浅谈Java自定义类加载器及JVM自带的类加载器之间的交互关系

import java.io.*;
public class MyClassLoader extends ClassLoader {
  private String name;  //类加载器的名字
  private String path;  //加载类的路径
  private final String fileType = ".class"; //class文件的扩展名
  public MyClassLoader(String name){
    super(); //让系统类加载器成为该类加载器的父 类加载器，该句可省略不写
    this.name = name;
  }
  public MyClassLoader(ClassLoader parent,String name){
    super(parent); //显示指定该类加载器的父 类加载器
    this.name = name;
  }
  @Override
  public String toString() {
    return this.name;
  }
  public String getPath() {
    return path;
  }
  public void setPath(String path) {
    this.path = path;
  }
  //实现自定义的类加载器必须重写findClass方法，否则ClassLoader类中的findClass()方法是抛出了异常
  @Override
  public Class findClass(String name)throws ClassNotFoundException{
    byte[] data = this.loadClassData(name);
    return this.defineClass(name,data,0,data.length);
  }
  private byte[] loadClassData(String name){
    InputStream is = null;
    byte[] data = null;
    ByteArrayOutputStream baos = null;
    try {
      this.name = this.name.replace(".","\\"); //com.dream.it---->com\dream\it
      is = new FileInputStream(new File(path + name + fileType));
      int ch;
      while(-1 != (ch = is.read())){
        baos.write(ch);  //将数据写入到字节数组输出流对象中去
      }
      data = baos.toByteArray();
    } catch (Exception e) {
      e.printStackTrace();
    }finally {
      try {
        is.close();
        baos.close();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
    return data;
  }
  public static void main(String[] args) throws Exception {
    MyClassLoader loader1 = new MyClassLoader("loader1");
    loader1.setPath("d:/myapp/serverlib/");
    MyClassLoader loader2 = new MyClassLoader(loader1,"loader2"); //loader1作为loader2的父 类加载器
    loader2.setPath("d:/myapp/clientlib");
    MyClassLoader loader3 = new MyClassLoader(null,"loader3");//父类加载器为null，表明其父类加载器为根类加载器
    loader3.setPath("d:/myapp/otherlib");
    test(loader2);
    test(loader3);
  }
  public static void test(ClassLoader cl) throws Exception {
    Class clazz = cl.loadClass("Sample");
    Object object = clazz.newInstance();
  }
}

protected Class<?> findClass(String name) throws ClassNotFoundException
Finds the class with the specified binary name. 
This method should be overridden by class loader
implementations that follow the delegation model 
for loading classes, and will be invoked by the 
loadClass method after checking the parent class
loader for the requested class. The default 
implementation throws a ClassNotFoundException. 

Sample:loader1
Dog:sun.misc.Launcher$AppClassLoader@1b84c92
Sample:loader3
Dog:loader3

Class clazz = loader1.loadClass("Sample");
Object obj = clazz.newInstance();
Sample sample = (Sample)obj;
System.out.println(sample.v1);

Class clazz = loader1.loadClass("Sample");
Object obj = clazz.newInstance();
Field field = clazz.getField("v1");
int v1 = field.getInt(obj);
System.out.println(v1);