打破双亲委派机制

再来一个沙箱安全机制示例，尝试打破双亲委派机制，用自定义类加载器加载我们自己实现的 java.lang.String.class

public class MyClassload1 extends ClassLoader {


    private String classPath;

    public MyClassload1(String classPath) {
        this.classPath = classPath;
    }

    @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();
                if (!name.startsWith("com.example.jvm")) {
                    c = this.getParent().loadClass(name);
                } else {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);
                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        String pacakge = name.replaceAll("\\.", "/");
        FileInputStream fis = null;
        byte[] data = null;
        try {
            fis = new FileInputStream(classPath + "/" + pacakge
                    + ".class");
            int len = fis.available();
            data = new byte[len];
            fis.read(data);
            fis.close();
        } catch (FileNotFoundException e) {
            throw new RuntimeException(e);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        //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);
    }

    public static void main(String[] args) throws Exception {
        MyClassload1 myClassload = new MyClassload1("C:/test");
        Class<?> aClass = myClassload.loadClass("java.lang.String");
        Object user = aClass.newInstance();
        Method sout = aClass.getDeclaredMethod("sout", null);
        sout.invoke(user, null);
        System.out.println(aClass.getClassLoader().getClass().getName());
    }
}