## The plug-in class

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

### Plug-in definition

The readme tool plug-in is defined in the MANIFEST.MF file.

    Manifest-Version: 1.0
    Bundle-ManifestVersion: 2
    Bundle-Name: %contributions.Activator.name
    Bundle-SymbolicName: org.eclipse.ui.examples.contributions; singleton:=true
    Bundle-Version: 3.3.100.qualifier
    Bundle-Activator: org.eclipse.ui.examples.contributions.Activator
    Require-Bundle: org.eclipse.ui;bundle-version="[3.3.0,4.0.0)",
     org.eclipse.core.runtime,
     org.eclipse.core.expressions;bundle-version="[3.3.0,4.0.0)"
    Bundle-Vendor: %contributions.Activator.providerName
    Bundle-Localization: plugin
    Bundle-RequiredExecutionEnvironment: J2SE-1.4
    Bundle-ActivationPolicy: lazy

The plug-in definition includes the Bundle-Name, Bundle-SymbolicName (plug-in id), Bundle-Version, and Bundle-Vendor of the plug-in. We saw most of these parameters before in our hello world plug-in. The contribution plug-in also defines a specialized plug-in class, org.eclipse.ui.examples.contributions.Activator.

The Require-Bundle element informs the platform of the contribution plug-in's dependencies. The workbench UI plug-ins are listed as required plug-ins, along with the various core and expression plug-ins.

### AbstractUIPlugin

The Activator class represents the contribution 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 Activator.

AbstractUIPlugin uses the generic startup and shutdown methods to manage images, dialog settings, and a preference store during the lifetime of the plug-in.