Javac for performance reasons has its own implementation of a ZipFile reader, note javac does not require the Zip/Jar writers.
This was because certain benchmarks were sped up when reading large jars such as rt.jar. The implementation is pure java
by-passing JNI (which causes performance degradation/state transitions when interfacing with the Zip library to fetch various zip archive 
attributes). The files of interest would be: c.s.t.j.file.Zip*.java.

There is an effort in the JDK Zip library team to come up with a pure Java Zip file reader, it would be prudent, to work closely
with the library and performance team to make this happen for jdk9, and eliminate this specific implementation in javac.