This post describes how you can have your lost source codes from the Jar.
For this we need to decompile all the classes which are present inside the jar to obtain your lost source codes.
A software called “JD-GUI” does this. You just need to open it and drag your executable jar to it and it will show you the decompiled source codes.
You can download the decompiler from http://java.decompiler.free.fr/?q=jdgui. The page contain all relevant information you will need to decompile.
Note : Make sure you don’t use it for illegal purpose. If the codes are obfuscated then decompiler won’t be able to fully decompile the jar.