Applets

Applets

The objectives of this chapter are:

To describe applets and their purpose

To discuss embedding applets in HTML pages

An applet is a subclass of PanelIt is a container which can hold GUI componentsIt has a graphics context which can be used to draw images

An applet embedded within an HTML pageApplets are defined using the <applet> tagIts 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.

• Unfortunately, most browsers have a very old version of the JVM• 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

What is an applet

Java applications are executed from the command lineA Java VM must be installedThe VM is given the name of a class which contains a main() methodThe main method instantiates the objects necessary to start the application

Applets are executed by a browserThe browser either contains a VM or loads the Java pluginThe programmer must implement a class which is a subclass of appletThere is no main method. Instead, the applet contains an init() method which is invoked by the Browser when the applet starts

Applications 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>

HTML and Applets

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}

}

Java applets execute within a SandboxApplets cannot access the local file systemApplets cannot connect to systems other than the server from which they were downloadedApplets cannot listen for inbound network connectionsApplets cannot spawn processes or load local jar filesApplets cannot terminate the Java VMApplets cannot change the security policy

Signed applets can be granted more access rightsThese rights are controlled by the SecurityManager

Applet Security

Unfortunately, Applets are not heavily usedBrowsers support is limited

Browsers often contain outdated or buggy Java virtual machinesEach browser has its own compatibility issues. This usually means that the programmer has to implement workarounds for the various browsersMicrosoft 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.

Browser Issues

Applets

Documents

Transcript of Applets