What you need to do is download the source code for Struts and point Eclipse to that. I think it will accept the source as a zip file, or you can unzip it and point Eclipse to the base source directory.
P.S. If you get rid of the line numbers, nothing will work. (How can Eclipse show you line on code in question if it cannot find it by line number?)
I have downloaded struts source to a directory in c:\Program Files... I've created 2 classes in a new package (strutssource) in my project and copied in ActionServlet and RequestProcessor classes respectively.
After resolving all the missing imports, I'm left with many "...not applicable for arguments..." compilation errors, which I've identified by placing at the end of each affected line "//here".
For example, there's 1 in ActionServlet.initModulePlugIns(ModuleConfig config) on the line "plugIns[i].init(this, config);" where the error message is "The method init(ActionServlet, ModuleConfig) in the type PlugIn is not applicable for the arguments (ActionServlet, ModuleConfig)".
In RequestProcessor class I have 6 likewise errors.
I don't believe this is an exercise in futility because I wish to "step through" parts of code and what if I wanted to override some behavior in one of the methods producing the compilation errors - I'd quite possibly have to resolve one or more of these errors to do so - wouldn't I?