Just installed Eclipse LUNA, and converted my old JUNO workspace. It worked on one of my computers fine (win7), but using the same project on my other computer (win8, synced via dropbox) created this issue for all opened files, and nothing shows up in my package explorer. Always worked fine with JUNO.
Inside package explorer, right click -> import -> General -> Existing Projects into Workspace -> next -> find your project in "Select root directory" -> Finish.
If only some projects or files are missing or the above doesn't work then a similar method using General/File System can be used where you choose the specific files to import.
来源：https://stackoverflow.com/questions/28367940/eclipse-luna-cannot-determine-uri-for-project-src-com-classfile-java-win