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Trail: The Extension Mechanism
Lesson: Creating and Using Extensions

## Download Extensions

> Download extensions are classes, including classes in JAR files, that are specified in the `Class-Path` headers in the manifests of other JAR files. Assume for example that `a.jar` and `b.jar` are two JAR files in the same directory, and that the manifest of `a.jar` contains this header:

> >     Class-Path: b.jar

> Then the classes in `b.jar` serve as extension classes for purposes of the classes in `a.jar`. The classes in `a.jar` can invoke classes in `b.jar` without `b.jar`'s classes having to be named on the class path. `a.jar` may or may not itself be an extension. If `b.jar` weren't in the same directory as `a.jar`, then the value of the `Class-Path` header should be set to the relative pathname of `b.jar`.

> There's nothing special about the classes that are playing the role of a download extension. They are treated as extensions solely because they're referenced by the manifest of some other JAR file.

> To get a better understanding of how download extensions work, let's create one and put it to use.

### An Example

> Suppose you want to create an applet that makes use of the `RectangleArea` class of the previous section:

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

> In the previous section, you made the `RectangleArea` class into an installed extension by placing the JAR file containing it into the `lib/ext` directory of the JRE. By making it an installed extension, you enabled any application to use the `RectangleArea` class as if it were part of the Java platform.

> If you want to be able to use the `RectangleArea` class from an applet, the situation is a little different. Suppose, for example, that you have an applet, `AreaApplet`, that makes use of class `RectangleArea`:

> >     import java.applet.Applet;
> >     import java.awt.*;

> >     public class AreaApplet extends Applet {
> >         Rectangle r;

> >         public void init() {
> >             int width = 10;
> >             int height = 5;

> >             r = new Rectangle(width, height);
> >         }

> >         public void paint(Graphics g) {
> >             g.drawString("The rectangle's area is "
> >                           + RectangleArea.area(r), 10, 10);
> >         }
> >     }

> This applet instantiates a 10 x 5 rectangle and then displays the rectangle's area by using the `RectangleArea.area` method.

> However, you can't assume that everyone who downloads and uses your applet is going to have the `RectangleArea` class available on their system, as an installed extension or otherwise. One way around that problem is to make the `RectangleArea` class available from the server side, and you can do that by using it as a download extension.

> To see how that's done, let's assume that you've bundled `AreaApplet` in a JAR file called `AreaApplet.jar` and that the class `RectangleArea` is bundled in `RectangleArea.jar`. In order for `RectangleArea.jar` to be treated as a download extension, `RectangleArea.jar` must be listed in the `Class-Path` header in `AreaApplet.jar`'s manifest. `AreaApplet.jar`'s manifest might look like this, for example:

> >     Manifest-Version: 1.0
> >     Class-Path: RectangleArea.jar

> The value of the `Class-Path` header in this manifest is `RectangleArea.jar` with no path specified, indicating that `RectangleArea.jar` is located in the same directory as the applet's JAR file.

### More about the Class-Path Header

> If an applet or application uses more than one extension, you can list multiple URLs in a manifest. For example, the following is a valid header:

> >     Class-Path: area.jar servlet.jar images/

> In the `Class-Path` header any URLs listed that don't end with '`/`' are assumed to be JAR files. URLs ending in '`/`' indicate directories. In the preceding example, `images/` might be a directory containing resources needed by the applet or the application.

> You can also specify multiple extension URLs by using more than one `Class-Path` header in the manifest. For example:

> >     Class-Path: area.jar
> >     Class-Path: servlet.jar

> Download extensions can be "daisy chained", meaning that the manifest of one download extension can have a `Class-Path` header that refers to a second extension, which can refer to a third extension, and so on.

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