Pack200, specified in JSR 200[1] (J2SE 1.5), deprecated in JEP 336[2] (Java SE 11) and removed in JEP 367 (Java SE 14),[3] is a compacting archive format developed by Sun, capable of reducing JAR file sizes by up to a factor of 9,[4] with a factor of 3 to 4 seen in practice.
In HTTP compression, such content is specified by the Content-Encoding type pack200-gzip.
Pack200 modifies the contents of the Java class files in the JAR archive undergoing compression.
Transformations to the class files include merging constant pools and removing duplicated attributes.
[8] The format is not lossless; it is possible for a JAR file after unpacking to not be identical to how it was before packing.