代理模式（学习笔记）
1. 意图
提供对象的替代品或其占位符。代理控制着对于原对象的访问，允许在将请求提交给对象前后进行一些处理
2. 动机
在面向对象的系统中，有些对象由于某种原因（比如对象创建的开销很大，或者某些操作需要安全控制，或者需要进程外的访问等），直接访问对象会给使用者或者系统结构带来很多麻烦
3. 适用性
- 延迟初始化（虚拟代理）。如果你有一个偶尔使用的重量级服务对象，一直保持该对象运行会消耗系统资源时，可使用代理模式
- 访问控制（保护代理）。如果只希望特定客户端使用服务对象，这里的对象可以是操作系统中非常重要的部分，而客户端则是各种已启动的程序（包括恶意程序），此时可使用代理模式
- 本地执行远程服务 （远程代理）。适用于服务对象位于远程服务器上的情形。代理通过网络传递客户端请求，负责处理所有与网络相关的复杂细节
- 记录日志请求（日志记录代理）。适用于当你需要保存对于服务对象的请求历史记录时。代理可以在向服务传递请求前进行记录
- 缓存请求结果 （缓存代理）。适用于需要缓存客户请求结果并对缓存生命周期进行管理时，特别是当返回结果的体积非常大时。代理可对重复请求所需的相同结果进行缓存，还可使用请求参数作为索引缓存的键值
- 智能引用。可在没有客户端使用某个重量级对象时立即销毁该对象。
代理会将所有获取了指向服务对象或其结果的客户端记录在案。代理会时不时地遍历各个客户端，检查它们是否仍在运行。如果相应的客户端列表为空，代理就会销毁该服务对象，释放底层系统资源。代理还可以记录客户端是否修改了服务对象。其他客户端还可以复用未修改的对象
4. 结构
5. 效果
1）可以在客户端毫无察觉的情况下控制服务对象
2）客户端对服务对象的生命周期没有特殊要求， 你可以对生命周期进行管理
3）开闭原则。 你可以在不对服务或客户端做出修改的情况下创建新代理
4）Proxy模式可以对用户隐藏另一种称为copy-on-write的优化方式，该优化与根据需要创建对象有关。拷贝一个庞大而复杂的对象是一种开销很大的操作，如果这个拷贝并没有被修改，那么开销就没有必要。可以用代理延迟这一拷贝过程。在实现copy-on-write时必须对实体进行引用计数。只有当用户请求一个修改该实体的操作时，代理才会真正的拷贝它。此时，代理需要减少实体引用计数，当引用数目为零时，这个实体将被删除
5）服务响应可能会延迟
6. 代码实现
some_cool_media_library/ThirdPartyYouTubeLib.java: 远程服务接口
package proxy.some_cool_media_library; import java.util.HashMap; /** * @author GaoMing * @date 2021/7/20 - 19:25 */ public interface ThirdPartyYouTubeLib { HashMap<String, Video> popularVideos(); Video getVideo(String videoId); }
some_cool_media_library/ThirdPartyYouTubeClass.java: 远程服务实现
package proxy.some_cool_media_library; import java.util.HashMap; /** * @author GaoMing * @date 2021/7/20 - 19:22 */ public class ThirdPartyYouTubeClass implements ThirdPartyYouTubeLib{ @Override public HashMap<String, Video> popularVideos() { connectToServer("http://www.youtube.com"); return getRandomVideos(); } @Override public Video getVideo(String videoId) { connectToServer("http://www.youtube.com/" + videoId); return getSomeVideo(videoId); } // ----------------------------------------------------------------------- // Fake methods to simulate network activity. They as slow as a real life. private int random(int min, int max) { return min + (int) (Math.random() * ((max - min) + 1)); } private void experienceNetworkLatency() { int randomLatency = random(5, 10); for (int i = 0; i < randomLatency; i++) { try { Thread.sleep(100); } catch (InterruptedException ex) { ex.printStackTrace(); } } } private void connectToServer(String server) { System.out.print("Connecting to " + server + "... "); experienceNetworkLatency(); System.out.print("Connected!" + "\n"); } private HashMap<String, Video> getRandomVideos() { System.out.print("Downloading populars... "); experienceNetworkLatency(); HashMap<String, Video> hmap = new HashMap<String, Video>(); hmap.put("catzzzzzzzzz", new Video("sadgahasgdas", "Catzzzz.avi")); hmap.put("mkafksangasj", new Video("mkafksangasj", "Dog play with ball.mp4")); hmap.put("dancesvideoo", new Video("asdfas3ffasd", "Dancing video.mpq")); hmap.put("dlsdk5jfslaf", new Video("dlsdk5jfslaf", "Barcelona vs RealM.mov")); hmap.put("3sdfgsd1j333", new Video("3sdfgsd1j333", "Programing lesson#1.avi")); System.out.print("Done!" + "\n"); return hmap; } private Video getSomeVideo(String videoId) { System.out.print("Downloading video... "); experienceNetworkLatency(); Video video = new Video(videoId, "Some video title"); System.out.print("Done!" + "\n"); return video; } }
some_cool_media_library/Video.java: 视频文件
package proxy.some_cool_media_library; /** * @author GaoMing * @date 2021/7/20 - 19:22 */ public class Video { public String id; public String title; public String data; Video(String id, String title) { this.id = id; this.title = title; this.data = "Random video."; } }
proxy/YouTubeCacheProxy.java: 缓存代理
package proxy.proxy; import proxy.some_cool_media_library.ThirdPartyYouTubeClass; import proxy.some_cool_media_library.ThirdPartyYouTubeLib; import proxy.some_cool_media_library.Video; import java.util.HashMap; /** * @author GaoMing * @date 2021/7/20 - 19:23 */ public class YouTubeCacheProxy implements ThirdPartyYouTubeLib{ private ThirdPartyYouTubeLib youtubeService; private HashMap<String, Video> cachePopular = new HashMap<String, Video>(); private HashMap<String, Video> cacheAll = new HashMap<String, Video>(); public YouTubeCacheProxy() { this.youtubeService = new ThirdPartyYouTubeClass(); } @Override public HashMap<String, Video> popularVideos() { if (cachePopular.isEmpty()) { cachePopular = youtubeService.popularVideos(); } else { System.out.println("Retrieved list from cache."); } return cachePopular; } @Override public Video getVideo(String videoId) { Video video = cacheAll.get(videoId); if (video == null) { video = youtubeService.getVideo(videoId); cacheAll.put(videoId, video); } else { System.out.println("Retrieved video '" + videoId + "' from cache."); } return video; } public void reset() { cachePopular.clear(); cacheAll.clear(); } }
downloader/YouTubeDownloader.java: 媒体下载应用
package proxy.downloader; import proxy.some_cool_media_library.ThirdPartyYouTubeLib; import proxy.some_cool_media_library.Video; import java.util.HashMap; /** * @author GaoMing * @date 2021/7/20 - 19:23 */ public class YouTubeDownloader { private ThirdPartyYouTubeLib api; public YouTubeDownloader(ThirdPartyYouTubeLib api) { this.api = api; } public void renderVideoPage(String videoId) { Video video = api.getVideo(videoId); System.out.println("\n-------------------------------"); System.out.println("Video page (imagine fancy HTML)"); System.out.println("ID: " + video.id); System.out.println("Title: " + video.title); System.out.println("Video: " + video.data); System.out.println("-------------------------------\n"); } public void renderPopularVideos() { HashMap<String, Video> list = api.popularVideos(); System.out.println("\n-------------------------------"); System.out.println("Most popular videos on YouTube (imagine fancy HTML)"); for (Video video : list.values()) { System.out.println("ID: " + video.id + " / Title: " + video.title); } System.out.println("-------------------------------\n"); } }
Demo.java: 初始化代码
package proxy; import proxy.downloader.YouTubeDownloader; import proxy.proxy.YouTubeCacheProxy; import proxy.some_cool_media_library.ThirdPartyYouTubeClass; /** * @author GaoMing * @date 2021/7/20 - 19:21 */ public class Demo { public static void main(String[] args) { YouTubeDownloader naiveDownloader = new YouTubeDownloader(new ThirdPartyYouTubeClass()); YouTubeDownloader smartDownloader = new YouTubeDownloader(new YouTubeCacheProxy()); long naive = test(naiveDownloader); long smart = test(smartDownloader); System.out.print("Time saved by caching proxy: " + (naive - smart) + "ms"); } private static long test(YouTubeDownloader downloader) { long startTime = System.currentTimeMillis(); // User behavior in our app: downloader.renderPopularVideos(); downloader.renderVideoPage("catzzzzzzzzz"); downloader.renderPopularVideos(); downloader.renderVideoPage("dancesvideoo"); // Users might visit the same page quite often. downloader.renderVideoPage("catzzzzzzzzz"); downloader.renderVideoPage("someothervid"); long estimatedTime = System.currentTimeMillis() - startTime; System.out.print("Time elapsed: " + estimatedTime + "ms\n"); return estimatedTime; } }
执行结果
Connecting to http://www.youtube.com... Connected! Downloading populars... Done! ------------------------------- Most popular videos on YouTube (imagine fancy HTML) ID: sadgahasgdas / Title: Catzzzz.avi ID: asdfas3ffasd / Title: Dancing video.mpq ID: 3sdfgsd1j333 / Title: Programing lesson#1.avi ID: mkafksangasj / Title: Dog play with ball.mp4 ID: dlsdk5jfslaf / Title: Barcelona vs RealM.mov ------------------------------- Connecting to http://www.youtube.com/catzzzzzzzzz... Connected! Downloading video... Done! ------------------------------- Video page (imagine fancy HTML) ID: catzzzzzzzzz Title: Some video title Video: Random video. ------------------------------- Connecting to http://www.youtube.com... Connected! Downloading populars... Done! ------------------------------- Most popular videos on YouTube (imagine fancy HTML) ID: sadgahasgdas / Title: Catzzzz.avi ID: asdfas3ffasd / Title: Dancing video.mpq ID: 3sdfgsd1j333 / Title: Programing lesson#1.avi ID: mkafksangasj / Title: Dog play with ball.mp4 ID: dlsdk5jfslaf / Title: Barcelona vs RealM.mov ------------------------------- Connecting to http://www.youtube.com/dancesvideoo... Connected! Downloading video... Done! ------------------------------- Video page (imagine fancy HTML) ID: dancesvideoo Title: Some video title Video: Random video. ------------------------------- Connecting to http://www.youtube.com/catzzzzzzzzz... Connected! Downloading video... Done! ------------------------------- Video page (imagine fancy HTML) ID: catzzzzzzzzz Title: Some video title Video: Random video. ------------------------------- Connecting to http://www.youtube.com/someothervid... Connected! Downloading video... Done! ------------------------------- Video page (imagine fancy HTML) ID: someothervid Title: Some video title Video: Random video. ------------------------------- Time elapsed: 9354ms Connecting to http://www.youtube.com... Connected! Downloading populars... Done! ------------------------------- Most popular videos on YouTube (imagine fancy HTML) ID: sadgahasgdas / Title: Catzzzz.avi ID: asdfas3ffasd / Title: Dancing video.mpq ID: 3sdfgsd1j333 / Title: Programing lesson#1.avi ID: mkafksangasj / Title: Dog play with ball.mp4 ID: dlsdk5jfslaf / Title: Barcelona vs RealM.mov ------------------------------- Connecting to http://www.youtube.com/catzzzzzzzzz... Connected! Downloading video... Done! ------------------------------- Video page (imagine fancy HTML) ID: catzzzzzzzzz Title: Some video title Video: Random video. ------------------------------- Retrieved list from cache. ------------------------------- Most popular videos on YouTube (imagine fancy HTML) ID: sadgahasgdas / Title: Catzzzz.avi ID: asdfas3ffasd / Title: Dancing video.mpq ID: 3sdfgsd1j333 / Title: Programing lesson#1.avi ID: mkafksangasj / Title: Dog play with ball.mp4 ID: dlsdk5jfslaf / Title: Barcelona vs RealM.mov ------------------------------- Connecting to http://www.youtube.com/dancesvideoo... Connected! Downloading video... Done! ------------------------------- Video page (imagine fancy HTML) ID: dancesvideoo Title: Some video title Video: Random video. ------------------------------- Retrieved video 'catzzzzzzzzz' from cache. ------------------------------- Video page (imagine fancy HTML) ID: catzzzzzzzzz Title: Some video title Video: Random video. ------------------------------- Connecting to http://www.youtube.com/someothervid... Connected! Downloading video... Done! ------------------------------- Video page (imagine fancy HTML) ID: someothervid Title: Some video title Video: Random video. ------------------------------- Time elapsed: 5875ms Time saved by caching proxy: 3479ms
7. 与其他模式的关系
- 适配器模式能为被封装对象提供不同的接口，代理模式能为对象提供相同的接口，装饰模式则能为对象提供加强的接口
- 外观模式与代理的相似之处在于它们都缓存了一个复杂实体并自行对其进行初始化。但是，代理与其服务对象遵循同一接口，其与服务对象可以互换
- 装饰和代理有着相似的结构，但是其意图却非常不同。这两个模式的构建都基于组合原则，也就是说一个对象应该将部分工作委派给另一个对象。两者之间的不同之处在于代理通常自行管理其服务对象的生命周期，而装饰的生成则总是由客户端进行控制
8. 已知应用
java.lang.reflect.Proxy
识别方法：代理模式会将所有实际
工作委派给一些其他对象。除非代理是某个服务的子类，否则每个代理方法最后都应该引用一个服务对象
