jvm1ClassLoadingDemo

1package pkg6jvm;2 3/*4 * jvm1ClassLoadingDemo.java5 * ---------------------6 * Shows the class loader hierarchy and the order of static initialization.7 *8 * CLASS LOADING PHASES:9 *   Loading -> Linking (Verify -> Prepare -> Resolve) -> Initialization10 * LOADER DELEGATION (parent-first):11 *   Bootstrap (core JDK) -> Platform -> Application (your classpath)12 *13 * INITIALIZATION ORDER (on first active use of a class):14 *   static fields & static blocks run top-to-bottom, exactly once.15 */16public class jvm1ClassLoadingDemo {17 18    static class Config {19        static final String NAME;20        static int counter;21        static {                                   // static initializer block22            System.out.println("  [Config static block] runs once on first use");23            NAME = "JavaMastery";24            counter = 100;25        }26        Config() { System.out.println("  [Config constructor] runs per instance"); }27    }28 29    public static void main(String[] args) {30        // 1) Class loader hierarchy31        ClassLoader app = jvm1ClassLoadingDemo.class.getClassLoader();32        System.out.println("Application loader : " + app);33        System.out.println("Platform loader    : " + app.getParent());34        System.out.println("Bootstrap loader   : " + null + " (represented as null; loads java.* core)");35 36        // Core classes are loaded by the bootstrap loader (null)37        System.out.println("String's loader    : " + String.class.getClassLoader() + " (bootstrap)");38 39        // 2) Static initialization happens on first ACTIVE use40        System.out.println("\nBefore touching Config (not initialized yet)...");41        System.out.println("Touching Config.NAME now:");42        System.out.println("  Config.NAME = " + Config.NAME);   // triggers static block43        System.out.println("Creating two instances:");44        new Config(); new Config();                              // static block does NOT rerun45        System.out.println("Config.counter = " + Config.counter);46    }47}