Using jar files

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Using jar files

• Dave Elliman

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Why jar

• When you program in Java you find the number of
  class files grows rapidly
• Computer programmers are tidy people
• Collect them in jar
• jar cvf myclasses.jar .class
• A jar compresses the files too
• A jar can be signed and verified beyond PRG.

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Things to look out for

• You need to be able to find the jar command
  your PATH should have something like
• PATHC\Program Files\Java\jdk1.5.0_05\bin
• Note and \ on Windows and / on unix
• Jar cvf is just like tar cvf on unix

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When do I make a jar?

• A good time is when some classes are stable (they
  work!)
• You can have several jars
• It can be helpful to look inside a jar this
  lists the files
• Jar tvf myfiles.jar
• Or use WinRar
• www.win-rar.com - 29

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Where do I put a jar file?

• In the classpath.
• CLASSPATHCmyhome\libs\myfiles.jar ..
• The lazy-mans way is
• drop it in
• C\Program Files\Java\jre1.5.0_05\lib\ext

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Another good reason for using a jar

• You can package your application in a jar and
  make it run directly from the jar
• You have to tell the jar file which class has
  main()
• Create Manifest.txt which contains
• Main-Class MyPackage.MyClass

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Then

• Just include Manifest.txt in the list of files
  going into the jar
• Then you can run it as
• java jar myfiles.jar
• Or in Windows you could set up java jar as the
  default command for .jar files
• Explorer-Tools-Folder Options
• Double click on the jar file and it runs

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Or

• Build it in Visual J and you get a .exe file
  that runs
• But that is not real Java

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How do I extract stuff from a jar

• jar xvf myfiles.jar
• Again just like tar xvf
• There is a u flag for update but I never bother.
  Delete it and create it again.