shadow

Shadow is how the JVM world ships a single runnable jar. It's the Gradle plugin for building fat/uber JARs: your code plus every dependency in one file, with package relocation to prevent version conflicts and optional R8/ProGuard shrinking. It's the Gradle counterpart to Maven's Shade plugin, free and open source.

Nothing to run; it's a build plugin. The one operational thing to know is the identity change: the plugin moved from the old com.github.johnrengelman.shadow coordinates to the GradleUp organization, so new builds should use com.gradleup.shadow to keep receiving fixes.

Anyone distributing JVM CLIs, Spark jobs, or plugins needs this or something like it. Relocation is the feature that separates it from a naive merged jar: two libraries demanding different Guava versions stops being your problem.

The catch: fat jars are a distribution choice with costs. Merged service files and duplicate resources need configuring, builds get slower, and Spring Boot apps don't need it because Boot's own packaging already does this job.