What is a fat JAR?

java

maven

gradle

dependency-management

byAnton Shumikhin·Mar 10, 2025

A fat JAR is an independent JAR file with all the classes, resources of your app and its dependencies for a friction-free execution environment. Employ Maven with maven-assembly-plugin or Gradle with shadowJar to construct a fat JAR.

Maven snippet:

<!-- Consider this "All you can pack" Maven airlines -->
<plugin>
    <artifactId>maven-assembly-plugin</artifactId>
    <configuration>
        <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
        </descriptorRefs>
    </configuration>
</plugin>

Gradle snippet:

// Plugin to the rescue! Pack light, or rather...pack large with shadowJar
plugins {
    id 'com.github.johnrengelman.shadow' version '5.2.0'
}

Name your Main-Class in the manifest, build, and sprint with java -jar your-fat-jar.jar. Easy-peasy!

Varieties of the JAR-kind

Without knowing your JAR types, you'd be packaging blind. A fat JAR is the omnibus of Java archives, a complete package ready for deployment. Its understudies, perform more specific roles:

Skinny JARs are the minimalists, containing only your classes, and shunning external libs.
Thin JARs bundle your classes and their significant others, the direct dependencies.
Hollow JARs are minimalist runtimes, yearning for more JARs to fulfill their purpose.

Each JAR serves its niche, and picking the right one can streamline your development and deployment.

Advantages of deploying fat JARs

Deploying fat JARs simplifies development, you also skip out on dependency conflicts and the infamous ClassNotFoundException associated with classpath mishaps. Here, you nurture a self-contained environment. This approach benefits:

Banishing of version conflicts with dependencies.
Assurance of consistency across different environments.
A user-friendly experience with no extra setup needed for users.

Building a fat JAR

For Maven users, your best tools are maven-assembly-plugin and maven-shade-plugin. The first allows configurability through assembly descriptors, while the second neatly repacks your JAR alongside its dependencies and releases a version ready for distribution.

Gradle enthusiasts can employ shadowJar, which blends project artifacts and dependencies with nifty conflict resolution. Furthermore, Gradle's zipTree() function provides custom Gradle tasks to assemble fat JARs.

Problems and Solutions with fat JARs

Sure, fat JARs could have a few issues:

Larger size: Can lead to longer startup times and distribution difficulties.
Dependency conflicts: Redundant classes due to dependencies overlapping.

Here's how you can overcome:

Use a tool like ProGuard to minimize JAR size by removing unused classes.
Practice class shading to prevent conflicts by adjusting package names within the JAR.

Fat JAR: best practices

Following best practices ensures you get the most from fat JARs:

Regularly update dependencies to avoid security threats.
Understand the nuanced relationship between Class Loaders and JAR architecture types.
Leverage continuous integration systems to automate builds and provide consistency.

Another name: Uber JAR

The term uber JAR might be used in place of fat JAR and emphasizes its superiority and self-sufficient nature. If you prefer a deep dive, visit imagej.net for an imagej-full of uber JAR knowledge.

explain-codes / Java / What is a fat JAR?

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