This page has a good description of what's in the decoder and what isn't. You get bytecode, but not Java source. Free for testing and non-commercial use. One of the authors of this code reports that this implementation was built around an older version of the standard. This page will eventually be updated to detail the differences, but in the meantime, caveat emptor .
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The author says this formed the framework for the JPEG2000 compression algorithm. This paper is to appear in IEEE Transactions on Image Processing, submitted March 1999. An article on JPEG 2000 in WebReview. A pretty good description of what's going to be in the standard. An analytical study of JPEG 2000 functionalities Rate by Diego Santa-Cruz and Touradj Ebrahimi.