A Spring Boot repackaged jar contains your compiled application classes under `BOOT-INF/classes`, all dependency jars nested (not flattened) under `BOOT-INF/lib`, and a special `org.springframework.boot.loader.JarLauncher` as its main class. Running `java -jar` invokes JarLauncher, which sets up a custom classloader capable of loading classes directly from those nested jars, then hands off control to your actual `@SpringBootApplication` main method — solving the 'jars can't nest inside jars' limitation of the standard JVM classloader.

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#Executable Jar#JarLauncher

What is inside a Spring Boot executable jar, and how does `java -jar app.jar` actually run it?

This Custom Loader Is Why a Naive Class-Path Extraction Doesn't Just Work

You can't simply unzip a Spring Boot jar and run `java -cp` against the extracted files directly without extra classpath setup — the nested-jar structure and JarLauncher's custom classloading are specifically what let the standard java command load a jar containing other jars, which regular java -cp classpath resolution doesn't support on its own.

BOOT-INF/classes (your code) + BOOT-INF/lib (nested dependency jars) + JarLauncher main class — custom classloader loads it all