Java Archive (JAR) Files

Expert View

As a software engineer, I often rely on JAR files to bundle and distribute my Java applications efficiently. JARs (Java Archives) act as portable containers that hold compiled classes, metadata, resources, and libraries — making deployment simple and modular. They help achieve reusability and scalability across multiple projects.

Though the topic of JARs is not included in the syllabus, I thought it’s better to give a brief introduction about this. Understanding JAR files builds a bridge between your knowledge of packages and the real-world deployment of Java software.

Introduction to JAR Files

A JAR (Java Archive) file is a compressed file (similar to ZIP) that aggregates multiple .class files, package directories, and resources like images, configuration files, and metadata (MANIFEST.MF). JARs make distribution and reuse of Java programs much easier.

Relation Between Packages and JARs

Packages provide the logical organization of Java classes, while JAR files provide the physical packaging for distribution. You can think of a JAR as a “portable folder” containing your entire package structure, enabling seamless code sharing and reuse.

Creating and Exporting JAR Files

To create a JAR file, use the jar tool:

jar cf MyLibrary.jar com/nbkrist/cse/*.class

• c – create a new archive
• f – specify the output filename
• MyLibrary.jar – the final archive
• com/nbkrist/cse/*.class – your compiled class files

Importing and Using JAR Files

After creating a JAR, it can be added to your project or executed with the -cp (classpath) option:

java -cp .;MyLibrary.jar com.nbkrist.cse.MainApp

In IDEs like Eclipse or IntelliJ, add JARs via “Project Properties → Libraries → Add External JARs”.

Setting JAR Files in the Classpath

Set the classpath so the JVM can locate your JARs automatically:

set CLASSPATH=C:\MyJavaLibs\MyLibrary.jar;%CLASSPATH%