13_不同的类加载器作用与加载动作分析

类加载器结构

代码：

public class MyTest13 {

    public static void main(String[] args) {
        ClassLoader classLoader = ClassLoader.getSystemClassLoader();
        System.out.println("classLoader = " + classLoader);
        while (null!=classLoader){
            classLoader = classLoader.getParent();
            System.out.println("classLoader = " + classLoader);
        }
    }

}

输出：

classLoader = sun.misc.Launcher$AppClassLoader@58644d46
classLoader = sun.misc.Launcher$ExtClassLoader@4554617c
classLoader = null

从javadoc可知，HotSpot中classLoader为null表示BootStrap类加载器。

获得ClassLoader的途径

代码：

public class MyTest14 {

    public static void main(String[] args) throws IOException {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        String resourceName = "top/tomxwd/classloader/MyTest13.class";
        Enumeration<URL> urls = classLoader.getResources(resourceName);
        while (urls.hasMoreElements()){
            URL url = urls.nextElement();
            System.out.println(url);
        }
        System.out.println("--------------------");
        Class<?> clazz = MyTest14.class;
        System.out.println("clazz.getClassLoader() = " + clazz.getClassLoader());
        System.out.println("--------------------");
        clazz = String.class;
        System.out.println("clazz.getClassLoader() = " + clazz.getClassLoader());
    }

}

输出：

file:/E:/IDEA-workspace/jvm-test-01/target/classes/top/tomxwd/classloader/MyTest13.class
--------------------
clazz.getClassLoader() = sun.misc.Launcher$AppClassLoader@58644d46
--------------------
clazz.getClassLoader() = null