问六：ClassLoader有什么用？写一个？

需要了解：

回答：

手写一个ClassLoader

package com.liu.socket;

import java.io.ByteArrayOutputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.IOException;

import java.io.InputStream;

public class Test extends ClassLoader{

    private String path;

    private String className;

    public Test(String path,String className) {

     this.path = path;

     this.className = className;

    }

    @Override

    public Class findClass(String name){

        byte[] b = null;

         try {

             b = loadClassData(name);

         } catch (IOException e) {

             // TODO Auto-generated catch block

             e.printStackTrace();

         }

        return defineClass(className, b, 0,  b.length);

    }

    private byte[] loadClassData(String name) throws  IOException {

        String namePath = path+name+".class";

        InputStream in = null;

        ByteArrayOutputStream out = null;

        try {

            in = new FileInputStream(new  File(namePath));

            out = new ByteArrayOutputStream();

            int i = 0;

            while((i = in.read()) != -1){

                out.write(i);

            }

        } catch (FileNotFoundException e) {

            e.printStackTrace();

        } catch (IOException e) {

            e.printStackTrace();

        } catch (Exception e){

            e.printStackTrace();

        }finally {

            in.close();

            out.close();

            

        }

        return out.toByteArray();

    }

}

package com.liu.socket;

public class ClassLoaderCheck {

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

         Test test = new Test("D:\\","Test");

         Class class1 = test.loadClass("test");

         System.out.println(class1.getName());

         class1.newInstance();

    }

}