Difference Between jar Files Generated by Gradle "bootJar" and "jar" Tasks

The bootJar Task Creates an Executable jar File

Run the bootJar task and inspect the generated file with the tree command.

The tree command must be installed separately, so if it is not available, install it with Homebrew.

$ brew install tree 

The file generated by the bootJar command is named {project-name}.jar under {project-dir}/build/libs/.

Unzip the jar file and run the tree command as follows.

% tree -L 3 --filelimit 10
.
├── BOOT-INF
│   ├── classes
│   │   ├── META-INF
│   │   ├── application.yml
│   │   ├── com
│   │   │   └── devkuma
│   │   │       └── api
│   │   └── logback-spring.xml
│   ├── classpath.idx
│   ├── layers.idx
│   └── lib  [294 entries exceeds filelimit, not opening dir]
├── META-INF
│   ├── MANIFEST.MF
│   └── spring-configuration-metadata.json
└── org
    └── springframework
        └── boot
            └── loader  [19 entries exceeds filelimit, not opening dir]

There are BOOT-INF, META-INF, and the loader directory that starts the jar.

The bootJar task creates an executable jar file, so it is often used for application deployment and similar tasks.

The jar Task Creates Class Files

Run the jar task and inspect the generated file with the tree command.

The generated file is named {project-name}-plain.jar under {project-dir}/build/libs/.

% tree -L 5 --filelimit 10
.
├── META-INF
│   ├── MANIFEST.MF
│   └── spring-configuration-metadata.json
├── application.yml
├── com
│   └── devkuma
│       └── api
│           ├── DevkumaApiApplication.class
│           ├── config
│           ├── controller
│           ├── param
│           ├── service
│           └── support
└── logback-spring.xml

Only META-INF and class files are generated.

This file cannot be run as a standalone application, but it can be used when distributing a library.

The build Task Creates Two jars

When you run the build task, both {project-name}.jar and {project-name}-plain.jar are created.

As mentioned above, application deployment requires an executable jar file, so plain.jar is not useful for that purpose.

If it is not specifically needed, it is also a good idea to configure build.gradle so that it is not generated.

Prevent plain.jar from Being Generated in build.gradle.kts

tasks.getByName("jar") {
    enabled = false
}

Prevent plain.jar from Being Generated in build.gradle

jar {
    enabled = false
}

With only this setting, plain.jar will not be generated even when the build task is run.

It will also not be generated when the jar task is run.

Summary

• The bootJar task creates an executable jar file.
• The jar task creates class files.
• The build task creates both jars.
• For ordinary deployment, use the executable {project-name}.jar.

If you need a jar for application deployment, run the bootJar or build task and use the generated jar file.