# How to reduce the Jar-File size with Java5 and 6

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:\Programme\Java\jdk1.6.0\jre\lib>pack200 -J-Xmx256m rt.jar.gz rt.jarThe flag -J-X.. is needed, because otherwise OutOfMemory exception can occur (pack200 is written in Java...)The results are amazing:Origin size: 43.8 MBCompressed size: 5.81 MBFor unpacking the tool "unpack200" can be used.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...