public class ClassLoadTest {
    public static void main(String[] args) {
        // 核心rt.jar中的类加载器 是C++加载的，因此这里为null 
        System.out.println(String.class.getClassLoader());
        // 扩展包的加载器 ExtClassLoader
        System.out.println(com.sun.crypto.provider.DESKeyFactory.class.getClassLoader());
        // 应用加载器 AppClassLoader
        System.out.println(ClassLoadTest.class.getClassLoader());
        System.out.println("");
        // 获取系统ClassLoader
        ClassLoader appClassLoader = ClassLoader.getSystemClassLoader();
        // appClassLoader的父加载器
        ClassLoader extClassLoader = appClassLoader.getParent();
        // extClassLoader的父加载器
        ClassLoader boostrapClassLoader = extClassLoader.getParent();
        System.out.println("the bootstrapLoader : " + boostrapClassLoader);
        System.out.println("the extClassloader : " + extClassLoader);
        System.out.println("the appClassLoader : "+ appClassLoader);
        System.out.println("");
        System.out.println("==============bootstrapLoader加载的文件====================");
        URL[] urLs = Launcher.getBootstrapClassPath().getURLs();
        for (int i = 0; i < urLs.length; i++) {
            System.out.println(urLs[i]);
        }
        System.out.println("");
        System.out.println("==============extClassloader加载的文件====================");
        System.out.println(System.getProperty("java.ext.dirs"));
        System.out.println("");
        System.out.println("==============appClassLoader 加载的文件====================");
        System.out.println(System.getProperty("java.class.path"));
    }
}

  public Launcher() {
        Launcher.ExtClassLoader var1;
        try {
           //构造扩展类加载器，在构造的过程中将其父加载器设置为null
            var1 = Launcher.ExtClassLoader.getExtClassLoader();
        } catch (IOException var10) {
            throw new InternalError("Could not create extension class loader", var10);
        }
        try { 
             //构造应用类加载器，在构造的过程中将其父加载器设置为ExtClassLoader，
             //Launcher的loader属性值是AppClassLoader，我们一般都是用这个类加载器来加载我们自己写的应用程序
            this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
        } catch (IOException var9) {
            throw new InternalError("Could not create application class loader", var9);
        }
        Thread.currentThread().setContextClassLoader(this.loader);
          .....
         .....
         .....
        }
    }

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
           //   检查当前类加载器是否已经加载了该类 ,加载直接返回
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    //如果当前加载器父加载器不为空则委托父加载器加载该类
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else { 
                       //如果当前加载器父加载器为空则委托引导类加载器加载该类
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                }
                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    //调用URLClassLoader的findClass方法在加载器的类路径里查找并加载该类
                    c = findClass(name);
                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }