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Extension Points allow that functionality is contributed to a specific plugin which defines this extension point. The functionality can then be contributed by any plugin which defines an extension for the defined extension point.

So an overview:

Functionality can be code but also non-code related contributions, e.g. help context.

The plugin which defines extension points defines a contract for other plugins how these plugins could provide functionality via this extension point.

The plugin which defines the extension point is also responsible for evaluating the contributions.

To define a extension point you perform the following:

Extension points are defined via the file plugin.xml. For example the following show the extension point which will defined in the following example.

				
<?xml version="1.0" encoding="UTF-8"?>
<?eclipse version="3.2"?>
<plugin>
   <extension-point id="de.vogella.extensionpoint.greeters" name="Greeters" schema="schema/de.vogella.extensionpoint.greeters.exsd"/>

   <extension
         id="application"
         point="org.eclipse.core.runtime.applications">
      <application>
         <run
               class="de.vogella.extensionprovider.simple.Application">
         </run>
      </application>
   </extension>

</plugin>
			

The extension point refers to an XML Schema which defines the details of the extension point.

Eclipse provides for both files a powerful editor which the following example will demonstrate.