This is an old question, but seeing as the decompiler landscape has changed significantly in the past year, I feel it's worth resurrecting.
Procyon is an open source framework that contains, among other things, a Java decompiler. It is written in Java, and the decompiler APIs can be integrated into another application fairly easily. In fact, there are already two third-party GUI front-ends, including the SecureTeam Java Decompiler.
CFRにはまだ利用可能なソースコードがありませんが、優れた逆コンパイラーでもあります。これもJavaで書かれており、既存のアプリケーションとの統合は試みていませんが、確かに可能であるはずです。