Java Applet Basics: Understanding Applets in Web Browsers

Learn the fundamentals of Java applets, how they run within web browsers, and their key differences from standalone Java applications. Discover how applets extend the java.applet.Applet class, follow sandbox security rules, and are embedded in HTML pages. Understand the requirements for running applets, including JVM integration and JAR file support.

Java - Applet Basics

An applet is a Java program that runs within a web browser. It can function as a full-fledged Java application with access to the complete Java API. However, there are some key differences between an applet and a standalone Java application:

• An applet is a Java class that extends java.applet.Applet.
• An applet does not have a main() method.
• Applets are embedded in an HTML page.
• The code for an applet is downloaded to the user's machine upon loading the HTML page.
• A Java Virtual Machine (JVM) is needed to run an applet, which may be integrated into the browser or installed separately.
• Applets follow strict security rules enforced by the web browser, known as "sandbox security."
• Classes required by the applet can be included in a JAR file.

Life Cycle of an Applet in Java

The Applet class provides methods that help structure the life cycle of an applet:

• init: Called once for initialization when the applet is first loaded.
• start: Automatically called after init(), and each time the user returns to the page containing the applet.
• stop: Automatically called when the user navigates away from the applet’s page.
• destroy: Called only when the browser shuts down.
• paint: Invoked whenever the applet needs to repaint itself on the page.

A Simple Java Applet Example: "Hello, World"

Here's a basic example of a Java applet:

import java.applet.*;
import java.awt.*;

public class HelloWorldApplet extends Applet {
public void paint (Graphics g) {
    g.drawString ("Hello World", 25, 50);
}
}

Hello World

Invoking an Applet in HTML

An applet is embedded in HTML using the <applet> tag. Below is an example:

    Your browser does not support applets.

Life Cycle Flow of an Applet

The flow of an applet's life cycle methods looks like this:

init() -> start() -> paint() -> stop() -> destroy()

Event Handling in Applets

Applets inherit event-handling methods like processMouseEvent and processKeyEvent. Here's an example of handling mouse events:

import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.applet.Applet;
import java.awt.Graphics;

public class ExampleEventHandling extends Applet implements MouseListener {
StringBuffer strBuffer;

public void init() {
    addMouseListener(this);
    strBuffer = new StringBuffer();
    addItem("initializing the applet ");
}

public void start() {
    addItem("starting the applet ");
}

public void stop() {
    addItem("stopping the applet ");
}

public void destroy() {
    addItem("unloading the applet");
}

void addItem(String word) {
    strBuffer.append(word);
    repaint();
}

public void paint(Graphics g) {
    g.drawRect(0, 0, getWidth() - 1, getHeight() - 1);
    g.drawString(strBuffer.toString(), 10, 20);
}

public void mouseClicked(MouseEvent event) {
    addItem("mouse clicked! ");
}

public void mouseEntered(MouseEvent event) {}
public void mouseExited(MouseEvent event) {}
public void mousePressed(MouseEvent event) {}
public void mouseReleased(MouseEvent event) {}
}

initializing the applet
starting the applet
mouse clicked!

Displaying Images in Applets

Applets can display images in GIF, JPEG, BMP, and other formats. Here's an example:

import java.applet.*;
import java.awt.*;
import java.net.*;

public class ImageDemo extends Applet {
private Image image;

public void init() {
    String imageURL = getParameter("image");
    try {
        URL url = new URL(getDocumentBase(), imageURL);
        image = getImage(url);
    } catch (MalformedURLException e) {
        e.printStackTrace();
    }
}

public void paint(Graphics g) {
    g.drawImage(image, 0, 0, this);
}
}

Playing Audio in Applets

Applets can play audio using the AudioClip interface. Here's an example:

import java.applet.*;
import java.net.*;

public class AudioDemo extends Applet {
private AudioClip clip;

public void init() {
    try {
        URL url = new URL(getDocumentBase(), "test.wav");
        clip = getAudioClip(url);
    } catch (MalformedURLException e) {
        e.printStackTrace();
    }
}

public void start() {
    if (clip != null) {
        clip.loop();
    }
}

public void stop() {
    if (clip != null) {
        clip.stop();
    }
}
}