## The plug-in class

So far, we've been looking at the different extensions that are provided by the readme tool. Let's look at the general definition of the readme tool plug-in.

### Plug-in definition

The readme tool plug-in is defined at the top of the plugin.xml file.

    <?xml version="1.0" encoding="UTF-8"?>
    <?eclipse version="3.0"?>
    <plugin
       id="org.eclipse.ui.examples.readmetool"
       name="%Plugin.name"
       version="2.1.0"
       provider-name="%Plugin.providerName"
       class="org.eclipse.ui.examples.readmetool.ReadmePlugin">

       <runtime>
          <library name="readmetool.jar"/>
       </runtime>
       <requires>
          <import plugin="org.eclipse.ui"/>
          <import plugin="org.eclipse.core.resources"/>
          <import plugin="org.eclipse.core.runtime.compatibility"/>
          <import plugin="org.eclipse.ui.views"/>
          <import plugin="org.eclipse.ui.ide"/>
          <import plugin="org.eclipse.jface.text"/>
          <import plugin="org.eclipse.text"/>
          <import plugin="org.eclipse.ui.workbench.texteditor"/>
          <import plugin="org.eclipse.ui.editors"/>
       </requires>
       ...

The plug-in definition includes the name, id, version, and provider name of the plug-in. We saw most of these parameters before in our hello world plug-in. The readme tool also defines a specialized plug-in class, ReadmePlugin.

The name of the jar file is also provided. File names specified in a plugin.xml file are relative to the plug-in's directory, so the readme tool's jar file should be located directly in the plug-in's directory.

The requires element informs the platform of the readme tool's dependencies. The workbench UI plug-ins are listed as required plug-ins, along with the various core, jface, and text plug-ins.

### AbstractUIPlugin

The ReadmePlugin class represents the readme tool plug-in and manages the life cycle of the plug-in. As we saw in the Hello World example, you don't have to specify a plug-in class. The platform will provide one for you. In this case, our plug-in needs to initialize UI related data when it starts up. The platform class AbstractUIPlugin provides a structure for managing UI resources and is extended by ReadmePlugin.

AbstractUIPlugin uses the generic startup and shutdown methods to manage images, dialog settings, and a preference store during the lifetime of the plug-in. We'll look at the specifics of the ReadmePlugin class when we work with dialogs and preferences.