Oracle Pack200 packed JAR archive
File type: Archive and compressed file
A jar.pack file is a JAR archive compressed with Oracle Pack200. It contains a packed representation of the JAR file.
The format was used with Java Web Start to reduce the size of JAR files distributed over a network.
File type category:
Archive and compressed file
Date updated: October 17, 2026
Pack200 was an Oracle Java technology for compressing JAR archives, particularly those containing Java class files. Its compression efficiency varied with the size and contents of the JAR.
For a JAR containing only class files and measuring several megabytes, Pack200 could reduce the packed file to about one-ninth of the original JAR size.
Java Web Start used Pack200 files as downloadable application resources and was available across multiple platforms. Pack200 and Java Web Start are no longer included in current Java distributions, so this format is now obsolete.
Use the Pack200 unpacking tool, unpack200, from an older compatible Java installation to restore the original JAR archive. Java Web Start could also process packed application resources automatically when supported by the installed Java environment.
After unpacking, open or run the resulting JAR with compatible Java software. Current Java installations generally do not include Pack200, so a legacy Java environment or another compatible Pack200 implementation may be required.
Convert a jar.pack file by unpacking it with unpack200. The resulting standard JAR can then be stored or distributed using another archive or compression utility.
Creating a new jar.pack file requires a compatible Pack200 packing tool. Current Java distributions generally do not provide one, so conversion back to Pack200 is not usually practical.
Software applications that can open jar.pack files:
Multiplatform
Language
