java双亲委派模型如何递归实现

以下是Classloader的中最重要的方法，也就是所谓的双亲委派模型。这个模型第一次在周志明的书上看到，当时看了只知道是类加载过程是首先是委托给父加载器，否则父不能加载，则自己加载，整个过程实则是一个很简单的递归过程，本文以实例讲解这个模型到底是咋实现的？

protected Class<?> loadClass(String name, boolean resolve)

        throws ClassNotFoundException

    {

        synchronized (getClassLoadingLock(name)) {

            // First, check if the class has already been loaded

            Class c = findLoadedClass(name);

            if (c == null) {

                long t0 = System.nanoTime();

                try {

                    if (parent != null) {

                        c = parent.loadClass(name, false);

                    } else {

                        c = findBootstrapClassOrNull(name);

                    }

                } catch (ClassNotFoundException e) {

                    // ClassNotFoundException thrown if class not found

                    // from the non-null parent class loader

                }



                if (c == null) {

                    // If still not found, then invoke findClass in order

                    // to find the class.

                    long t1 = System.nanoTime();

                    c = findClass(name);



              

                }

            }

            if (resolve) {

                resolveClass(c);

            }

            return c;

        }

    }

类加载器用来测试的Class文件对应的类

public class TestLoaderClass {

    public TestLoaderClass()

    {

        System.out.println("加载器为"+TestLoaderClass.class.getClass());

        

        

    }



    



}

//自定义类加载器
import java.io.ByteArrayOutputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.IOException;

import java.io.InputStream;

import java.nio.ByteOrder;



public class MyClassLoader extends ClassLoader {



    //类加载器名称

    private String name;

    //加载类的路径

    private String path = "D:/";

    private final String fileType = ".class";

    public MyClassLoader(String name){

        //让系统类加载器成为该 类加载器的父加载器

        super();

        this.name = name;

    }

 

   //显示设置类加载器的父类加载器

    public MyClassLoader(ClassLoader parent, String name){

        //显示指定该类加载器的父加载器

        super(parent);

        this.name = name;

    }



    public String getPath() {

        return path;

    }



    public void setPath(String path) {

        this.path = path;

    }



    @Override

    public String toString() {

        return this.name;

    }



    /**

     * 获取.class文件的字节数组

     * @param name

     * @return

     */

   



    /**

     * 获取Class对象

     * @throws IOException 

     */

    private byte[] getBytes(String name) throws IOException

    {

        //name=name.replace(".", "/");

        File f=new File(path+name+fileType);

        InputStream in=new FileInputStream(f);

        System.out.println(in.available()+"file");

        ByteArrayOutputStream bao=new ByteArrayOutputStream();

        int r=in.read();

        while(r!=-1)

        {

            bao.write(r);

            r=in.read();

            

        }

        System.out.println(bao.toByteArray().toString());

        return bao.toByteArray();

        

        

        

    }

    public Class<?> findClass(String name){

        byte[] data = null;

        try {

            data = getBytes(name);

        } catch (IOException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

        System.out.println();

        return this.defineClass(name, data, 0, data.length);

    }



    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {

        //loader1的父加载器为系统类加载器

        MyClassLoader loader1 = new MyClassLoader("loader1");

        loader1.setPath("D:/lib1/");

        //loader2的父加载器为loader1

        MyClassLoader loader2 = new MyClassLoader(loader1, "loader2");

        loader2.setPath("D:/lib2/");

        //loader3的父加载器为根类加载器

        MyClassLoader loader3 = new MyClassLoader(loader2, "loader3");

        loader3.setPath("D:/lib3/");



        Class<?> clazz = loader3.loadClass("TestLoaderClass");

        System.out.println(clazz.getClassLoader());

        ClassLoader cl=loader3;

       

        

    }

}