public void abc(){
    //每一种读取方法,使用某个类获取Appclassloader
    ClassLoader cl = ReadFile.class.getClassLoader();
    URL url = cl.getResource("a.txt");
    System.err.println("url1 is:"+url.getPath());
    
    //第二种方式，直接使用ClassLoader
    URL url2 = ClassLoader.getSystemResource("a.txt");
    System.err.println("url2 is:"+url2.getPath());
}

 public class OneServlet extends HttpServlet {
  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {
    ClassLoader loader = OneServlet.class.getClassLoader();//WebAppClassLoader
    int index=1;
    while(loader!=null){
      System.err.println((index++)+"类加载器是:"+loader.getClass());
   loader=loader.getParent();//获取父类加载器
    }
  }
}

1类加载器是:class org.apache.catalina.loader.WebappClassLoader
2类加载器是:class org.apache.catalina.loader.StandardClassLoader
3类加载器是:class sun.misc.Launcher$AppClassLoader
4类加载器是:class sun.misc.Launcher$ExtClassLoader

package cn.hx.demo;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.net.URL;
public class MyClassLoader2 extends ClassLoader {
  /**
   * name:cn.itcast.demo.Person
   * 根据包名找到.class文件
   * cn.itcast.demo.person = > cn/itcast/demo/Person.class
   */
  public Class<?> findClass(String name ) throws ClassNotFoundException {
    String classNameWithPackage=name;
    Class<?> cls = null;
    try {
      //先将
      name = name.replace(".","/");
      name +=".class";
       //确定目录
      URL url = MyClassLoader2.class.getClassLoader().getResource(name);
      System.err.println(">>:"+url.getPath());
      File file = new File(url.getPath());
      InputStream in = new FileInputStream(file);
      //读取这个.class文件的字节码
      byte[] b = new byte[in.available()];//直接声明这个字节大小为这个文件的大小
      int len = in.read(b);//len=621
      System.err.println(len);
      /**
       * 第一个参数是类名
       */
      cls = defineClass(classNameWithPackage,b,0,len);
    } catch (Exception e) {
      e.printStackTrace();
    }
    return cls;
  }
}

public class ClassLoaderDemo {
  public static void main(String[] args) throws Exception {
    MyClassLoader2 mc = new MyClassLoader2();
    Class cls = mc.findClass("cn.itcast.demo.Person");
    Object o = cls.newInstance();
    System.err.println("toString:"+o+","+o.getClass().getClassLoader());
    //直接使用peron是 AppClassLoader
    System.err.println(">>:"+Person.class.getClassLoader());
    //由于o是由mc加载的。而Person是由App加载的，所有不可以转换＝来自于两个不同的加载器
    //Person p = (Person) o;//类型转换错误ClassCastException
    //System.err.println(p);
  }
}