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Revision as of 15:58, 20 April 2006 by Aniefer (Talk | contribs)

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Pack200 is a compression technology included in Java 1.5.0. It was designed for compressing jars and works most efficiently on Java class files. Using Pack200 compression can reduce the size of a jar by about 60%.

Pack200 is not a lossless compression. Packing and unpacking will produce a jar that is semantically the same as the original, but classfile structures will be rearranged; the resulting jar will not be identical to the original. However, this reordering is idempotent so a second pack-unpack will not further change the jar.


Signing a jar hashes the contents and stores the hash codes in the manifest. Since packing and unpacking a jar will modify the contents, the jar must be normalized prior to signing. Normalizing the jar will also be refered to as repacking the jar.

Jar Processor

External Links

Pack200 and Compression
JAR Packing tool
JAR Unpacking tool
JAR Signing and Verification tool

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