Spring Boot uses Spring Boot Gradle Plugin or Spring Boot Maven Plugin to package application as a runnable standalone jar file.

Spring Boot uses the Spring Boot Loader to lauch the Spring Boot jar file usingjava -jar.

Let's uncompress the jar file and see its structure(jar is actually a zip file):

We see three sub-folders in the uncompressed folder：BOOT-INF,META-INF,org

example.jar
 |
 +-META-INF
 |  +-MANIFEST.MF
 +-org
 |  +-springframework
 |     +-boot
 |        +-loader
 |           +-<spring boot loader classes>
 +-BOOT-INF
    +-classes
    |  +-mycompany
    |     +-project
    |        +-YourClasses.class
    +-lib
       +-dependency1.jar
       +-dependency2.jar

BOOT-INF

1. BOOT-INF/classes: application classes
2. BOOT-INF/lib: nested dependency jars
3. BOOT-INF/classpath.idx: the ordering that jars should be added to the classpath
4. BOOT-INF/layers.idx: allows a jar to be split into logical layers for Docker/OCI image creation

META-INF

META-INF/MANIFEST.MF: contains information about the files contained in the JAR file

org

spring boot loader classes

Explore the Magic

We will explore the steps of running a standalone jar file:

1. java -jar
2. Looking for the executable jar’s main entry point in the META-INF/MANIFEST.MF

   Manifest-Version: 1.0
   Main-Class: org.springframework.boot.loader.JarLauncher
   Start-Class: top.wisely.springasync.SpringAsyncApplication
   Spring-Boot-Version: 2.7.2
   Spring-Boot-Classes: BOOT-INF/classes/
   Spring-Boot-Lib: BOOT-INF/lib/
   Spring-Boot-Classpath-Index: BOOT-INF/classpath.idx
   Spring-Boot-Layers-Index: BOOT-INF/layers.idx
   

3. Main-Class: org.springframework.boot.loader.JarLauncher is the main entry point. JarLauncher

4. BecauseJarLauncher extends ExecutableArchiveLauncher and ExecutableArchiveLauncher extends Launcher

5. JarLauncher's main method:

   public class JarLauncher extends ExecutableArchiveLauncher {
   //...
    public static void main(String[] args) throws Exception {
        new JarLauncher().launch(args);
    }
   //...
   }
   

6. So the real launch(args) method is in Launcher class:

   public abstract class Launcher {
    protected void launch(String[] args) throws Exception {
       if (!isExploded()) {
           JarFile.registerUrlProtocolHandler(); 
       }
       ClassLoader classLoader = createClassLoader(getClassPathArchivesIterator()); //①
       String jarMode = System.getProperty("jarmode"); 
       String launchClass = (jarMode != null && !jarMode.isEmpty()) ? JAR_MODE_LAUNCHER : getMainClass(); //②
       launch(args, launchClass, classLoader);  //③
   }
   }
   

① : classLoader - Create a new LaunchedURLClassLoader instance. all BOOT-INF/classes and BOOT-INF/jar classes are loaded by LaunchedURLClassLoader .

② : launchClass - get the Start-Class from META-INF/MANIFEST.MF.

③：launch Application using args,launchClass,classLoader.

That'a all for my first English article.