CountdownClock bug

when I use CountdownClock add-ons with the demo source, it show me the error:

Widgetset does not contain implementation for org.vaadin.kim.countdownclock.CountdownClock. Check its @ClientWidget mapping, widgetsets GWT module description file and re-compile your widgetset. In case you have downloaded a vaadin add-on package, you might want to refer to add-on instructions. Unrendered UIDL:
-Unrendered UIDL
-org.vaadin.kim.countdownclock.CountdownClock(NO CLIENT IMPLEMENTATION FOUND) id=PID19 height=40px
-countdown time=26837267056 format=<span style='font: bold 13px Arial; margin: 10px'>The end of the world will come on December 21, 2012<br />Which is %d days, %h hours, %m minutes and %s seconds from now.

can anyone tell me why?

Did you recompile the widgetset after adding the add-on ?

yes， I recompiled the whole project.

I use springroo generate my project,then add the CountdownClock add-ons dependency in pom.xml

this is the code in my java:

CountdownClock clock1 = new CountdownClock();
        Calendar c = Calendar.getInstance();
        c.set(2012, 12, 21, 0, 0, 0);
        clock1.setDate(c.getTime());
        clock1
                .setFormat("<span style='font: bold 13px Arial; margin: 10px'>"
                        + "The end of the world will come on December 21, 2012<br />"
                        + "Which is %d days, %h hours, %m minutes and %s seconds from now.</span>");
        clock1.setHeight("40px");

        mainLayout.addComponent(clock1);

Compiling project and widgetset are different tasks.
See this chapter of the book

thank you very much!I'll try it!