SpringMVC在@RequestMapping配置两个相同路径

　　这篇博客来自这个问题： 在SpringMVC中@RequestMapping可以配置两个相同的url路径吗。

　　首先，这个问题会点SpringMVC的人可能都知道答案，但是上次面试中我就回答了可以。。。可以。。Spicy Chicken!!!

　　参考文章： http://lgbolgger.iteye.com/blog/2105108

　　这个问题要从 RequestMappingHandlerMapping 和 RequestMappingHandlerAdapter 讲起了。　

　　首先，在配置文件中声明了 <mvc:annotation-driven /> 注解之后， 在 initStrategies() 方法中注册了处理的类：

 1 protected void initStrategies(ApplicationContext context) {
 2         initMultipartResolver(context);
 3         initLocaleResolver(context);
 4         initThemeResolver(context);
 5      initHandlerMappings(context);
 6         initHandlerAdapters(context);
 7         initHandlerExceptionResolvers(context);
 8         initRequestToViewNameTranslator(context);
 9         initViewResolvers(context);
10         initFlashMapManager(context);
11     }  

　　容器对 @ReqeustMapping 便签的处理的简化流程就是首先 RequestMappingHandlerMapping 类去查找有 @Controller 或 @RequestMapping 的类， 然后为含有其中一个注解的类还有类中含有 @RequestMapping 注解的方法构建 HandlerMethod 对象， 之后 RequestMappingHandlerAdapter 判断是否 support 对应的方法并执行对应的方法。

　　具体的过程如下：

　　1， RequestMappingHandlerMapping 遍历所有的 bean， 判断是否有 @Controller 或 @RequestMapping 注解

 1     protected void initHandlerMethods() {
 2         if (logger.isDebugEnabled()) {
 3             logger.debug("Looking for request mappings in application context: " + getApplicationContext());
 4         }
 5
 6         String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
 7                 BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
 8                 getApplicationContext().getBeanNamesForType(Object.class));
 9
10         for (String beanName : beanNames) {
11             if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX) &&
12           isHandler(getApplicationContext().getType(beanName))){
13                 detectHandlerMethods(beanName);
14             }
15         }
16         handlerMethodsInitialized(getHandlerMethods());
17     }  

　　isHandler() 判断的方法：

1     protected boolean isHandler(Class<?> beanType) {
2         return ((AnnotationUtils.findAnnotation(beanType, Controller.class) != null) ||
3                 (AnnotationUtils.findAnnotation(beanType, RequestMapping.class) != null));
4     }  

　　注意方法中的判断是用 || 逻辑，说明两者当中的其中一个符合即可。

　　2，处理含有 @RequestMapping 的方法，遍历第一步中找到的类中的所有方法，用一个 MethodFilter 查找所有的 @ReqeustMapping 注解的方法，并为找到的 @RequestMapping 的方法构建 ReqeustMappingInfo 对象

 1 /**
 2      * Look for handler methods in a handler.
 3      * @param handler the bean name of a handler or a handler instance
 4      */
 5     protected void detectHandlerMethods(final Object handler) {
 6         Class<?> handlerType =
 7                 (handler instanceof String ? getApplicationContext().getType((String) handler) : handler.getClass());
 8
 9         // Avoid repeated calls to getMappingForMethod which would rebuild RequestMappingInfo instances
10         final Map<Method, T> mappings = new IdentityHashMap<Method, T>();
11         final Class<?> userType = ClassUtils.getUserClass(handlerType);
12
13         Set<Method> methods = HandlerMethodSelector.selectMethods(userType, new MethodFilter() {
14             @Override
15             public boolean matches(Method method) {
16                 T mapping = getMappingForMethod(method, userType);
17                 if (mapping != null) {
18                     mappings.put(method, mapping);
19                     return true;
20                 }
21                 else {
22                     return false;
23                 }
24             }
25         });
26
27         for (Method method : methods) {
28             registerHandlerMethod(handler, method, mappings.get(method));
29         }
30     }  

　　其中查找 @RequestMapping 注解的方法的 getMappingForMethod() 方法：

 1 /**
 2      * Uses method and type-level @{@link RequestMapping} annotations to create
 3      * the RequestMappingInfo.
 4      * @return the created RequestMappingInfo, or {@code null} if the method
 5      * does not have a {@code @RequestMapping} annotation.
 6      * @see #getCustomMethodCondition(Method)
 7      * @see #getCustomTypeCondition(Class)
 8      */
 9     @Override
10     protected RequestMappingInfo getMappingForMethod(Method method, Class<?> handlerType) {
11         RequestMappingInfo info = null;
12         RequestMapping methodAnnotation = AnnotationUtils.findAnnotation(method, RequestMapping.class);
13         if (methodAnnotation != null) {
14             RequestCondition<?> methodCondition = getCustomMethodCondition(method);
15             info = createRequestMappingInfo(methodAnnotation, methodCondition);
16             RequestMapping typeAnnotation = AnnotationUtils.findAnnotation(handlerType, RequestMapping.class);
17             if (typeAnnotation != null) {
18                 RequestCondition<?> typeCondition = getCustomTypeCondition(handlerType);
19                 info = createRequestMappingInfo(typeAnnotation, typeCondition).combine(info);
20             }
21         }
22         return info;
23     }  

　　构建 RequestMappingInfo 对象的 createRequestMappingInfo() 方法：

 1 /**
 2      * Created a RequestMappingInfo from a RequestMapping annotation.
 3      */
 4     protected RequestMappingInfo createRequestMappingInfo(RequestMapping annotation, RequestCondition<?> customCondition) {
 5         String[] patterns = resolveEmbeddedValuesInPatterns(annotation.value());
 6         return new RequestMappingInfo(
 7                 new PatternsRequestCondition(patterns, getUrlPathHelper(), getPathMatcher(),
 8                         this.useSuffixPatternMatch, this.useTrailingSlashMatch, this.fileExtensions),
 9                 new RequestMethodsRequestCondition(annotation.method()),
10                 new ParamsRequestCondition(annotation.params()),
11                 new HeadersRequestCondition(annotation.headers()),
12                 new ConsumesRequestCondition(annotation.consumes(), annotation.headers()),
13                 new ProducesRequestCondition(annotation.produces(), annotation.headers(), this.contentNegotiationManager),
14                 customCondition);
15     }  

　　3，构建完 RequestMappingInfo 对象后， 存储到 Map 类型的 handlerMethods 对象中

 1 /**
 2      * Register a handler method and its unique mapping.
 3      * @param handler the bean name of the handler or the handler instance
 4      * @param method the method to register
 5      * @param mapping the mapping conditions associated with the handler method
 6      * @throws IllegalStateException if another method was already registered
 7      * under the same mapping
 8      */
 9     protected void registerHandlerMethod(Object handler, Method method, T mapping) {
10         HandlerMethod newHandlerMethod = createHandlerMethod(handler, method);
11         HandlerMethod oldHandlerMethod = this.handlerMethods.get(mapping);
12         if (oldHandlerMethod != null && !oldHandlerMethod.equals(newHandlerMethod)) {
13             throw new IllegalStateException("Ambiguous mapping found. Cannot map ‘" + newHandlerMethod.getBean() +
14                     "‘ bean method \n" + newHandlerMethod + "\nto " + mapping + ": There is already ‘" +
15                     oldHandlerMethod.getBean() + "‘ bean method\n" + oldHandlerMethod + " mapped.");
16         }
17
18         this.handlerMethods.put(mapping, newHandlerMethod);
19         if (logger.isInfoEnabled()) {
20             logger.info("Mapped \"" + mapping + "\" onto " + newHandlerMethod);
21         }
22
23         Set<String> patterns = getMappingPathPatterns(mapping);
24         for (String pattern : patterns) {
25             if (!getPathMatcher().isPattern(pattern)) {
26                 this.urlMap.add(pattern, mapping);
27             }
28         }
29     }  

　　 ***在这个地方， 给出了文章开头的问题的答案， 如果存在 @RequestMapping 注解有相同的访问路径时，会在此处抛出异常 （注释很重要）*************

　　4， 构建完了  private final Map<T, HandlerMethod> handlerMethods = new LinkedHashMap(); 这个对象之后，该对象就包含了所有的它应该包含的 bean 了，它的key为RequestMappingInfo对象，value为handler和它中含有@RequestMapping注释的方法method构建的HandlerMethod。

　　5， url 匹配路径时， RequestMappingHandlerAdapter 依据为是否是HandlerMethod 类型判断是否执行该方法。（这点我不太懂）

1 public final boolean supports(Object handler) {
2         return handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler);
3     }  

　　声明两个相同路径的 @RequestMapping 注解并查看错误：

　　查看错误：

　　正好是步骤三中抛出异常的位置，问题结束！！