jar檔可以用壓縮軟體壓縮

http://download.oracle.com/javase/1.4.2/docs/guide/extensions/spec.html

Each optional package or application consists of at least one JAR file containing an optional manifest, code and assorted resources. As described below, this primary JAR file can also include additional information in its manifest to describe dependencies on other JAR files. The jar command line tool included with the Java 2 SDK provides a convenient means of packaging optional packages, although any ZIP-compatible archiving tool should work as well.

由官方規格即知jar檔只要是ZIP相容即可,基本上我都用7-zip壓
只是jar工具程式壓的可能包含了一些相依資訊
另外放在$/JAVA_HOME/jre/lib/ext的套件也是可以壓成jar或zip檔