .html web page we are accessing is on the
Web Server computer

It is on this machine that the code (the Java
.class file) for the Java applet resides which the
.html file imports
.html web page
MyApplet.class
Client running Netscape
Web server
Example – A “Hello World!” applet

The following example applet draws a “Hello
World” string



It is essentially a very simple Swing program
However, there is no main program


http://www.eee.bham.ac.uk/spannm/Java%20St
uff/HelloWorldApplet/HelloWorldApplet.html
Instead there is an init() method within a subclass of JApplet
We also need a .html file to tell the browser to
download and run the applet
class HelloWorldPanel extends JPanel
{ public void paintComponent(Graphics g)
{
super.paintComponent(g);
Font f=new Font("SansSerif",Font.BOLD,36);
g.setFont(f);
g.drawString("Hello World!", 50, 120);
}
}
public class HelloWorldApplet extends JApplet
{
public void init()
{
Container contentPane = getContentPane();
contentPane.add(new HelloWorldPanel());
}
}

The only other thing required is a .html which tells
the browser to load a particular Java class file
containing the applet code (plus the size of the
window in which to run the applet)
<APPLET CODE = "HelloWorldApplet.class"
WIDTH = 300 HEIGHT = 300 >
</APPLET>

This simple .html file is fine if we don’t need a
Java plug-in (see) later with our browser
 OK for Netscape 6.2 and later versions
 .html file more cumbersome for use with
earlier versions of Netscape
Converting an application to an applet

An applet is essentially a graphics container
(rather like the JFrame) class

The JApplet class replaces JFrame as the
outer container
public class HelloWorldApplet extends JApplet

JApplet is a sub-class of Container
as is JFrame and contains many of
the same methods


An applet has no main() method

This is replaced by the init() method and is
called by the browser to perform initializations

Usually an applet has no constructor
The browser determines the graphics container
title and the size is determined in the SIZE
parameters of the .html file
 No need for calls to setSize() and setTitle()

Other methods of JApplet that you may need to
implement
 start()
 Automatically called by the browser after
init() and when the user returns to the web
page (whereas init() only called once)
 stop()
 Automatically called when the user moves
off the web-page.
 destroy()
 Automatically called when the browser shuts
down (after it calls the stop() method)
 Normally put code to reclaim resources here

Lots of other example applets can be found
at
http://javaboutique.internet.com/applet_inde
x/d.html
 A favourite of mine is
http://javaboutique.internet.com/Date2Da
y/
Browsers and plug-ins


Internet browsers such as Explorer and Netscape
have not kept up with the development of Java

Both contain a Java Virtual Machine (JVM) but
earlier versions of the browsers can only run
Java 1.0 programs not the more recent Java 2

This is always going to be a problem as Java is
developing much faster than internet browsers
To combat his problem, Sun developed a Java
plug-in which ‘plugs into’ either IE or Netscape

Allows the browser to run the JVM installed on
the machine running the browser and not its
internal version
Client computer
Browser
Java virtual
machine
Java virtual
machine
html file
<… code for plug-in …>

Plug-ins allow the user to select any of the
installed JVM’s but it makes the .html file
much more complex


For an example see Core Java 2 page 37
Fortunately, the simple .html file delimited
between the the APPLET and /APPLET
tags is sufficient for more recent browsers
compatible with Java 2 (such as Netscape 6
or Netscape 7)
<APPLET CODE = "HelloWorldApplet.class"
WIDTH = 300 HEIGHT = 300 >
</APPLET>

Also, a .html converter can automatically
generate the code for the plug-in from this
simple .html file

The appletviewer program can understand
the basic .html file and is a good tool for
testing applets outside of the browser
 Can be called from the Textpad editor
Applet attributes

These are specified after the APPLET tag which
tell the browser how to load and display the applet
 The general form is as follows (attributes in [..]
are optional)
< APPLET
[CODEBASE = codebaseURL]
CODE = appletFile
[ARCHIVE = jarFile]
[ALT = alternateText]
[NAME = appletInstanceName]
WIDTH = pixels HEIGHT = pixels
[ALIGN = alignment]
[VSPACE = pixels]
[HSPACE = pixels]
</APPLET>

We will only look at the most common
attributes
 WIDTH, HEIGHT
 The width and height of the applet
window in pixels
 Note that in a browser, the applet
cannot be resized so you need to make
a good guess at these values

The CODE attribute is simply the name of the
applet class file

The CODEBASE attribute to tells the browser in
which directory the applet's files are located

If this attribute is not specified, the .class file is
in the same directory as the .html file

If it is specified, it is the relative path from the
directory of the .html file
“…/myDirectory/myHTML.html”
<APPLET CODE=myApplet.class
CODEBASE=“appletDir/"
WIDTH=500 HEIGHT=500> </APPLET>
/myDirectory
myHTML.html
/appletDir
myApplet.class
NAME
 Used when applets on the same page
want to communicate and calling applets
from JavaScript
 ARCHIVE
 Used for more efficiently downloading
multiple .class files in one Java archive
file (see later)

Passing parameters to applets

Applets can use parameters passed to them from the
.html file

This is done using the PARAM tag in the .html
file

The NAME and VALUE tags then specify the
parameter name and value
<APPLET CODE=AppletClass.class
WIDTH=500 HEIGHT=500>
<PARAM NAME=parameter1Name VALUE=aValue>
<PARAM NAME=parameter2NameVALUE=anotherValue>
</APPLET>

The getParameter() method of JApplet then
retrieves that (string) parameter

Parameters allow the applet versatility to be
increased without the need to recompile the applet
code

The following example is the ‘Hello World’ applet
with the printed message passed as a parameter

http://www.eee.bham.ac.uk/spannm/Java%20St
uff/MessageApplet/MessageApplet.html
The html file is as follows

<APPLET CODE = "MessageApplet.class" WIDTH = 500
HEIGHT = 300 >
<PARAM NAME=message VALUE="Another Message">
</APPLET>

Note that parameters are strings but they can be
converted to other types (eg integers) using
string parsing
String messageValue=getParameter("message");

The parameter is retrieved using the following
line in the Java program
class MessagePanel extends JPanel
{ public void paintComponent(Graphics g)
{ super.paintComponent(g);
Font f=new Font("SansSerif",Font.BOLD,36);
g.setFont(f);
g.drawString(messageValue, 50, 120);
}
public MessagePanel(String m)
{
messageValue=m;
}
}
private String messageValue;
public class MessageApplet extends JApplet
{ public void init()
{ Container contentPane = getContentPane();
String messageValue=getParameter("message");
contentPane.add(new MessagePanel(messageValue));
}
}
Applet security

Applets are programs downloaded from a
remote site and run automatically on the
users home computer
 They constitute potentially a huge
security risk!
 For this reason, applets are extremely
limited in their functionality
 Attempts to perform other functions leads
to a SecurityException being generated

Applets cannot
 Read or write to the local computer’s file
system
 If this were not the case, applets could
potentially read sensitive data (credit
card info!) or delete important files
 Applets cannot find information about the
local computer. For example user names
and email addresses
 Applets cannot run a local executable
program (for obvious reasons!)

Applets cannot
 Communicate with any host except its
originating host
 Applets can only phone home!
 If this were not the case, applets could
access web pages behind corporate
firewalls
 They could then read potentially
sensitive company information
Corporate intranet
Executing
applet
applet
Corporate
web
server
Applet originating host
Firewall
JAR files


The previous MessageApplet example actually
creates 2 class files
 MessageApplet.class
 MessagePanel.class
The browser needs to load both classes to run the
applet
 The browser must make 2 connections to the
server
 Typically an applet consists of many .class files

JAR files enable the .class files for an
applet to be downloaded using a single
connection to the server
 The class files are loaded into a JAR file
using the following command
jar cf MessageApplet.jar
MessageApplet.class MessagePanel.class

The MessageApplet.jar JAR file then
contains the two class files

The JAR file is then referenced in the APPLET tag
of the html file using the ARCHIVE tag
<APPLET CODE = "MessageApplet.class"
ARCHIVE=“MessageApplet.jar”
WIDTH = 500 HEIGHT = 300 >
<PARAM NAME=message VALUE="Another Message">
</APPLET>

JAR files are extensively used in large Java
projects as repositories of classes which can be
included in the CLASSSPATH
And finally…..
Applets are very useful but are basically
Java programs with a bit of extra
‘wrapping’
 Great for me as I have used them
extensively in this course to demonstrate
Java programs as I can run them from
Powerpoint
 Their limited functionality makes them
difficult to write if we want to do ‘real’
work
