代理（Proxy）模式

为其他对象提供一种代理以控制对这个对象的访问。

远程代理（remote proxy）为一个对象在不同的地址空间提供局部代表。

保护代理（Protection proxy）控制对原始对象的访问。

虚拟代理（Virtual proxy）根据需要创建开销很大的对象。

智能指引（Smart Reference）取代简单的指针，它在访问对象时执行一些附加操作。包括对指向对象的引用计数，当第一次引用一个持久对象时，将它载入内存。

参与者：

Proxy（ImageProxy）

保存一个引用使得代理可以访问实体。若realSubject和Subject的接口相同，Proxy会引用Subject。

提供一个与Subject的接口相同的接口，这样代理就可以用来代替实体。

控制对实体的存取，并可能负责创建和删除它。

其他功能依赖于代理的类型:

Remote proxy负责对请求及其参数进行编码，并向不同地址空间中的实体发送以编码的请求。

Virtual Proxy可以缓存实体的附加信息，以便延迟对它的访问。

Protection Proxy检查调用者是否具有实现一个请求所必需的访问权限。

Subject（Graphic）

定义RealSubject和Proxy的公用接口，这样就在任何使用RealSubject的地方都可以使用Proxy。

RealSubject（Image）

            定义Proxy所代表的实体。

代码

Subject

 

/**

The Subject defines the common interface for RealSubject and Proxy so that a

 * Proxy can be used anywhere a RealSubject is expected.

 * @modelguid {F53E01CF-38BC-4532-8A7C-E49269EA26B0}

*/

public interface GraphicSubject {

       /** @modelguid {3F5433C7-4967-4588-B350-A5B6089108EE} */

       public void Draw();

       /** @modelguid {98193698-60AB-44A5-9A90-80637E1171FF} */

       public void Save();

       /** @modelguid {5A8063C8-8849-4126-9A6B-676635E93F75} */

       public void Load(Object o);     

 

}

 

RealSubject

 

/**

The RealSubject defines the real object that the proxy represents.

 * @modelguid {29F3D9A9-6FB1-4518-8B5C-B213F1E2FBE7}

*/

public class ImageRealSubject implements GraphicSubject {

       /** @modelguid {408A6002-D939-4ACF-8D01-F694F69A5E11} */

       public ImageRealSubject() {

       }

 

       /** @modelguid {A3D4D0AA-EDC1-468A-BFC1-195927B1FF5F} */

       public void Draw() {

             

              // add your codes here...

              // be sure you update the return type based on your custom operation signature

              System.out.println("Draw here");

              return;

             

       }

      

       /** @modelguid {E3BF6A53-9276-47FB-AD54-639B29D260CD} */

       public void Save(){

              System.out.println("Save here");

       }

      

       /** @modelguid {FB73AB03-4D33-43E5-9031-5797878FBA83} */

       public void Load(Object o){

              System.out.println("Load here");

       }

 

}

 

Proxy

 

/**

Proxy controls access to the real subject and may be responsible for creating

 * and deleting it.

 

The Proxy maintains a reference that lets the proxy  access the real subject.

 * Proxy may refer to a Subject if the RealSubject and Subject interfaces are

 * the same.  It provides an interface identical to Subject's so that a proxy

 * can be substituted for the real subject.  It also controls access to the

 * real subject and may be responsible for creating and deleting it.

 

Other responsibilities depend on the kind of proxy:

- remote proxies are responsible for encoding a request and its arguements and

 * for sending the encoded request to the real subject in a different address

 * space.

virtual proxies may cache additional informatin about the real subject so that

 * they can postpone accessing it.  For example, the ImageProxy for the

 * Motivation caches the real image's extent.

- protection proxies check that the caller has the access permissions required

 * to perform a request.

 * @modelguid {1B76A4ED-C987-47A1-94B5-BDF5A3F19938}

*/

public class ImageProxy implements GraphicSubject {

       /** @modelguid {4EE79008-471B-4A7E-AE49-16406E804CBB} */

       private ImageRealSubject theRealSubject;

 

       /**

       This is the overloaded constructor of the Proxy class.

        * @modelguid {7D49A0C9-F9D5-4C24-B6D9-99D667E46623}

       */

       public ImageProxy(ImageRealSubject subject) {

             

              this.theRealSubject = subject;

             

       }

 

       /**

       This is the default constructor of the Proxy class.

        * @modelguid {69DB530A-E37E-4DB6-AABC-6A96F1048363}

       */

       public ImageProxy() {

       }

 

       /**

       This operation is setting "theRealSubject" attribute of the Proxy class.

        * @modelguid {E88BF991-2B8C-47B8-BA98-D65819F5B73F}

       */

       public void setRealSubject(ImageRealSubject realSubject) {

             

              theRealSubject = realSubject;

             

       }

 

       /**

       This is any custom operation from the model.   It can implement any logic

        * required by your application.

        * @modelguid {356AF68C-1BF2-4F8B-83D5-8DEC58BC2341}

       */

       public void Draw() {

              if (theRealSubject != null)

               (theRealSubject).Draw();

      

       }

      

       /** @modelguid {1A9CDEDD-75E2-4C26-9272-C7203804CB73} */

       public void Save(){

              if (theRealSubject != null)

               (theRealSubject).Save();

       }

      

       /** @modelguid {BB51C2C8-4302-4493-86DC-7D1F1BDD7DD5} */

       public void Load(Object o){

              if (theRealSubject != null)

               (theRealSubject).Load(o);       

       }

 

}