JAR files in Java

A JAR (Java Archive) file is a platform-independent package file format used to bundle Java class files, metadata, and resources (such as images, text files, and configuration files) into a single compressed archive. JAR files simplify distribution, deployment and reuse of Java applications and libraries. In simple terms, a JAR file is a compressed (.zip-like) archive that contains: Compiled .class files, Resource files (images, audio, text, etc.) and Metadata such as the manifest file.

Note: Since JAR files follow the ZIP specification, tools like WinZip, WinRAR, or 7-Zip can be used to view or extract their contents. However, Java provides the jar tool, which is the recommended way to create and manage JAR files.

Features of JAR Files

• Combines multiple files into a single unit
• Reduces file size through compression
• Enables easy distribution and deployment
• Supports executable applications via manifest
• Platform independent

The jar tool is included with the JDK and is used to create, view, extract, update, and execute JAR (Java ARchive) files. Below are examples demonstrating common operations on JAR files using the jar tool.

1. Creating a JAR File

To create a JAR file, use the c (create) and f (file name) options.

Syntax:

jar cf jarfilename.jar inputfiles

Example: Assume a package named pack exists in C:\directory:

C:\> jar cf pack.jar pack

This command creates a JAR file named pack.jar containing the pack directory and its contents.

2. Viewing the Contents of a JAR File

To list the contents of a JAR file, use the t (table) option.

Syntax:

jar tf jarfilename.jar

Example:

C:/> jar tf pack.jar

META-INF/
META-INF/MANIFEST.MF
pack/
pack/Class1.class
pack/Class2.class

Understanding the Output

• META-INF/MANIFEST.MF: This is the manifest file, which stores metadata about the archive.
• pack/: Directory representing the Java package
• .class files: Compiled Java classes inside the package

Manifest File Notes

• Every JAR file contains only one manifest file
• Path is always:

META-INF/MANIFEST.MF

Used to store information such as: Main class, Classpath, Versioning details

3. Extracting a JAR file

To extract the contents of a JAR file, use the x (extract) option.

Syntax:

jar xf jarfilename.jar

Example:

C:\> jar xf pack.jar

This extracts the contents into the current directory, recreating the original folder structure.

4. Updating a JAR File 

To update an existing JAR file by adding or replacing files, use the u (update) option.

Syntax:

jar uf jarfilename.jar inputfiles

Example:

C:\> jar uf pack.jar NewClass.class

This command adds or updates NewClass.class in pack.jar.

5. Running a JAR file

A JAR file can be executed if it contains a Main-Class entry in its manifest file.

Syntax:

java -jar jarfilename.jar

Example:

C:\> java -jar pack.jar

The manifest must include:

Main-Class: fully.qualified.ClassName

Common JAR Options Summary

Related Article

• Working with JAR and Manifest files In Java
  
• What are zip files