From Eclipsepedia

The extension registry is a means for bundles to register and access extension points.

Eclipse 3.x API

The existing IExtensionRegistry can currently either be retrieved directly via a static method (Platform.getExtensionRegistry()) or queried through via OSGi as a service.

e4 (Java)

The extension registry can be easily queried for by asking the Eclipse context.

private IExtensionRegistry getRegistry(IEclipseContext context) {
  return (IExtensionRegistry) context.get(IExtensionRegistry.class.getName());
}