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JAR

What is a JAR?

A JAR (Java ARchive) is a package file format (based on ZIP) that aggregates compiled Java class files, associated metadata, and resources (text, images, configuration files, etc.) into a single distributable unit.

Thin JAR vs Fat (Uber) JAR

Thin JAR : Contains only the project’s compiled classes and resources.
Third-party dependencies are not included and must be supplied separately on the runtime classpath.
Thin JARs are small and are commonly used with application servers or environments that manage the classpath externally.

Fat (Uber) JAR : Bundles the project classes and all third-party dependencies into a single executable JAR.
This simplifies distribution (one file to run) but increases size and may introduce duplicate resources or class-path conflicts.
Commonly built using the Maven Shade Plugin or Maven Assembly Plugin.

Troubleshooting from Local Maven Runs

A minimal example exists under JAR/src/main/java/ (JarExample.java) and was used to build a thin JAR with Maven.
During experimentation, the following issues were encountered.

1) No Source Files → Empty JAR

If no .java files exist under src/main/java/, Maven reports “No sources to compile” and produces an effectively empty JAR.

  • No Sources to Compile
  • Empty JAR

Fix

  • Ensure all production sources are placed under src/main/java.

2) Invalid main Method Signature

A runnable JAR requires a valid entry point:

public static void main(String[] args)

If the signature is incorrect, the runtime cannot locate a proper entry point.

  • Define the main method

Fix

  • Use the exact main method signature shown above.

Understanding the target/ Directory

The target/ directory is Maven’s build output directory.
Everything inside it is generated, re-creatable, and must not be committed to version control.

Important

The target/ directory is created automatically when you run Maven goals such as: mvn compile, mvn package, or mvn install.

Common Contents

Path Purpose
target/classes/ Compiled .class files produced from src/main/java and copied resources from src/main/resources
target/generated-sources/annotations/ Java source files generated by annotation processors
target/maven-archiver/ Metadata describing the built artifact
target/maven-status/ Incremental build state used by Maven plugins
target/<artifactId>-<version>.jar Final packaged artifact (thin JAR by default)

target/maven-archiver/

This directory is created by maven-archiver, a utility library used internally by:

  • maven-jar-plugin
  • maven-war-plugin
  • maven-shade-plugin
  • spring-boot-maven-plugin

Contents

  • pom.properties; Stores Maven coordinates:
    • groupId
    • artifactId
    • version

Use cases

  • Allows tooling and runtime environments to identify:
    • Which project produced the artifact
    • The exact version of the artifact

Note

maven-archiver does not compile code and does not package classes. Its sole responsibility is to write metadata that can be inspected later.

How to run Thin JAR file

java -cp "target/jar-example-1.jar;C:\Users\<you>\.m2\repository\org\apache\commons\commons-lang3\3.14.0\commons-lang3-3.14.0.jar" JAR.JarExample

How this works?

-cp / -classpath

Classpath includes:

  • Compiled application JAR
  • External dependency JARs

Java then:

  • Loads JAR.JarExample
  • Sees StringUtils
  • Finds it in commons-lang3.jar
  • Executes main()

Why java -jar DOES NOT work for thin JARs?

Java behaviour

  • Ignores -cp
  • Uses ONLY what’s inside the JAR
  • Tries to run JarExample.java 👉 Dependency JARs are invisible → 💥 NoClassDefFoundError

Thin JAR File output

Fat (Uber) JAR using Maven Shade Plugin

A Fat JAR packages:

  • Application classes
  • All third-party dependencies
  • This allows the application to be executed using a single command.

What Happens During mvn package

  • maven-compiler-plugin compiles Java sources
  • maven-jar-plugin creates the original thin JAR
  • maven-shade-plugin
    • Renames the thin JAR to: original-jar-example-1.jar
    • Creates a new fat JAR: jar-example-1.jar
    • Embeds all dependency classes

Running the Fat JAR

java -jar target/jar-example-1.jar

  • ✔ No classpath required

  • ✔ All dependencies bundled

  • ✔ Single-file deployment

  • Fat JAR - output

Important

maven-jar-plugin is a default Maven plugin that runs automatically during the package phase for projects with:

<packaging>jar</packaging>

If packaging is not specified, Maven defaults to jar.