Java SE 5 and 6 comes already with a packing tool called "pack200". This tool is able to compress already existing jar files which can be then used for network distribution. I compressed the rt.jar with the following command:

C:\Java\jdk1.6.0\jre\lib>pack200 -J-Xmx256m rt.jar.gz rt.jar

参数-J-X是必须的，否则你可能会收到一个OutOfMemoryError的错误，因为这工具本身也是用Java语言写的，来看看结果吧，神奇的结果：

Origin size: 43.8 MB
Compressed size: 5.81 MB

要解压的话可以使用工具"unpack200"

The tool pack200 is especially interesting for the distribution of WebStart applications to achieve faster download times. There is already one another well known sample - glassfish. In the second installation stage, glassfish unpacks internal libraries using the unpack200 tool...

认真的看看最后这段话便可知这工具有何用途：）