Understanding Spring Boot Executable JAR Packaging and the Role of JarLauncher

Spring Boot provides the spring-boot-maven-plugin to package an application as an executable JAR. Adding the plugin to the <build><plugins> section of the pom enables creation of a fat JAR that contains the application classes, dependencies and Spring Boot loader classes.

The generated JAR has a specific internal layout: a META-INF folder with MANIFEST.MF , a lib directory with third‑party JARs, Spring Boot loader classes, and the application’s own classes.

The manifest’s Main-Class is org.springframework.boot.loader.JarLauncher . When the JAR is started with java -jar … , JarLauncher’s main method is invoked, which registers the custom URL protocol handler, builds a class loader from the lib entries, reads the Start-Class attribute and launches the application.

Spring Boot Loader defines several abstractions such as Launcher , Archive , JarFileArchive and LaunchedURLClassLoader . The latter overrides loadClass to first try a root class loader, then search locally in the JAR URLs, and finally delegate to the parent loader.

Custom URLs use the jar:file:/…!/ syntax and are handled by org.springframework.boot.loader.jar.Handler , allowing nested JARs and classes to be accessed inside the executable archive.

Overall, the plugin and the Spring Boot Loader together provide a self‑contained executable JAR that can be run directly, with a well‑defined class‑loading mechanism and clear separation of metadata, dependencies, loader code, and application code.