The default class viewer doesn't decompile the class file so you cannot open and check the source code, you may down third part App to do that while it is not convenient for debugging. The JAD plugin unable you open it directly in Eclipse.
you just follow below three stages to set the JAD plugin in Eclipse.
First, down plugin. the JAD plugin consists of two files, JAD.exe and net.sf.jadclipse_3.3.0.jar and you can download them from http://download.csdn.net/detail/sushengmiyan/6885693 (thx contribution of this blog. http://blog.csdn.net/sushengmiyan/article/details/18798473 )
Then put JAD.exe to any folder, of cause \Java\jdk1.xx\jre\bin\ is recommended. put the net.sf.jadclipse_3.3.0.jar to your Eclipse plugin folder.
At last, you need to do some setting in window->preferences. you need to restart your Eclipse first.
General->Editors->File Associations. add "JadClipse Class File Viewer" for both file type *.class and *.class without source, set as default at the same time.
Java->JadClipse. set the "Path to decompiler" path with the JAD.exe you have set precedently.
OK, now enjoy to view the source of class file directly in Eclipse.
