<extension point="org.eclipse.ui.menus">
      <menuContribution
            locationURI="popup:org.eclipse.emf.codegen.ecore.genmodelMenuID?after=additions">
         <command
               commandId="someCommandId"
               id="someId"
               label="DoIt">
         </command>
      </menuContribution>
   </extension>
   <extension
         point="org.eclipse.ui.commands">
      <command
            defaultHandler="java.path.to.the.handler.class.Handlerclass"
            description="Some description"
            id="someCommandId"
            name="DoIt">
      </command>
   </extension>

public class Handlerclass extends AbstractHandler {

    @Override
    public Object execute(ExecutionEvent event) throws ExecutionException {
        System.out.println("DoStuff");
        return null;
    }

    @Override
    public boolean isEnabled() {
        return true;
    }
    
    @Override
    public boolean isHandled() {
        return true;
    }
}