How to Create and Use JAR Files in Java

For convenient application distribution, Java provides a mechanism called JAR (Java Archive) files. JAR files are used for archiving and compressing data.

Basic command format for creating a JAR file is as follows:

jar -cf jar-file input-file(s)

The generated JAR file will be placed in the current directory.

Let's examine the keys and arguments used in this command:

• The flag c indicates that a JAR file should be created.
• The flag f specifies that the output should be directed to a file rather than standard output.
• jar-file is the name to be assigned to the resulting JAR file. You can use any name for the JAR file. By convention, the JAR file name is given a .jar extension, though this is not mandatory.
• The argument input-file(s) is a space-separated list of one or more files you want to include in your JAR file. The input-file(s) argument may also contain a wildcard *. If any of the input files are directories, their contents are recursively added to the JAR archive.

Rules regarding JAR file structure

• The jar command automatically creates the META-INF directory.
• The jar command automatically generates the MANIFEST.MF file and places it in the META-INF directory.
• The exact directory structure is preserved.
• The java and javac commands can use a JAR file just like a normal directory tree.
• Searching for JAR files using the -cp flag is similar to searching for package files. The only difference is that the path to the JAR file must include the JAR file name (e.g., classes/project1.jar).

Let's see an example of how JAR files are created and used.

Suppose our application has the following directory structure:

We will create a JAR file named project1.jar, which will contain the first and second packages:

cd project1/classes
jar –cf project1.jar first second

We can view the contents of the project1.jar file using the following command:

jar -tf project1.jar

The output will be approximately as follows:

META-INF/
META-INF/MANIFEST.MF
first/
first/Example1.class
second/
second/Example2.class

Now, we will move the created project1.jar file to the lib directory and run the first.Example1 program:

cd lib
java -cp project1.jar first.Example1