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Exploring the Magic of Spring Boot runnable standalone jar

Exploring the Magic of Spring Boot runnable standalone jar

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yunfei wang
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2 min read

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):

image.png

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

image.png

META-INF

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

image.png

org

spring boot loader classes

image.png

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.

Springboot