Transcript Applets
Applets
The objectives of this chapter are:
To describe applets and their purpose
To discuss embedding applets in HTML pages
What is an applet
An applet is a subclass of Panel
It is a container which can hold GUI components
It has a graphics context which can be used to draw images
An applet embedded within an HTML page
Applets are defined using the <applet> tag
Its size and location are defined within the tag
The browser contains a Java Virtual Machine which executes
the applet
The applet .class file is downloaded, through the net, into the Virtual
machine.
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Unfortunately, most browsers have a very old version of the
JVM
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The standard is either Java 1.1.4 or 1.1.5
Sun has released a Java 1.2 plugin which can be used instead
Applications and Applets
Java applications are executed from the command line
A Java VM must be installed
The VM is given the name of a class which contains a main() method
The main method instantiates the objects necessary to start the
application
Applets are executed by a browser
The browser either contains a VM or loads the Java plugin
The programmer must implement a class which is a subclass of applet
There is no main method. Instead, the applet contains an init() method
which is invoked by the Browser when the applet starts
HTML and Applets
The HTML applet tag contains the following parameters:
<Applet code="name of .class file"
codebase="URL where code is loaded from"
name="applet identifier"
align="LEFT|RIGHT|CENTER
width="size in pixels"
height="size in pixels“>
<param name=“aName1” value=“aValue”>
<param name=“aName2” value=“aValue”>
</Applet>
Example HTML file
<HTML>
<HEAD>
<TITLE> Sample Applet</TITLE>
</HEAD>
<BODY>
<APPLET code="Sample.class" WIDTH=100 HEIGHT=200>
This text will display if the browser does not support applets
</APPLET>
</BODY>
</HTML>
Sample Applet
import java.applet.*;
import java.awt.*;
public class Sample extends Applet implements ActionListener
{
private Button okButton = new Button("OK");
private Button cancelButton = new Button("Cancel");
public void init()
{
okButton.addActionListener(this);
cancelButton.addActionListener(this);
add(okButton);
add(cancelButton);
}
public void actionPerformed(ActionEvent x)
{
// ...
}
...
}
Passing Parameters To Applets
<Applet code="Menu.class“>
<param name="MenuName" value="My Web Site“>
<param name="Font" value="Serif">
</Applet>
public class Menu extends Applet
{
public void init()
{
String menuName = getParameter("MenuName");
String theFont = getParameter("Font");
// do something with parameters
}
}
Applet Security
Java applets execute within a Sandbox
Applets cannot access the local file system
Applets cannot connect to systems other than the server from which they
were downloaded
Applets cannot listen for inbound network connections
Applets cannot spawn processes or load local jar files
Applets cannot terminate the Java VM
Applets cannot change the security policy
Signed applets can be granted more access rights
These rights are controlled by the SecurityManager
Browser Issues
Unfortunately, Applets are not heavily used
Browsers support is limited
Browsers often contain outdated or buggy Java virtual
machines
Each browser has its own compatibility issues. This usually means that
the programmer has to implement workarounds for the various browsers
Microsoft has not been very cooperative in terms of ensuring that IE
correctly implements applets. Even the Java plugin has issues under IE
Because the AWT and Swing are not highly regarded, use of
Java in the client is minimal.