动态代理

Object invoke(Object proxy, Method method, Object[] args) throws Throwable
proxy:　　    指代我们所代理的那个真实对象
method:　　   指代的是我们所要调用真实对象的某个方法的Method对象
args:　　 指代的是调用真实对象某个方法时接受的参数

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException
loader:　　       一个ClassLoader对象，定义了由哪个ClassLoader对象来对生成的代理对象进行加载
interfaces:　　   一个Interface对象的数组，表示的是我将要给我需要代理的对象提供一组什么接口
h:　　            一个InvocationHandler对象，表示的是会关联到哪一个InvocationHandler对象上

public interface Subject {
    public void rent();

    public void hello(String str);
}

public class RealSubject implements Subject {

    @Override
    public void rent() {
        System.out.println("I want to rent my house");
    }

    @Override
    public void hello(String str) {
        System.out.println("hello: " + str);
    }

}

public class DynamicProxy implements InvocationHandler {

    // 这个就是我们要代理的真实对象
    private Object subject;

    // 构造方法，给我们要代理的真实对象赋初值
    public DynamicProxy(Object subject) {
        this.subject = subject;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Exception {
        //在代理真实对象前我们可以添加一些自己的操作
        //System.out.println("before rent house");

        //当代理对象调用真实对象的方法时，其会自动的跳转到代理对象关联的handler对象的invoke方法来进行调用
        method.invoke(subject, args);

        //在代理真实对象后我们也可以添加一些自己的操作
        //System.out.println("after rent house");

        return null;
    }

}

    public static void main(String[] args) {
        // 我们要代理的真实对象
        Subject realSubject = new RealSubject();

        // 我们要代理哪个真实对象，就将该对象传进去，最后是通过该真实对象来调用其方法的
        InvocationHandler handler = new DynamicProxy(realSubject);

        /*
         * 通过Proxy的newProxyInstance方法来创建我们的代理对象，我们来看看其三个参数 第一个参数
         * handler.getClass().getClassLoader()
         * ，我们这里使用handler这个类的ClassLoader对象来加载我们的代理对象
         * 第二个参数realSubject.getClass().getInterfaces()，我们这里为代理对象提供的接口是真实对象所实行的接口
         * ，表示我要代理的是该真实对象，这样我就能调用这组接口中的方法了 第三个参数handler， 我们这里将这个代理对象关联到了上方的
         * InvocationHandler 这个对象上
         */
        Subject subject = (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(),
                realSubject.getClass().getInterfaces(), handler);
        // 替代静态代理的new

        subject.rent();
        subject.hello("world");
    }

public class Proxy implements java.io.Serializable {

    /** parameter types of a proxy class constructor */
    private static final Class<?>[] constructorParams =
        { InvocationHandler.class };

    /**
     * a cache of proxy classes
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());


    public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)
        throws IllegalArgumentException
    {
        final Class<?>[] intfs = interfaces.clone();

        /*
         * Look up or generate the designated proxy class.
         */
        Class<?> cl = getProxyClass0(loader, intfs);
        //******************返回proxyClassCache.get(loader, interfaces);有必要研究一下这个proxyClassCache如何初始化的

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            final Constructor<?> cons = cl.getConstructor(constructorParams);//获得动态代理的构造函数
            final InvocationHandler ih = h;
            return cons.newInstance(new Object[]{h});//生成实例
        } catch (Exception e) {
        } 
    }
}

    private static final class ProxyClassFactory implements BiFunction<ClassLoader, Class<?>[], Class<?>>{
        // prefix for all proxy class names
        private static final String proxyClassNamePrefix = "$Proxy";

        // next number to use for generation of unique proxy class names
        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            for (Class<?> intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this
                 * interface to the same Class object.
                 */
                Class<?> interfaceClass = null;
                try {
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                ......
            }

            String proxyPkg = null;     // package to define proxy class in
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;

            /*
             * Record the package of a non-public proxy interface so that the
             * proxy class will be defined in the same package.  Verify that
             * all non-public proxy interfaces are in the same package.
             */
            for (Class<?> intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    accessFlags = Modifier.FINAL;
                    String name = intf.getName();
                    int n = name.lastIndexOf('.');
                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                    if (proxyPkg == null) {
                        proxyPkg = pkg;
                    } else if (!pkg.equals(proxyPkg)) {
                        throw new IllegalArgumentException(
                            "non-public interfaces from different packages");
                    }
                }
            }

            /*
             * Choose a name for the proxy class to generate.
             */
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * Generate the specified proxy class.
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);//********************************
            try {
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
            }
        }
    }

    public static byte[] generateProxyClass(final String name,
                                             Class<?>[] interfaces,
                                             int accessFlags)
     {
         ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
         final byte[] classFile = gen.generateClassFile();//-----------------------------------

         if (saveGeneratedFiles) {
             java.security.AccessController.doPrivileged(
             new java.security.PrivilegedAction<Void>() {
                 public Void More ...run() {
                     try {
                         int i = name.lastIndexOf('.');
                         Path path;
                         if (i > 0) {
                             Path dir = Paths.get(name.substring(0, i).replace('.', File.separatorChar));
                             Files.createDirectories(dir);
                             path = dir.resolve(name.substring(i+1, name.length()) + ".class");
                         } else {
                             path = Paths.get(name + ".class");
                         }
                         Files.write(path, classFile);
                         return null;
                     } catch (IOException e) {
                         throw new InternalError(
                             "I/O exception saving generated file: " + e);
                     }
                 }
             });
         }

         return classFile;
     }

     private byte[] generateClassFile() {

         /* ============================================================
          * Step 1: Assemble ProxyMethod objects for all methods to
          * generate proxy dispatching code for.
          */
         addProxyMethod(hashCodeMethod, Object.class);
         addProxyMethod(equalsMethod, Object.class);
         addProxyMethod(toStringMethod, Object.class);
         for (Class<?> intf : interfaces) {
             for (Method m : intf.getMethods()) {
                 addProxyMethod(m, intf);//------------------------------这里会调用invoke的接口方法
             }
         }
         ......
         return bout.toByteArray();
     }