What is a Fat JAR and When Should You Use One?

Understanding the Concept of Fat JAR

Fat JAR is a type of Java archive file commonly used for deploying Java applications. Unlike standard JARs that only package class files, fat JARs also embed the Java Runtime Environment (JRE) or a subset of it, making it a stand-alone deployment package.

Types of JAR Files

In the context of Java packaging, there are several different types of JARs that vary in their contents:

• Skinny JAR: Contains only the source code of the application and no dependencies.
• Thin JAR: Includes the application code along with its direct dependencies (e.g., third-party libraries, database drivers).
• Hollow JAR: Provides the runtime environment but does not contain the application itself.
• Fat/Uber JAR: Embeds the application code, its dependencies, and the necessary runtime components to run the application independently.

Benefits of Fat JARs

Fat JARs offer several advantages over other packaging methods:

• Self-contained: By bundling all necessary components, fat JARs eliminate the need for separate JRE or application server installations.
• Simplified deployment: They enable easy deployment of applications to any machine that supports Java, regardless of the JRE version installed.
• Reduced dependencies: Fat JARs reduce the risk of runtime errors caused by version conflicts or missing dependencies.

When to Use Fat JARs

Fat JARs are particularly suitable for deploying applications:

• That need to be run independently on different machines
• That have complex dependencies that can be difficult to manage individually
• Where a lightweight and easy-to-deploy solution is desired

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