Pack is a custom archive format for Java class files, developed by William Pugh . It exploits regularities in the Java class file format to achieve a high degree of compression, and is designed as an alternative to Jar files. Like Jar files, the Pack format has the advantage that a large number of files can be delivered in response to a single client request, meaning that the request latency cost is paid only once for the entire archive, rather than per-file. It shares the disadvantage that unneeded files may be sent, and that the files may not arrive in the correct order.
Pack must be downloaded prior to use. The decompressor requires 36 kilobytes when downloaded as a Jar file.
Pack's decompressor is slower than zlib's. On a 333 MHz Sun Ultra 5 workstation it can decompress about 75-120 kilobytes per second, limiting Pack's effectiveness for fast networks.