代理模式

３.Proxy ^{_{(Provide a surrogate or placeholder for another object to control access to it.)}}

　　　　－－代理模式(为其他对象提供一种代理以控制对这个对象的访问)

public class Proxy implements java.io.Serializable {

    /**
     * the invocation handler for this proxy instance.
     * @serial
     */
    protected InvocationHandler h;

     /**
     * Constructs a new {@code Proxy} instance from a subclass
     * (typically, a dynamic proxy class) with the specified value
     * for its invocation handler.
     *
     * @param  h the invocation handler for this proxy instance
     *
     * @throws NullPointerException if the given invocation handler, {@code h},
     *         is {@code null}.
     */
    protected Proxy(InvocationHandler h) {
        Objects.requireNonNull(h);
        this.h = h;
    }

    /**
     * Returns an instance of a proxy class for the specified interfaces
     * that dispatches method invocations to the specified invocation
     * handler.
     *
     * <p>{@code Proxy.newProxyInstance} throws
     * {@code IllegalArgumentException} for the same reasons that
     * {@code Proxy.getProxyClass} does.
     *
     * @param   loader the class loader to define the proxy class
     * @param   interfaces the list of interfaces for the proxy class
     *          to implement
     * @param   h the invocation handler to dispatch method invocations to
     * @return  a proxy instance with the specified invocation handler of a
     *          proxy class that is defined by the specified class loader
     *          and that implements the specified interfaces
     * @throws  IllegalArgumentException if any of the restrictions on the
     *          parameters that may be passed to {@code getProxyClass}
     *          are violated
     * @throws  SecurityException if a security manager, <em>s</em>, is present
     *          and any of the following conditions is met:
     *          <ul>
     *          <li> the given {@code loader} is {@code null} and
     *               the caller's class loader is not {@code null} and the
     *               invocation of {@link SecurityManager#checkPermission
     *               s.checkPermission} with
     *               {@code RuntimePermission("getClassLoader")} permission
     *               denies access;</li>
     *          <li> for each proxy interface, {@code intf},
     *               the caller's class loader is not the same as or an
     *               ancestor of the class loader for {@code intf} and
     *               invocation of {@link SecurityManager#checkPackageAccess
     *               s.checkPackageAccess()} denies access to {@code intf};</li>
     *          <li> any of the given proxy interfaces is non-public and the
     *               caller class is not in the same {@linkplain Package runtime package}
     *               as the non-public interface and the invocation of
     *               {@link SecurityManager#checkPermission s.checkPermission} with
     *               {@code ReflectPermission("newProxyInPackage.{package name}")}
     *               permission denies access.</li>
     *          </ul>
     * @throws  NullPointerException if the {@code interfaces} array
     *          argument or any of its elements are {@code null}, or
     *          if the invocation handler, {@code h}, is
     *          {@code null}
     */
    @CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /*
         * Look up or generate the designated proxy class.
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) {
            Throwable t = e.getCause();
            if (t instanceof RuntimeException) {
                throw (RuntimeException) t;
            } else {
                throw new InternalError(t.toString(), t);
            }
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString(), e);
        }
    }         
    

}