类加载

1 public static void main(String[] args) {
2         ClassLoader loader=Loader.class.getClassLoader();
3         System.err.println(loader);
4         String path =loader.getResource("a.txt").getPath();
5         System.err.println(path);
6         System.err.println("=============================");
7         path= ClassLoader.getSystemResource("a.txt").getPath();
8         System.err.println(path);
9     }

1 ClassLoader loader=LoaderServlet.class.getClassLoader();
2         System.err.println(loader);
3         String path= loader.getResource("a.txt").getPath();
4         System.err.println(path);
5         System.err.println("===================");
6         //path= ClassLoader.getSystemResource("a.txt").getPath();
7         System.err.println(path);

1 ClassLoader loader = OneServlet.class.getClassLoader();//WebAppClassLoader
2         int index=1;
3         while(loader!=null){
4             System.err.println((index++)+"类加载器是:"+loader.getClass()); 
                    loader=loader.getParent();//获取父类加载器

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 //name接受类名

ClassNotFoundException {
        String classNameWithPackage=name;
        Class<?> cls = null;
        try {
            //先将
            name = name.replace(".","/");//处理成路径名
            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;
    }
}

测试类自定义类加载器“
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);
    }
}