I have searching in search Engine regarding convert dex file to jar file but I got failure to find dex2jar file in code.Google.com
I have getting basic idea from here.>
Extract the dex2jar-0.0.9.8.zip file and move to desktop folder like dex2jar-0.0.9.7...then take the android app classes.dex file and move to this directory ...open the terminal and change to your dex2jar directory..type this command
./d2j-dex2jar.sh classes.dex
you will get classesdex2.jar file and you can open this file using java decompiler...