What is a Fat JAR and How Does It Simplify Java Deployment?

Demystifying Fat JARs

In the realm of Java deployment, the term "fat JAR" is frequently encountered. But what exactly does it signify?

Defining a Fat JAR

A fat JAR, also known as an Uber JAR, encapsulates the following components:

• Your source code
• Direct dependencies of your application (e.g., database drivers, libraries)
• Additional bits required to run the application independently

Unlike a "skinny" JAR, which contains only your source code, and a "thin" JAR, which includes direct dependencies, a fat JAR comprises a complete standalone execution environment.

Hollow vs. Fat JARs

The inverse of a thin JAR is a "hollow" JAR. It contains the runtime components needed to execute an application, but not the application itself. This approach resembles traditional Java EE application servers, but has distinct characteristics.

In contrast, a fat JAR combines the components of a thin JAR with the runtime environment, creating a self-contained package.

Real-World Applications

Fat JARs are beneficial in scenarios where you need to:

• Deploy a lightweight standalone application
• Run applications on machines without pre-installed Java environments
• Simplify deployment by bundling all necessary components into a single artifact

By offering a single, portable package, fat JARs streamline deployment and reduce the burden of managing multiple dependencies.

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