The jar Utility

The jar utility provided with the JDK is a simple tool for creating and reading JAR files. Its user interface isn’t particularly friendly. It mimics the Unix tar (tape archive) command. If you’re familiar with tar, you’ll recognize the following incantations:

 jar -cvf jarFile path [ path ] [ ... ]  

Create jarFile containing path(s).

 jar -tvf jarFile [ path ] [ ... ] 

List the contents of jarFile, optionally showing just path(s).

 jar -xvf jarFile [ path ] [ ... ]

Extract the contents of jarFile, optionally extracting just path(s). In these commands, the flag letters c, t, and x tell jar whether it is creating an archive, listing an archive’s contents, or extracting files from an archive. The f means that the next argument is the name of the JAR file on which to operate. The optional v flag tells jar to be verbose when displaying information about files. In verbose mode, you get information about file sizes, modification times, and compression ratios. Subsequent items on the command line (i.e., anything aside from the letters telling jar what to do and the file on which jar should operate) are taken as names of archive items. If you’re creating an archive, the files and directories you list are placed in it. If you’re extracting, only the filenames you list are extracted from the archive. (If you don’t list any files, jar extracts everything in the archive.) For example, let’s say we have just completed our new game, spaceblaster. All the files associated with the game are in three directories. The Java classes themselves are in the spaceblaster/game directory, spaceblaster/images contains the game’s images, and spaceblaster/ docs contains associated game data. We can pack all this in an archive with this command:

 % jar -cvf spaceblaster.jar spaceblaster

Because we requested verbose output, jar tells us what it is doing:

 adding:spaceblaster/ (in=0) (out=0) (stored 0%)
adding:spaceblaster/game/ (in=0) (out=0) (stored 0%)
adding:spaceblaster/game/Game.class (in=8035) (out=3936) (deflated 51%)
adding:spaceblaster/game/Planetoid.class (in=6254) (out=3288) (deflated 47%)
adding:spaceblaster/game/SpaceShip.class (in=2295) (out=1280) (deflated 44%)
adding:spaceblaster/images/ (in=0) (out=0) (stored 0%)
adding:spaceblaster/images/spaceship.gif (in=6174) (out=5936) (deflated 3%)
adding:spaceblaster/images/planetoid.gif (in=23444) (out=23454) (deflated 0%)
adding:spaceblaster/docs/ (in=0) (out=0) (stored 0%)
adding:spaceblaster/docs/help1.html (in=3592) (out=1545) (deflated 56%)
adding:spaceblaster/docs/help2.html (in=3148) (out=1535) (deflated 51%)

jar creates the file spaceblaster.jar and adds the directory spaceblaster, adding the directories and files within spaceblaster to the archive. In verbose mode, jar reports the savings gained by compressing the files in the archive. We can unpack the archive with this command:

 % jar -xvf spaceblaster.jar  

Likewise, we can extract an individual file or directory with:

 % jar -xvf spaceblaster.jar filename  

But, of course, you normally don’t have to unpack a JAR file to use its contents; Java tools know how to extract files from archives automatically. We can list the contents of our JAR with the command:

 % jar -tvf spaceblaster.jar  

Here’s the output; it lists all the files, their sizes, and their creation times:

 0 Thu May 15 12:18:54 PDT 2003 META-INF/  
 1074 Thu May 15 12:18:54 PDT 2003 META-INF/MANIFEST.MF  
 0 Thu May 15 12:09:24 PDT 2003 spaceblaster/  
 0 Thu May 15 11:59:32 PDT 2003 spaceblaster/game/  
 8035 Thu May 15 12:14:08 PDT 2003 spaceblaster/game/Game.class  
 6254 Thu May 15 12:15:18 PDT 2003 spaceblaster/game/Planetoid.class  
 2295 Thu May 15 12:15:26 PDT 2003 spaceblaster/game/SpaceShip.class  
 0 Thu May 15 12:17:00 PDT 2003 spaceblaster/images/  
 6174 Thu May 15 12:16:54 PDT 2003 spaceblaster/images/spaceship.gif  
 23444 Thu May 15 12:16:58 PDT 2003 spaceblaster/images/planetoid.gif  
 0 Thu May 15 12:10:02 PDT 2003 spaceblaster/docs/  
 3592 Thu May 15 12:10:16 PDT 2003 spaceblaster/docs/help1.html  
 3148 Thu May 15 12:10:02 PDT 2003 spaceblaster/docs/help2.html