先看一个例子

package com.cqs.learning.jvm;

import jdk.nashorn.tools.Shell;

import java.io.IOException;
import java.io.InputStream;

/**
 * @author lixiaowen
 * @create 19-2-28
 */
public class ClassLoaderDemo {

    public static void main(String[] args) {

        // bootstrap class loader example &&
        System.out.println(Object.class  + " classloader: "+ Object.class.getClassLoader());

        //ext class loader example
        //System.out.println(System.getProperty("java.ext.dirs"));
        System.out.println(Shell.class + " classloader: " + Shell.class.getClassLoader());

        //App class loader example
        System.out.println(ClassLoaderDemo.class + " classloader： " + ClassLoaderDemo.class.getClassLoader());

        // self define class loader example
        SelfDefineClassLoader selfDefineClassLoader = new SelfDefineClassLoader();
        try {
            //指定自定义加载器
            Class<?> aClass = selfDefineClassLoader.loadClass("com.cqs.learning.jvm.ClassLoaderDemo");
            Object o = aClass.newInstance();
            System.out.println(o.getClass() +"\t classloader: " + o.getClass().getClassLoader());
            //System.out.println(o instanceof com.cqs.learning.jvm.ClassLoaderDemo);
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }

   static class SelfDefineClassLoader extends ClassLoader{

       @Override
       public Class<?> loadClass(String name) throws ClassNotFoundException {
           String fileName = name.substring(name.lastIndexOf(".")+1) + ".class";
           InputStream is = getClass().getResourceAsStream(fileName);
           if (is == null){
               return super.loadClass(name);
           }
           try {
               byte[] bytes = new byte[is.available()];
               is.read(bytes);
               return defineClass(name,bytes,0, bytes.length);
           } catch (IOException e) {
               throw new RuntimeException();
           }
       }
   }
}

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