JAAD is an AAC decoder and MP4 demultiplexer library written completely in Java. It uses no native libraries, is platform-independent and portable.
It can read MP4 container from almost every input-stream (files, network sockets etc.) and decode AAC-LC (Low Complexity) and HE-AAC (High Efficiency/AAC+).
Java version 5 or higher is required.
The library is released open-source as public domain.
download section. You can also browse the source code in the SVN-repository.