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Trail: The Extension Mechanism
Lesson: Creating and Using Extensions

## Installed Extensions

> Installed extensions are JAR files in the `lib/ext` directory of the Java Runtime Environment (JRE) software. As its name implies, the JRE is the runtime portion of the Java Development Kit containing the platform's core API but without development tools such as compilers and debuggers. The JRE is available either by itself or as part of the Java Development Kit.

> In the 1.2 platform, the JRE is a strict subset of the JDKTM software. The JDK 1.2 software directory tree looks like this:

> JRE 1.2 consists of those directories within the highlighted box in the diagram. Whether your JRE is stand-alone or part of the JDK software, any JAR file in the JRE's `lib/ext` directory is automatically treated by the runtime environment as an extension.

### A Simple Example

> Let's create a simple installed extension. Our extension consists of one class, `RectangleArea`, that computes the areas of rectangles:

> >     public final class RectangleArea {
> >         public static int area(java.awt.Rectangle r) {
> >             return r.width * r.height;
> >         }
> >     }

> This class has a single method, `area`, that takes an instance of `java.awt.Rectangle` and returns the rectangle's area.

> Suppose that you want to test `RectangleArea` with an application called `AreaApp`:

> >     import java.awt.*;

> >     public class AreaApp {
> >         public static void main(String[] args) {
> >             int width = 10;
> >             int height = 5;

> >             Rectangle r = new Rectangle(width, height);
> >             System.out.println("The rectangle's area is "
> >                                + RectangleArea.area(r));
> >         }
> >     }

> This application instantiates a 10 x 5 rectangle, and then prints out the rectangle's area using the `RectangleArea.area` method.

> ### Running AreaApp Without the Extension Mechanism

> Let's first review how you would run the `AreaApp` application without using the extension mechanism. We'll assume that the `RectangleArea` class is bundled in a JAR file named `area.jar`.

> The `RectangleArea` class is not part of the Java platform, of course, so you would need to place the `area.jar` file on the class path in order to run `AreaApp` without getting a runtime exception. If `area.jar` was in the directory `/home/user`, for example, you could use this command:

> >     java -classpath .:/home/user/area.jar AreaApp

> The class path specified in this command contains both the current directory, containing `AreaApp.class`, and the path to the JAR file containing the `RectangleArea` package. You would get the desired output by running this command:

> >     The rectangle's area is 50

> ### Running AreaApp by Using the Extension Mechanism

> Now let's look at how you would run `AreaApp` by using the `RectangleArea` class as an extension.

> To make the `RectangleArea` class into an extension, you place the file `area.jar` in the `lib/ext` directory of the JRE. Doing so automatically gives the `RectangleArea` the status of being an installed extension.

> With `area.jar` installed as an extension, you can run `AreaApp` without needing to specify the class path:

> >     java AreaApp

> Because you're using `area.jar` as an installed extension, the runtime environment will be able to find and to load the `RectangleArea` class even though you haven't specified it on the class path. Similarly, any applet or application being run by any user on your system would be able to find and use the `RectangleArea` class.

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