Working with application JAR files

A JAR (Java Application aRchive) is a common way to package and distribute Java applications and libraries. It includes Java class files, resources, and other files.

Working

Working with JAR files includes various tasks, such as

• Creating JAR file.

• Viewing JAR file content.

• Extracting files from JAR.

• Executing JAR file.

• Updating JAR file content.

We can implement these tasks using jar commands in cmd, but to use jar, we need to install JDK (Java Development Kit).

We can download JDK from here.

Let's explore these tasks in detail.

Creating JAR file

We can create a JAR file by running the following command.

Arguments

The arguments used in the command are described below.

• c indicates that we want to create JAR file.

• f indicates that we want to create a file, not just to show output in stdout.

• jar-file can be any name that we want to give to the JAR file.

• input-file(s) can be a single or multiple files separated by space that we want to include in JAR file.

Options

We also have some arguments we can include in our command instead of cf or in addition to them without any space. These options are:

• v creates an output on stdout showing the name of each file while the JAR file is being built.

• 0 (zero) indicates that we don't want the JAR file to be compressed.

• M indicates that default manifest file should not be produced.

• m indicates that we want to include information from the previous existing manifest file in the new manifest file. Its format is:

Viewing JAR file content

We can view the JAR file content by running the following command.

Arguments

• t indicates that we want to view the JAR file's content table.

• f indicates the JAR file whose content is to be viewed.

• jar-file indicates the path or the name of the JAR file to be viewed. If we replace it with a JAR file name such as "myApplicatiom.jar" then it will show the content of the file to stdout as:

Updating JAR file content

We can update the JAR file content by running the following command.

After making a few changes, such as changing jar-file to "myApplicatiom.jar" and input-file(s) to "images/newfile.gif" and then running the above command, a file name "newfile.gif" will be added to the JAR file in the images folder, and the table of content is shown on the command line as:

Arguments

• u indicates that we want to update the JAR file.

• f indicates the JAR file whose content is to be viewed.

• jar-file is the existing JAR file that we want to update.

• input-file(s) is the file name to be added; we can also write multiple files separated by space.

Extracting files from JAR

We can extract the JAR file content by running the following command.

If we run the above command after making a few changes, such as jar xf myApplication.jar, it will extract all the files of the JAR file into the same directory.

Arguments

• x option indicates that we want to extract files from the JAR file.

• f indicates the JAR file whose content is to be viewed.

• jar-file is the existing JAR file that we want to update.

• archived-file(s) is an optional argument consisting of the number of files separated by spaces we want to extract from the JAR file.

Executing JAR file

We can execute the JAR file by running the following command.

Arguments

• -jar indicates that the application is in JAR format.

• jar-file is the existing JAR file that we want to execute.

We must also specify which class is the entry point to run the application, so we must add a Main-Class header to the JAR file's manifest.mf file.