Java - Applet - Introduction

Java applets were a popular way of building web applications in the 1990s and early 2000s. Although they have since fallen out of favor due to security issues and lack of support, understanding their concepts can still be useful. Here is a tutorial on Java applet concept:

Java Applet Concept

Java applets are small programs written in the Java programming language and designed to run within a web browser. Applets can be embedded in web pages and used to provide interactive content, such as games, animations, and calculators. They were widely used in the past, but have since been largely replaced by other technologies, such as HTML5 and JavaScript.

Creating a Java Applet

To create a Java applet, you need to follow these steps:

  • Write the applet code: This code should be written in Java programming language and compiled to create a .class file.
  • Create an HTML file: This file should be used to embed the applet in a web page.
  • Compile the HTML file: This file should be compiled to ensure that it is compatible with the web browser.
  • Test the applet: The applet should be tested to ensure that it works as expected in the web browser.
  • Publish the applet: Once the applet has been tested and verified, it can be published on the web for others to use.

Applet Lifecycle

An applet has four stages in its lifecycle: initialization, start, stop, and destroy.

  1. Initialization: The init() method is called when the applet is first loaded into the web browser. This method is used to initialize the applet and set its properties.
  2. Start: The start() method is called when the applet is about to be displayed on the web page. This method is used to start any background threads or animations that the applet may use.
  3. Stop: The stop() method is called when the applet is no longer visible on the web page. This method is used to stop any background threads or animations.
  4. Destroy: The destroy() method is called when the applet is removed from the web page. This method is used to release any resources used by the applet.

Applet Tags

The <applet> tag is used to embed the applet in an HTML page. The following attributes can be used with the <applet> tag:

  • code: The name of the .class file that contains the applet code.
  • archive: The name of the .jar file that contains any other classes or resources used by the applet.
  • width: The width of the applet in pixels.
  • height: The height of the applet in pixels.
  • align: The alignment of the applet within the web page.
  • hspace: The amount of horizontal space to be added around the applet.
  • vspace: The amount of vertical space to be added around the applet.

Applet Security

Java applets run within a security sandbox, which restricts their access to system resources such as the file system and network. This is done to prevent malicious applets from causing harm to the user's computer. Applets can also be digitally signed and verified to ensure their authenticity.

Java applets were once a popular way of building interactive web applications, but have since been largely replaced by other technologies. Understanding the concepts behind applets, such as their lifecycle and security, can still be useful for historical purposes or for maintaining legacy systems.