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.>
Navigate to extracted folder in command prompt and run command "dex2jar your.apk" and its output will be jar file. This jar file can be viewed in jd-gui window.
check the link below , it shows with snapshots
http://a4apphack.com/security/sec-code/extract-android-apk-from-market-and-decompile-it-to-java-source