The jar output stream of the compiler currently relies on the Apache Ant zip output stream for historical reasons. The main reason for this has been support of pack file data compression as bzip2, which actually doesn't consistently work in IzPack 5.0.
Furthermore, the compiler and installer support an approach of automatically packing the appropriate code from ant.jar to the installer, additional code is packed when bzip2 should be chosen as bzip2 pack compressor format. This code was mostly unused and should be no longer supported.
There is also a couple of unused wrapper *Compressor classes which can be removed along with this.
Later, we will try to go to commons-compress, which offers more choices of compression algorithms in a generic way.