JAD
Jad (Java Decompiler) is, as of August 2011, an unmaintained decompiler for the Java programming language. Jad provides a command-line user interface to extract source code from class files.
As http://www.kpdus.com is no longer accessible, JAD Java Decompiler download is extremely hard to find. Here is a mirror where you can get JAD for various platforms.
Example
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jad -o -r -sjava -dsrc D:\down\classes_dex2jar\**\*.class -
src디렉터리에 모든 class 파일을 java로 디컴파일 한다.