What is the PACK file format?
A .pack file is a Pack200 data archive container used for storing compressed JAR files for easier network download. .pack files are typically saved with an additional .gz suffix - the standard delivery bundle is named .jar.pack.gz and produced by Java's pack200 command-line tool.
A JAR is a ZIP archive that also serves as the executable format for the Java virtual machine. Pack200, standardised in 2004 under JSR 200, re-encodes a JAR's class-file constant pools using a specialised binary algorithm; combined with GZIP, a .pack.gz file can be up to 60% smaller than the original JAR and hence significantly faster to download - the primary goal of the format for Java Web Start and browser applet delivery.
Pack200 is now obsolete. Oracle deprecated the pack200 and unpack200 command-line tools in Java 11 (JEP 336, 2018) and removed them entirely in Java 14 (2020). Legacy .pack or .pack.gz files can only be decompressed using a JDK 8-13 environment; the raw .pack stream begins with the magic number 0xCAFED00D.
Security & safety
RISK: MEDIUMA .pack itself is inert data, but it decompresses to a Java JAR containing executable bytecode - only unpack and run JARs from sources you trust, since a malicious JAR can run arbitrary code under the JRE. Pack200 was a known historical attack surface (CVEs in the unpacker), so use a current JDK build when unpacking. Avoid third-party "PACK opener" download portals; use Oracle/Adoptium JDKs for unpack200 instead.
Format details
in a nutshell- Git packfile (.pack) - Inside .git/objects/pack, Git stores objects in binary .pack files (with a .idx index); managed by Git, never opened by hand.
- Minecraft / game resource pack (.pack) - Some games and modpacks use .pack for bundled resources or texture packs; structure is engine-specific (often ZIP-based).
- Skype / app data pack (.pack) - Various applications write internal '.pack' data blobs; meaning depends entirely on the program.
Programs that open PACK files
Windows3 apps
7-Zip
Open-source Opens the outer GZIP of a .pack.gz (giving the raw .pack stream); the Pack200 layer still needs unpack200 to become a JAR. 
unpack200 (JDK 8-13)
Free Run: unpack200 app.pack app.jar (decompress .pack.gz first if needed). Tool exists only in JDK 8 through 13.
Adoptium Temurin JDK 8/11
Open-source Install a free OpenJDK 8 or 11 that still includes unpack200, then run unpack200 on the file. 
macOS1 app
unpack200 (JDK 8-13)
Open-source Install Temurin JDK 8/11 and run unpack200 app.pack app.jar in Terminal. 
Linux2 apps
unpack200 (OpenJDK 8-13)
Open-source unpack200 app.pack app.jar - packaged with OpenJDK 8/11 (e.g. apt install openjdk-11-jdk-headless).
Java Web Start (IcedTea-Web)
Open-source Open-source Web Start replacement that still handles Pack200-compressed JNLP resources. Technical details
deep spec| Format type | Binary Pack200 compressed-JAR container |
| Magic number | 0xCAFED00D at byte offset 0 in the raw .pack stream; GZIP header 0x1F8B at offset 0 in the common .pack.gz form |
| Byte order | Big-endian |
| Compression | Pack200 constant-pool re-encoding algorithm; outer GZIP/DEFLATE applied in the standard .pack.gz delivery form |
| Compression ratio | Up to ~60% smaller than the equivalent plain-GZIP-compressed JAR |
| MIME type | application/x-java-pack200 |
| Specification | JSR 200 - Network Transfer Format for JAR Files (Java SE 5.0, introduced 2004) |
| Max archive entries | Inherits the JAR/ZIP 65,535-entry ceiling |
| Encryption | None; existing JAR code-signing signatures are preserved and round-tripped by pack200/unpack200 |
| Integrity checking | GZIP CRC-32 on the .pack.gz outer wrapper; JAR signatures verified independently after unpacking |
| Typical file size | 50 KB - 20 MB (always smaller than the source JAR it encodes) |
| Status | Deprecated in Java 11 (JEP 336, 2018); pack200 and unpack200 tools removed in Java 14 (2020) |
| Unpack tools | unpack200 (JDK 8-13 only); also 7-Zip on Windows for inspection |
| Primary use case | Minimising JAR download size for Java Web Start applications and browser applets |
| Compatible platforms | Windows, Linux, macOS - any platform with JDK/JRE 8-13 installed |
| Released | 2004 (Java SE 5.0 / J2SE 1.5 - JSR 200 'Network Transfer Format for JAR Files') |
| Latest version | Pack200 as specified through JSR 200; deprecated in Java 11 (2018), removed in Java 14 (2020) |
| Open standard | Yes · royalty-free |
| Specification | docs.oracle.com |
PACK conversions
Community Q&A
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