创建自定义ClassLoader，绕过双亲委派

MyClassLoader.java 中定义了两个加载class的方法，其中findClass(String name)实现双亲委派逻辑，createClass(String name)绕过双亲委派逻辑

package com.xinxin.classloader;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;

public class MyClassLoader extends ClassLoader {
    private String mLibPath;

    public MyClassLoader(String path) {
        mLibPath = path;
    }

    /**
     * 双亲委派逻辑，父加载器读不到Class才会调用此方法
     */
    @Override
    protected Class findClass(String name) throws ClassNotFoundException{
        byte[] data;
        try {
            data = readClassFile( name);
            return defineClass(name,data,0,data.length);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return super.findClass(name);
    }

    /**
     * 绕过双亲委派逻辑，直接获取Class
     */
    public Class createClass(String name) throws Exception{
        byte[] data;
        data = readClassFile(name);
        return defineClass(name,data,0,data.length);
    }

    /**
     * 读取Class文件
     */
    private byte[] readClassFile(String name) throws Exception{
        String fileName = getFileName(name);
        File file = new File(mLibPath,fileName);
        FileInputStream is = new FileInputStream(file);
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        int len = 0;
        while ((len = is.read()) != -1) {
            bos.write(len);
        }
        byte[] data = bos.toByteArray();
        is.close();
        bos.close();
        return data;
    }

    //获取要加载 的class文件名
    private String getFileName(String name) {
        int index = name.lastIndexOf('.');
        if(index == -1){ 
            return name+".class";
        }else{
            return name.substring(index+1)+".class";
        }
    }
}

测试：

    static void testClassLoader() throws Exception{
      MyClassLoader diskLoader = new MyClassLoader("D:\\tmp\\");
      //双亲委派      
      //Class c = diskLoader.loadClass("com.xinxin.classloader.Student");

      //绕过双亲委派
      Class c = diskLoader.createClass("com.xinxin.classloader.Student");
      if(c != null){
          try {
              Object obj = c.newInstance();
              Method method = c.getDeclaredMethod("hello",null);
              method.invoke(obj, null);
          } catch (Exception e){
            e.printStackTrace();
          }
      }
    }