究 Spring 3.1之无web.xml式 基于代码配置的servlet3.0应用(转)

??大家应该都已经知道Spring 3.1对无web.xml式基于代码配置的servlet3.0应用。通过spring的api或是网络上高手们的博文，也一定很快就学会并且加到自己的应用中去了。PS：如果还没，也可以小小参考一下鄙人的上一篇文章＜＜探 Spring 3.1之无web.xml式 基于代码配置的servlet3.0应用＞＞。?

??????? 经过一天的深度research, 我了解，理解以及重现了springframework的那一小段代码。?

????????OK,第一步，入手点，WebApplicationInitializer接口。因为我们只需实现这个接口覆写它的一个方法，就可以做到配置web.xml同样的功效。看它的源码，其实看和不看没什么两样：?

Java代码??

1. package?org.springframework.web;??
2. ??
3. import?javax.servlet.ServletContext;??
4. import?javax.servlet.ServletException;??
5. public?interface?WebApplicationInitializer?{??
6. ????void?onStartup(ServletContext?servletContext)?throws?ServletException;??
7. }??

???就这么点儿，有效代码5行，弄地我一头雾水，就是一个普通接口，声明了一个方法。连注解都没有，server是怎么找到实现了它的类的？如果这样，何不找我定义的其它接口（的实现类完成配置工作）呢。可见现在java的解耦技术，真令人汗颜。?
?? 第二步，这个接口旁边(同包)有个SpringServletContainerInitializer, 看下它是何方神圣吧：?
Java代码??

1. package?org.springframework.web;??
2. ??
3. import?java.lang.reflect.Modifier;??
4. import?java.util.Collections;??
5. import?java.util.LinkedList;??
6. import?java.util.List;??
7. import?java.util.ServiceLoader;??
8. import?java.util.Set;??
9. import?javax.servlet.ServletContainerInitializer;??
10. import?javax.servlet.ServletContext;??
11. import?javax.servlet.ServletException;??
12. import?javax.servlet.annotation.HandlesTypes;??
13. ??
14. import?org.springframework.core.annotation.AnnotationAwareOrderComparator;??
15. ??
16. @HandlesTypes(WebApplicationInitializer.class)??
17. public?class?SpringServletContainerInitializer?implements?ServletContainerInitializer?{??
18. public?void?onStartup(Set<Class<?>>?webAppInitializerClasses,?ServletContext?servletContext)??
19. ????????????throws?ServletException?{??
20. ??
21. ????????List<WebApplicationInitializer>?initializers?=?new?LinkedList<WebApplicationInitializer>();??
22. ????????if?(webAppInitializerClasses?!=?null)?{??
23. ????????????for?(Class<?>?waiClass?:?webAppInitializerClasses)?{??
24. ????????????????//?Be?defensive:?Some?servlet?containers?provide?us?with?invalid?classes,??
25. ????????????????//?no?matter?what?@HandlesTypes?says...??
26. ????????????????if?(!waiClass.isInterface()?&&?!Modifier.isAbstract(waiClass.getModifiers())?&&?????????????WebApplicationInitializer.class.isAssignableFrom(waiClass))?{??
27. ????????????????????try?{??
28. ????????????????????????initializers.add((WebApplicationInitializer)?waiClass.newInstance());??
29. ????????????????????}??
30. ????????????????????catch?(Throwable?ex)?{??
31. ????????????????????????throw?new?ServletException("Failed?to?instantiate?WebApplicationInitializer?class",?ex);??
32. ????????????????????}??
33. ????????????????}??
34. ????????????}??
35. ????????}??
36. ??
37. ????????if?(initializers.isEmpty())?{??
38. ????????????servletContext.log("No?Spring?WebApplicationInitializer?types?detected?on?classpath");??
39. ????????????return;??
40. ????????}??
41. ??
42. ????????Collections.sort(initializers,?new?AnnotationAwareOrderComparator());??
43. ????????servletContext.log("Spring?WebApplicationInitializers?detected?on?classpath:?"?+?initializers);??
44. ??
45. ????????for?(WebApplicationInitializer?initializer?:?initializers)?{??
46. ????????????initializer.onStartup(servletContext);??
47. ????????}??
48. ????}??
49. ??
50. }??

???? 以上的有效代码28行。刚看时也很迷茫，其实慢慢就理解了。拟个伪代码吧，方便大家理解：?
????? 1，定义一个类SpringServletContainerInitializer，并标明该类要操作的一个类WebApplicationInitializer?
????? 2, 该类会行使ServletContainerInitializer接口的一个行为onStartup,从而将一个集合中的初始化设置 全部配置到ServletContext的实例中。?
????? 3，具体的onStartup方法中，建立合格配置列表，?
????? 4，如果确定集合中有配置，逐一检查配置是否是合格配置，具体判断依据：这个类不是接口，不是抽象类，而且是所要操作的那个接口的一个实现类。满足此依据，合格。将合格的配置类实例化放入合格配置列表。过程中有错要通知控制台。?
???? 5，如若执行完步骤4，发现没有合格配置，在ServletContext记录该结果，并结束onStartup行为。?
???? 6，将找到配置按一定排列方式（AnnotationAwareOrder）排序。?
???? 7，在ServletContext中记录找到结果。?
???? 8，逐一执行配置。 即驱动每一个WebApplicationInitializer的实现类行使其onStartup行为。?

???? 第三步很明显了，去research 接口ServletContainerInitializer和注解HandleType。在这里：http://docs.oracle.com/javaee/6/api/javax/servlet/ServletContainerInitializer.html?

????该接口允许一个库或运行时，(运行时应该指server)声明为一个web程序的启动状态，并执行任何所需的程序中注册的servlet,filter,listener来响应它......?
???? 其它也就不用看了，可以想象得到支持Servlet3机制的服务器，会找到这样接口的实现类，执行onStartup行为。至于如何找，无非也是这样一系列的反射机制的应用。自己做一个试试吧：?
???? 自定义的WebApplicationInitializer:?

Java代码??

1. package?com.gxino.imagecapture.cfg;??
2. ??
3. import?javax.servlet.ServletContext;??
4. import?javax.servlet.ServletException;??
5. ??
6. public?interface?WebParameter?{??
7. ????public?void?loadInfo(ServletContext?servletContext)?throws?ServletException;??
8. }??

???? 自定义的ServletContainerInitializer,我做得很简单，直接去执行找到配置类中的loadInfo方法?

Java代码??

1. package?com.gxino.imagecapture.cfg;??
2. ??
3. import?java.lang.reflect.Modifier;??
4. import?java.util.Set;??
5. ??
6. import?javax.servlet.ServletContainerInitializer;??
7. import?javax.servlet.ServletContext;??
8. import?javax.servlet.ServletException;??
9. import?javax.servlet.annotation.HandlesTypes;??
10. ??
11. @HandlesTypes(WebParameter.class)??
12. public?class?WebConfiguration?implements?ServletContainerInitializer?{??
13. ??
14. ????@Override??
15. ????public?void?onStartup(Set<Class<?>>?webParams,?ServletContext?servletCtx)??
16. ????????????throws?ServletException?{??
17. ????????if?(webParams?!=?null)?{??
18. ????????????for?(Class<?>?paramClass?:?webParams)?{??
19. ????????????????if?(!paramClass.isInterface()?&&?!Modifier.isAbstract(paramClass.getModifiers())?&&??
20. ????????????????????????WebParameter.class.isAssignableFrom(paramClass))?{??
21. ????????????????????try?{??
22. ????????????????????????((WebParameter)?paramClass.newInstance()).loadInfo(servletCtx);??
23. ????????????????????}??
24. ????????????????????catch?(Throwable?ex)?{??
25. ????????????????????????throw?new?ServletException("Failed?to?instantiate?WebParam?class",?ex);??
26. ????????????????????}??
27. ????????????????}??
28. ????????????}//loop??
29. ????????}//Web?Params??
30. ????}//onStartup??
31. ??
32. }??

??????写个测试Servlet:?

Java代码??

1. package?com.gxino.imagecapture.ctrl;??
2. ??
3. import?java.io.IOException;??
4. ??
5. import?javax.servlet.ServletException;??
6. import?javax.servlet.http.HttpServlet;??
7. import?javax.servlet.http.HttpServletRequest;??
8. import?javax.servlet.http.HttpServletResponse;??
9. ??
10. import?com.gxino.imagecapture.cfg.WebParameter;??
11. ??
12. public?class?TestServlet?extends?HttpServlet?{??
13. ??????
14. ????public?void?doGet(HttpServletRequest?req,?HttpServletResponse?resp){??
15. ????????System.out.println("Some?client?access?once");??
16. ????????try?{??
17. ????????????req.getRequestDispatcher("/index.jsp").forward(req,?resp);??
18. ????????}?catch?(ServletException?|?IOException?e)?{??
19. ????????????//?TODO?Auto-generated?catch?block??
20. ????????????e.printStackTrace();??
21. ????????}??
22. ????}??
23. ??????
24. }??

???????实现WebParam配置接口来配置刚才的Servlet：?

Java代码??

1. package?com.gxino.imagecapture.cfg;??
2. ??
3. import?javax.servlet.ServletContext;??
4. import?javax.servlet.ServletException;??
5. import?javax.servlet.ServletRegistration;??
6. ??
7. ??
8. public?class?ServletParameter?implements?WebParameter?{??
9. ??
10. ????@Override??
11. ????public?void?loadInfo(ServletContext?servletContext)?throws?ServletException?{??
12. ????????ServletRegistration.Dynamic?testServlet=servletContext.addServlet("test","com.gxino.imagecapture.ctrl.TestServlet");??
13. ????????testServlet.setLoadOnStartup(1);??
14. ????????testServlet.addMapping("/index.html");??
15. ????}??
16. ??
17. }??

?????启动服务器，访问http://localhost:xxxx/xxxxx/index.html?
??
???? 失败。Debug. 发现没有走这些代码。应该还差关键环节。看来还得知道Servlet3中是怎么找ServletContainerInitializer的。再回刚才ServletContainerInitializer的api有这样一句：该接口的实现必须声明一个JAR资源放到程序中的META-INF/services下，并且记有该接口那个实现类的全路径，才会被运行时(server)的查找机制或是其它特定机制找到。那篇api需要仔细阅读啊。?
???? 到org.springframework.web-3.0.1.RELEASE.jar中能找到META-INF/services下的javax.servlet.ServletContainerInitializer文件，内容为org.springframework.web.SpringServletContainerInitializer同样，我们专门作这样一个包，在mkdir好的META-INF/services下vi 一个文件命名为javax.servlet.ServletContainerInitializer，内容为自定的那个WebConfiguration的全路径类名。 然后在META-INF的parent路径下运行jar cvf test.jar META-INF。一切完毕，将其放到WEB-INF/lib下。启动。?
?????
?????这回大功告成。?
?????
???? 访问http://localhost:xxxx/xxxxx/index.html。页面跳到了index.jsp下。?
???? 并且控制台打出： Some client access once?

???? 再使个劲，将Servlet和Servlet配置合二为一：?
Java代码??

1. package?com.gxino.imagecapture.ctrl;??
2. ??
3. import?java.io.IOException;??
4. ??
5. import?javax.servlet.ServletContext;??
6. import?javax.servlet.ServletException;??
7. import?javax.servlet.ServletRegistration;??
8. import?javax.servlet.http.HttpServlet;??
9. import?javax.servlet.http.HttpServletRequest;??
10. import?javax.servlet.http.HttpServletResponse;??
11. ??
12. import?com.gxino.imagecapture.cfg.WebParameter;??
13. ??
14. public?class?TestServlet?extends?HttpServlet?implements?WebParameter{??
15. ??
16. ????@Override??
17. ????public?void?loadInfo(ServletContext?servletContext)?throws?ServletException?{??
18. ????????ServletRegistration.Dynamic?testServlet=servletContext.addServlet("test",?"com.gxino.imagecapture.ctrl.TestServlet");??
19. ????????testServlet.setLoadOnStartup(1);??
20. ????????testServlet.addMapping("/index.html");??
21. ????}??
22. ????public?void?doGet(HttpServletRequest?req,?HttpServletResponse?resp){??
23. ????????System.out.println("Some?client?access?once");??
24. ????????try?{??
25. ????????????req.getRequestDispatcher("/index.jsp").forward(req,?resp);??
26. ????????}?catch?(ServletException?|?IOException?e)?{??
27. ????????????//?TODO?Auto-generated?catch?block??
28. ????????????e.printStackTrace();??
29. ????????}??
30. ????}??
31. ??????
32. }??

这回我们看到，配置文件与servlet放到了一起。这样将回节省大量时间。?

??? 以后直接运用Spring Framework的WebApplicationInitializer也知道是怎么一回事儿了。而且可以将Spring 的applicationContext.xml与web.xml融合在一个类中。即注解为@Configuration,并实现WebApplicationInitializer.回头试试。