M04 Q13 What is a fat jar? How is it different from the original jar?

Fat jar, also called “executable jar”, is a jar that contains compiled code for your application and also all dependencies. Spring Boot uses nested jars approach, that means that fat jar contains all dependencies as nested jars. This differs from other approach, which is uber jar that packs all dependencies into single jar archive. Uber jar approach is problematic because it is hard to see application dependencies and also causes issues when same filename in the same context is used in different jars.

Fat jar is often called “executable jar” because Spring Boot will generate MANIFEST.MF file which contains Main-Class and Start-Class entries together with JarLauncher code. This manifest together with launcher code will be used to execute standalone jar.

To create fat jar in your project, you need to use spring-boot-maven-plugin. Executing application is as simple as executing one command:

java -jar spring-boot-application-1.0-SNAPSHOT.jar

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <executions>
                    <execution>
                        <goals>
                            <goal>repackage</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
    

When repackage stage will be executed the spring-boot-maven-plugin will take original jar and put it inside "fat" jar.

The differences in comparison to original jar are following:

• Original jar does not contain all dependencies
• Original jar is not executable by default