How Spring Boot Packages Your App Into One Runnable JAR File

Spring Boot allows developers to run an entire web application with a single command by packaging all dependencies into one executable file, commonly called a fat jar. Standard Java cannot natively load classes from jars nested inside other jars, so Spring Boot's build plugin uses a custom launcher called JarLauncher to handle this limitation. Unlike older uber-jar approaches that merged all class files into a flat structure, Spring Boot keeps each dependency as an intact jar under BOOT-INF/lib, avoiding file-path conflicts between libraries. The fat jar and layered jar are the two main packaging options, each affecting how the application runs and how efficiently it can be deployed. Spring Boot's DevTools complement this setup by speeding up the code-and-test feedback loop during active development.