public class DropdownButtonHandler {

	@Execute
	public void execute(MToolItem toolItem,
			IPresentationEngine presentationEngine) {
		Menu menu = (Menu) toolItem.getMenu().getWidget();
		if (menu != null)
			menu.setVisible(true);
	}
}

@Execute
public void execute(EModelService modelService, MApplication application,
        @Named("toolItemId") String toolItemId) {
    MUIElement mToolItem = modelService.find(toolItemId, application);
    if (mToolItem != null && mToolItem instanceof MToolItem) {
        execute((MToolItem) mToolItem);
    }
}

@Execute
public void execute(MToolItem mToolItem) {
    ToolItem toolItem = (ToolItem) mToolItem.getWidget();
    Listener[] listeners = toolItem.getListeners(SWT.Selection);
    if (listeners.length > 0) {
        Listener listener = listeners[0];
        // prepare DropDown click event
        Event e = new Event();
        e.type = SWT.Selection;
        e.widget = toolItem;
        e.detail = SWT.DROP_DOWN;
        // send event to the widget
        listener.handleEvent(e);
    }
}

import org.eclipse.core.commands.AbstractHandler;
import org.eclipse.core.commands.ExecutionEvent;
import org.eclipse.core.commands.ExecutionException;
import org.eclipse.swt.SWT;
import org.eclipse.swt.widgets.Event;
import org.eclipse.swt.widgets.Listener;
import org.eclipse.swt.widgets.ToolItem;
import org.eclipse.swt.widgets.Widget;

/**
 * Handler for drop down commands contributed to a toolbar which opens the
 * corresponding drop down menu when the command is executed (i.e. toolbar item
 * is pressed).
 * 
 * <p>
 * Like <a href="http://www.eclipse.org/forums/index.php/t/483266/">this forum
 * post</a>, the implementation is (mostly) taken from WindowBuilders
 * "Create new visual classes" drop down command.
 *
 * @author Henno Vermeulen
 */
public class DropDownCommandHandler extends AbstractHandler {

	public Object execute(ExecutionEvent event) throws ExecutionException {
		if (event.getTrigger() != null && (event.getTrigger() instanceof Event)) {
			runWithEvent((Event) event.getTrigger());
		}
		return null;
	}

	public void runWithEvent(Event event) {
		// this action is represented as ToolItem
		Widget widget = event.widget;
		if (widget instanceof ToolItem) {
			ToolItem toolItem = (ToolItem) widget;
			Listener[] listeners = toolItem.getListeners(SWT.Selection);
			if (listeners.length > 0) {
				Listener listener = listeners[0];
				// prepare DropDown click event
				Event e = new Event();
				e.type = SWT.Selection;
				e.widget = toolItem;
				e.detail = SWT.DROP_DOWN;
				e.x = toolItem.getBounds().x;
				e.y = toolItem.getBounds().height;
				// send event to the widget
				listener.handleEvent(e);
			}
		}
	}
}