What is an Extension Point¶

An extension point is a formal declaration in a plugin.xml file where customization is allowed. Customization takes the place of other plug-ins (inclusive) providing a little chunk of XML describing the specific customization that is requested.

In practice, most extension points allow the XML to indicate a class of some sort (where all the magic occurs). If we were just doing straight-up object oriented programming, we would handle this kind of thing by allowing others to provide a strategy object that conformed to a specific interface.

It works the same way with most extension points wherein, when they allow a class contribution, they indicate what interface is required.

However, it is important to remember this is not straight-up object oriented programming, instead, that chunk of XML (the extension) also needs to correspond to a respective interface. In XML-speak, this is what is known as an XML Schema.

Here is a link to the extension points used by uDig right now: uDig Extension Points List. In addition to these extension points, you can use any of the extension points defined by the Eclipse RCP application (allowing you to do everything from additional views... to online help).

When the class/interface is extended to create a plug-in, the plugin.xml file must be updated with the appropriate information.

For example:

<?xml version="1.0" encoding="UTF-8"?>
<?eclipse version="3.0"?>
<plugin
    id="com.yourname.helloworld"
    name="HelloWorld Plugin"
    version="1.0.0"
    provider="EXAMPLE"
    class="com.yourname.helloworld.HelloWorldPlugin">

   <runtime>
    <library name="helloworld.jar">
     <export name="*"/>
    </library>
   </runtime>

   <requires>
    <import plugin="org.eclipse.ui"/>
    <import plugin="org.eclipse.core.runtime"/>
    <import plugin="org.eclipse.core.runtime.compatibility"/>
   </requires>

   <extension
         point="org.eclipse.ui.views">
      <category
            name="Hello Category"
            id="com.yourname.helloworld">
      </category>
      <view
            name="Hello View"
            icon="icons/sample.gif"
            category="com.yourname.helloworld"
            class="com.yourname.helloworld.HelloWorldView"
            id="com.yourname.helloworld.HelloWorldView">
      </view>
   </extension>

</plugin>

Notice the <extension> tag. The extension point for this example hello world plug-in is the org.eclipse.ui.views class.

The above example code can be found in the section:: “Plug it in: Hello World meets the Workbench”

of the “Platform Plug-in Developer Guide”:: org.eclipse.platform.doc.isv_3.0.1.pdf

Located on Eclipse’s documentation page:: http://www.eclipse.org/documentation/main.html

The Plugin.xml file contains the XML code that allows the Eclipse run-time engine to execute new plug-ins. By specifying an “Extension Point” in the XML file, Eclipse can run the class, and because the class implements an already existing interface, Eclipse knows what functions it must run.