Make A JAR File

JAR files are Java's version of ZIP files. In fact, JAR uses the ZIP file format. There are two main uses for JAR files which I shall explain here. The first use is to compress (make a smaller size) a number of files into one file (archiving). The second use is to make a Java executable JAR file.
  • Compress Files To A Java Archive (JAR)
  • Common Examples
  • Compress Files To An Executable Java Archive (JAR)
  • Create An Executable JAR
  • Running An Executable JAR From Command Line
  • Running An Executable JAR From Explorer (Windows)
  • Further Resources

Compress Files To A Java Archive (JAR)

This is by far the most common use for JAR files: to compress multiple files into a single JAR archive. JAR files can be opened with WinZip or WinRar. In terms of Java applications, the ability to archive any number of source or class files into one single archive represents the biggest advantage - distributing one file containing hundreds of files is so much easier than distributing hundreds of files separately!
The jar utility program is run from the command line (DOS prompt or bash for example, depending on your OS). Here is how to create a compressed JAR file:
   jar cf archive_name.jar files
Let's look at each part of that command line.
The command to run the jar utility.
Create a new archive with the file name specified. These two options are from this list of common options:
- c create new archive
- t list table of contents for archive
- x extract named (or all) files from archive
- u update existing archive
- v generate verbose output on standard output
- f specify archive file name
- m include manifest information from specified manifest file
- 0 store only; use no ZIP compression
- M do not create a manifest file for the entries
- i generate index information for the specified jar files
- C change to the specified directory and include the following file
Multiple options can be used together. They all must appear after the "jar" command with no white space separating them.
Name of the JAR file. This can only be included if you use the 'f' option.
Names of all the files you want to put in the jar file. This could be just one name or a list of multiple names separated by space. Names can use pattern matching characters to match multiple files.

Common Examples

Let's say I have a Java application consisting of three source files that I wish to distribute:
I also want to call my JAR file example.jar. To make a JAR file with just
   jar CF example.jar
To make a file with all three files listed separately:
   jar CF example.jar
To make a file with all three files using a pattern match:
   jar CF example.jar *.java
It goes (almost) without saying that the source files are in the same directory you are running the jar command in.

Compress Files To An Executable Java Archive (JAR)

It is also possible to make an archive that can be executed (run) by Java and even by a double click through your OS, if you have it set up correctly. Of course, to do this you must store compiled class files in the archive, as well as or instead of Java source files, since Java source files cannot be run!
Continuing the example above, I now compile my Java source files:
   javac *Java

Create An Executable JAR

All JAR files contain something called a manifest file which holds information Java wants to know. One piece of information a manifest file may contain is the name of a class that will be run if the JAR file is executed.
The first thing you must do is create a text file that lists the "main" class - the class that has the main method you want executed when the JAR is executed. Let's say that Three from the above example has the main method I want executed. I create a text file called "mainClass.txt" with the following text:
Main-Class: Three
IMPORTANT: the text file only needs the one line of text for this purpose. However, the file must end with a blank line or this will not work, ie the file has two lines in it - the second one is empty. Note too the class is called "Three" and not "" (the file containing the source code) or "Three.class" (the file containing the byte codes). If your class file is in a package hierarchy, you must use the fully qualified name of the class (eg "myPackage.MyClass").
I then run the jar utility with this command line:
   jar cmf mainClass.txt example.jar *.class
With this line, I told jar to create a JAR file (option c) with modifications to the manifest file (option m) as specified within mainClass.txt, naming the JAR file (option f) as example.jar and including everything that matches the pattern *Class

Running An Executable JAR From Command Line

I can run it from the command line like this:
   java -jar example.jar

Running An Executable JAR From Explorer (Windows)

Within Windows, it is also possible to set up Windows Explorer so that you can double click on the JAR file icon to execute the file (handy for GUI applications). The first thing you must do is set up the correct association with the 'javaw.exe' application that JDK for Windows will have. Click here for an older example with pictures! Open Windows Explorer and select Tools | Folder Options | File Types.
If there is no JAR file type, create it. Give it a description like
   jar - Executable Jar File
to ensure it sorts under 'jar'. Create or edit the action called "open" and associate it with the following action:
   "C:\Java\jdk1.4.0\bin\javaw.exe" -jar "%1"
Of course, you will replace "C:\Java\jdk1.4.0\bin\javaw.exe" with whatever path is correct on your machine.
IMPORTANT: include the double quotes to take care of names with spaces.
If you are using something other than Windows and you know how to set up an association in your OS, please contact me.

Further Resources

Java's Tutorial on Jar files:
Roedy Green's JAR file page:


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