Help with JAR files (Java ARchive)

Jar files are much like ZIP files. The ZIP file format is used to compress a number of files into 1 file (an archive). This is useful so that one need not worry about transfering or copying a number of .class or .java files but rather create 1 archive with all of the files contained (and compressed) into it.

Compressing several files into a JAR file

jar cf archive_file_name.jar file_to_be_put_in_archive1 file_to_be_put_in_archive2 file_to_be_put_in_archive3

Example:

jar cf lab7.jar StackV.java StackFrame.java StackL.java

Creates an archive file named lab7.jar that contains each of the files StackV.java, StackFrame.java, and StackL.java.

jar is the command to run the jar utility.

cf are the 2 options to use with the jar utility.

c is the option to create a new archive with the archive_name.jar (given) and f is the file name specified.

Here is a list of commonly used options with jar

-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.

Example

Assume there is a Java application consisting of three source files that need to be distributed:

File1.java
File2.java
File3.java

We will name the JAR file test.jar. To make a JAR file with just File1.java:
jar cf test.jar File1.java

To make a jar file named test.jar with all three files listed separately:
jar cf test.jar File1.java File2.java File3.java

To make a file with all java files using a pattern match:
jar cf test.jar *.java

This assumes that the source files are in the same directory you are running the jar command in and they are the only .java files in the directory (the * will put ALL files ending in .java in the jar).

To 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. For example, File3 from the above example has the main method we want to execute. Create a text file called "mainClass.txt" with the following text:

Main-Class: File3

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 "File3" and NOT "File3.java" (the file containing the source code) or "File3.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").

Now run the jar utility with this command line:

jar cmf mainClass.txt example.jar *.class

With this line, jar will 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

To run an executable JAR from the command line:

java -jar example.jar