This article will demonstrate how to start a Spring Boot application from another Java program. A Spring Boot application is typically built into a single executable JAR archive. It contains all dependencies inside, packaged as nested JARs.

Likewise, a Spring Boot project is usually built as an executable JAR file by a provided maven plugin that does all the dirty work. The result is a convenient, single JAR file that is easy to share with others, deploy on a server, and so on.

Starting a Spring Boot application is as easy as typing java -jar mySpringProg.jar, and the application will print on console some nicely formatted info messages.

But what if a Spring Boot developer wants to run an application from another Java program, without human intervention?

How Nested JARs Work

To pack a Java program with all dependencies into a single runnable JAR file, dependencies that are also JAR files have to be provided and somehow stored inside the final runnable JAR file.

“Shading” is one option. Shading dependencies is the process of including and renaming dependencies, relocating the classes, and rewriting affected bytecode and resources in order to create a copy that is bundled alongside with an application’s (project) own code.

Shading allows users to unpack all classes and resources from dependencies and pack them back into a runnable JAR file. This might work for simple scenarios, however, if two dependencies contain the same resource file or class with the exact same name and path, they will overlap and the program might not work.

Spring Boot takes a different approach and packs dependency JARs inside runnable JAR, as nested JARs.

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