There is a cool hidden feature in JDeveloper to preview the source of any file.
It is a bit tricky however, but you can give it a try.
Just click on the file in the file navigator
Now press shift and take away mouse out of file name
and keeping shift pressed bring again the mouse pointer on the file title.
It may not work immediately in that case just release the shift key and press again try again
to bring the mouse pointer on file name.
Here is the screenshot of how it looks like: