自定义类加载器、打破双亲委派

自定义类加载器、打破双亲委派

 

import java.io.FileInputStream;
import java.lang.reflect.Method;

public class MyClassLoaderTest {
    static class MyClassLoader extends ClassLoader {
        private String classPath;
        public MyClassLoader(String classPath) {
            this.classPath = classPath;
        }
        private byte[] loadByte(String name) throws Exception {
            name = name.replaceAll("\\.", "/");
            FileInputStream fis = new FileInputStream(classPath + "/" + name
                    + ".class");
            int len = fis.available();
            byte[] data = new byte[len];
            fis.read(data);
            fis.close();
            return data;
        }
        //打破双亲委派
        @Override
        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();
                    long t1 = System.nanoTime();
                    if (!name.startsWith("com.turingschool.demo")){
                        c = this.getParent().loadClass(name);
                    }else {
                        c = findClass(name);
                    }
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
                if (resolve) {
                    resolveClass(c);
                }
                return c;
            }
        }
        //自定义类加载器 重写findClass()方法
        @Override
        protected Class findClass(String name) throws ClassNotFoundException {
            try {
                byte[] data = loadByte(name);
                //defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节 数组。
                return defineClass(name, data, 0, data.length);
            } catch (Exception e) {
                e.printStackTrace();
                throw new ClassNotFoundException();
            }
        }
    }
    public static void main(String args[]) throws Exception {
        //初始化自定义类加载器，会先初始化父类ClassLoader，其中会把自定义类加载器的父加载 器设置为应用程序类加载器AppClassLoader
        MyClassLoader classLoader = new MyClassLoader("D:/test");
        // D盘创建 test/com/tuling/jvm 几级目录，将User类的复制类User1.class丢入该目录
        Class clazz = classLoader.loadClass("com.turingschool.demo.User1");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("sout", null);
        method.invoke(obj, null);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }
}