Using Java2Script Pacemaker
Using Java2Script Pacemaker won't require any extra APIs learning. All you have to do is to develop your Java applications in Eclipse as normal. Developing your application as "Java Application", you may debug into sources line by line by using Eclipse's debugging, and you may design your SWT dialogs by SWT UI designer, and you may do necessary refactoring, and you may format your codes and you may ..., you can do anything you are familiar with Eclipse JDT.
After you finishing your Java application, then you can "Enable Java2Script Compiler" to convert your RCP application into RIA. For more technical details, please read Getting Started Tutorials.
After something like "Bang!", you will get the your RIA running inside browsers.
After testing the application in standalone browser outside Eclipse platform, you can also deploy the RIA to server (Manually by coping *.js, *.css and *.html files). Then you will have both RCP and RIA!
Come to more detail: http://j2s.sourceforge.net/
Recommend the tutorial