Understanding the Applet Life Cycle in Java
Applet life cycle in Java is a special type of program that runs within a web browser. What happens to an applet from the time it is loaded until it is shut off is known as the applet life cycle.
This life cycle consists of several methods that are invoked at different stages of the applet’s execution. Understanding the applet life cycle is essential for developing robust and responsive Java applets. This article will examine the many steps in the Java applet life cycle. We will also see the order in which they are invoked.
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The Methods of the Applet Life Cycle
The applet life cycle in Java consists of five core methods:-
This method is invoked only once, when the applet is first loaded. It is used to perform initialization tasks such as setting up the applet’s graphical user interface (GUI) components and loading any necessary data. The `init()` method provides a suitable place to initialize variables, register event listeners, and perform other setup operations.
Following the initialization phase (‘init()’), the ‘start()’ function is triggered. To initiate the applet’s execution, use this function. When the ‘start()’ method is invoked, the applet becomes functional and begins to process events and input. The `start()` method is typically used to initiate animations, start timers, or begin any continuous processing required by the applet.
The `paint()` method is invoked whenever the applet needs to be repainted. This can happen when the applet’s size changes, when it is moved on the screen, or when a request for repainting is made explicitly. The `paint()` method is responsible for rendering the applet’s visual representation on the screen. It is where you would typically draw shapes, images, and text using the applet’s graphics context.
The `stop()` method is called when the applet is no longer visible or active. This can happen if the browser is minimized or if the user leaves the page on which the applet is running. The `stop()` method provides an opportunity to pause or suspend any ongoing operations or timers within the applet. It is crucial to release any system resources or stop any background threads that might be consuming system resources.
When the applet is about to be unloaded or terminated, the `destroy()` method is invoked. This takes place when either the browser is closed or the page containing the applet is closed. The `destroy()` method gives you a chance to release any resources, such as open files or network connections, that the applet may have acquired during its execution. It is also a suitable place to perform any final cleanup operations.
The Order of the Applet Life Cycle
Understanding the order in which these methods are invoked is vital for managing the applet’s behavior and ensuring proper initialization and cleanup. Here is the typical sequence of events in the applet life cycle:
- The `init()` method is invoked when the applet is first loaded. It is called only once during the entire life cycle and is used for initializing the applet.
- It calls the `start()` method after executing the `init()` method. This method starts the execution of the applet and makes it active.
- As the applet runs, the `paint()` method is invoked as needed. It is responsible for rendering the applet’s visual representation and is called when the applet needs to be redrawn.
- When the applet is no longer visible or active (For example – when the user navigates away from the web page) the `stop()` method is called. This method allows you to pause or suspend ongoing operations or timers.
- Finally, when the applet is about to be unloaded or terminated, the `destroy()` method is invoked. This method provides an opportunity to release any acquired resources and perform any necessary cleanup operations.
Table of Applet Life Cycle in Java
|init()||Invoked once when the applet is first loaded.||public void init()|
|Used for initializing the applet and its components.|
|Sets up GUI components and loads necessary data.|
|start()||Invoked after init() method.||public void start()|
|Starts the execution of the applet.|
|Applet becomes active and responds to user input or events.|
|paint()||Invoked whenever the applet needs to be repainted.||public void paint(Graphics g)|
|Renders the visual representation of the applet on the screen.|
|Updates the applet’s visual state based on changes.|
|stop()||Invoked when the applet is no longer visible or active.||public void stop()|
|Allows pausing or suspending ongoing operations or timers.|
|Releases system resources or stops background threads.|
|destroy()||Invoked when the applet is about to be unloaded or terminated.||public void destroy()|
|Performs final cleanup operations and releases resources.|
Example of the Applet Life Cycle
Let’s consider an example to illustrate how the applet life cycle works:
- When we load a web page containing an applet, the `init()` method is invoked. We use this method for initializing the applet, such as setting up GUI components and loading data.
- After the initialization, we call the `start()` method. This method starts the execution of the applet, making it responsive to user input or other events.
- As the user interacts with the applet, we invoke the `paint()` method whenever we need to repaint the applet. For example, if the applet’s size changes, the `paint()` method will be called to update its visual representation.
- If the user navigates away from the web page or minimizes the browser window, the `stop()` method is called. This allows the applet to pause or suspend any ongoing operations or timers.
- Finally, when the web page is closed or the browser is shut down, the `destroy()` method is invoked. This is the last opportunity for the applet to release resources and perform any necessary cleanup.
Understanding the applet life cycle is crucial for developing robust and responsive Java applets. By implementing the appropriate methods at each stage of the life cycle, you can ensure proper initialization, handle user interactions, and perform necessary cleanup operations.
The methods form the core of the applet life cycle and provide the necessary hooks for managing the applet’s behavior. By leveraging this life cycle effectively, you can create dynamic and interactive applets that enhance the user experience.