HubSpot’s backend services are almost all written in Java. We have over 1,000 microservices constantly being built and deployed. When it comes time to deploy and run one of our Java applications, its dependencies must be present on the classpath for it to work. Previously, we handled this by using the maven-shade-plugin to build a fat JAR. This takes the application and all of its dependencies and bundles them into one massive JAR. This JAR is immutable and has no external dependencies, which makes it easy to deploy and run. For years this is how we packaged all of our Java applications and it worked pretty well, but it had some serious drawbacks.