类似于Java Web应用程序世界中的RJS（在Ruby on Rails中使用）？

In Ruby on Rails, there's RJS templates that help do AJAX by returning javascript code that will be executed in the client browser. How can I do a similar thing in the Java/J2EE world? Is Google Widget Toolkit similar to RJS? What options do I have? In Ruby on Rails, there's RJS templates that he