RequestMappingHandlerMapping 详解

我们先理简单梳理一个关系

关系梳理

spring ioc 是spring的核心，用来管理spring bean的生命周期
MVC 是一种使用 MVC（Model View Controller 模型-视图-控制器）设计创建 Web 应用程序的模式
spring mvc 是spring的一个独立的模块，就像AOP一样

在spring mvc中把web框架和spring ioc融合在一起，是通过ContextLoaderListener监听servlet上下文的创建后来加载父容器完成的，然后通过配置一个servlet对象DispatcherServlet，在初始化DispatcherServlet时来加载具体子容器，详细的可以参考spring ioc & web宿主这篇文章

关于我们今天要讲的RequestMappingHandlerMapping也是在DispatcherServlet的初始化过程中自动加载的，默认会自动加载所有实现HandlerMapping接口的bean，且我们可以通过serOrder来设置优先级，系统默认会加载RequestMappingHandlerMapping、BeanNameUrlHandlerMapping、SimpleUrlHandlerMapping 并且按照顺序使用

 1private void initHandlerMappings(ApplicationContext context) {
 2        this.handlerMappings = null;
 3        if (this.detectAllHandlerMappings) {
 4            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
 5            Map<String, HandlerMapping> matchingBeans =
 6                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
 7            if (!matchingBeans.isEmpty()) {
 8                this.handlerMappings = new ArrayList<>(matchingBeans.values());
 9                // We keep HandlerMappings in sorted order.
10                AnnotationAwareOrderComparator.sort(this.handlerMappings);
11            }
12        }
13}

RequestMappingHandlerMapping 加载过程

RequestMappingHandlerMapping 实现了接口InitializingBean，在bean加载完成后会自动调用afterPropertiesSet方法，在此方法中调用了initHandlerMethods()来实现初始化
遍历所有bean，如果bean实现带有注解@Controller或者@RequestMapping 则进一步调用detectHandlerMethods处理，处理逻辑大致就是根据@RequestMapping配置的信息，构建RequestMappingInfo，然后注册到MappingRegistry中

 1protected void initHandlerMethods() {
 2        String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
 3                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(obtainApplicationContext(), Object.class) :
 4                obtainApplicationContext().getBeanNamesForType(Object.class));
 5        for (String beanName : beanNames) {
 6            if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
 7                Class<?> beanType = null;
 8                beanType = obtainApplicationContext().getType(beanName);
 9                if (beanType != null && isHandler(beanType)) {
10                    detectHandlerMethods(beanName);
11                }
12            }
13        }
14        handlerMethodsInitialized(getHandlerMethods());
15    }

 1protected void detectHandlerMethods(final Object handler) {
 2        Class<?> handlerType = (handler instanceof String ?
 3                obtainApplicationContext().getType((String) handler) : handler.getClass());
 4        if (handlerType != null) {
 5            final Class<?> userType = ClassUtils.getUserClass(handlerType);
 6            Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
 7                    (MethodIntrospector.MetadataLookup<T>) method -> {
 8                        try {
 9                            return getMappingForMethod(method, userType);
10                        }
11                        catch (Throwable ex) {
12                            throw new IllegalStateException("Invalid mapping on handler class [" +
13                                    userType.getName() + "]: " + method, ex);
14                        }
15                    });
16            methods.forEach((method, mapping) -> {
17                Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
18                registerHandlerMethod(handler, invocableMethod, mapping);
19            });
20        }
21    }

RequestMappingHandlerMapping 解析过程

在DispatcherServlet中，根据请求对象调用getHander方法获取HandlerExecutionChain对象
在getHander方法中也是遍历上面默认加载的三个HandlerMapping，当然第一个就是RequestMappingHandlerMapping对象，调用其getHandler方法,根据请求path，找到一个最为匹配的HandlerMethod来处理请求

 1protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
 2        if (this.handlerMappings != null) {
 3            for (HandlerMapping hm : this.handlerMappings) {
 4                if (logger.isTraceEnabled()) {
 5                    logger.trace(
 6                            "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
 7                }
 8                HandlerExecutionChain handler = hm.getHandler(request);
 9                if (handler != null) {
10                    return handler;
11                }
12            }
13        }
14        return null;
15    }

根据请求路径获取HandlerInterceptor，然后和上面获得的HandlerMethod一起构成HandlerExecutionChain返回给DispatcherServlet

DispatcherServlet得到HandlerExecutionChain也就获得了处理此次请求所需的Handler【即我们熟悉的Controller和对应的Action】，后续将会选择合适HandlerAdapter来执行对应的Handler，获取返回值，再根据返回值类型，进一步觉决定用什么方式展示给用户，下一遍将开启HandlerAdapter的讲解…….

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