SpringBoot之请求映射原理
前言
我们发出的请求，SpringMVC是如何精准定位到那个Controller以及具体方法？其实这都是 HandlerMapping 发挥的作用，这篇博文我们以 RequestMappingHandlerMapping 为例并结合源码一步步进行分析。
定义HandlerMapping
默认 HandlerMapping 主要定义在 EnableWebMvcConfiguration 和其祖父类 WebMvcConfigurationSupport 中
EnableWebMvcConfiguration
@Bean
@Primary
@Override
public RequestMappingHandlerMapping requestMappingHandlerMapping(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {// Must be @Primary for MvcUriComponentsBuilder to workreturn super.requestMappingHandlerMapping(contentNegotiationManager, conversionService,resourceUrlProvider);
}@Bean
public WelcomePageHandlerMapping welcomePageHandlerMapping(ApplicationContext applicationContext,FormattingConversionService mvcConversionService, ResourceUrlProvider mvcResourceUrlProvider) {WelcomePageHandlerMapping welcomePageHandlerMapping = new WelcomePageHandlerMapping(new TemplateAvailabilityProviders(applicationContext), applicationContext, getWelcomePage(),this.mvcProperties.getStaticPathPattern());welcomePageHandlerMapping.setInterceptors(getInterceptors(mvcConversionService, mvcResourceUrlProvider));welcomePageHandlerMapping.setCorsConfigurations(getCorsConfigurations());return welcomePageHandlerMapping;
}WebMvcConfigurationSupport
@Bean
public BeanNameUrlHandlerMapping beanNameHandlerMapping(@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {BeanNameUrlHandlerMapping mapping = new BeanNameUrlHandlerMapping();mapping.setOrder(2);PathMatchConfigurer pathConfig = getPathMatchConfigurer();if (pathConfig.getPatternParser() != null) {mapping.setPatternParser(pathConfig.getPatternParser());}else {mapping.setUrlPathHelper(pathConfig.getUrlPathHelperOrDefault());mapping.setPathMatcher(pathConfig.getPathMatcherOrDefault());}mapping.setInterceptors(getInterceptors(conversionService, resourceUrlProvider));mapping.setCorsConfigurations(getCorsConfigurations());return mapping;
}@Bean
public RouterFunctionMapping routerFunctionMapping(@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {RouterFunctionMapping mapping = new RouterFunctionMapping();mapping.setOrder(3);mapping.setInterceptors(getInterceptors(conversionService, resourceUrlProvider));mapping.setCorsConfigurations(getCorsConfigurations());mapping.setMessageConverters(getMessageConverters());PathPatternParser patternParser = getPathMatchConfigurer().getPatternParser();if (patternParser != null) {mapping.setPatternParser(patternParser);}return mapping;
}@Bean
@Nullable
public HandlerMapping resourceHandlerMapping(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {Assert.state(this.applicationContext != null, "No ApplicationContext set");Assert.state(this.servletContext != null, "No ServletContext set");PathMatchConfigurer pathConfig = getPathMatchConfigurer();ResourceHandlerRegistry registry = new ResourceHandlerRegistry(this.applicationContext,this.servletContext, contentNegotiationManager, pathConfig.getUrlPathHelper());addResourceHandlers(registry);AbstractHandlerMapping handlerMapping = registry.getHandlerMapping();if (handlerMapping == null) {return null;}if (pathConfig.getPatternParser() != null) {handlerMapping.setPatternParser(pathConfig.getPatternParser());}else {handlerMapping.setUrlPathHelper(pathConfig.getUrlPathHelperOrDefault());handlerMapping.setPathMatcher(pathConfig.getPathMatcherOrDefault());}handlerMapping.setInterceptors(getInterceptors(conversionService, resourceUrlProvider));handlerMapping.setCorsConfigurations(getCorsConfigurations());return handlerMapping;
}PS：WebMvcConfigurationSupport 中定义的 HandlerMapping 不止上述三个，但是有效的只有三个 （SpringBoot 版本 2.6.13），有的 HandlerMapping 需要满足一定条件才生效。
初始化
initHandlerMappings
private void initHandlerMappings(ApplicationContext context) {this.handlerMappings = null;if (this.detectAllHandlerMappings) {// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.Map<String, HandlerMapping> matchingBeans =BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);if (!matchingBeans.isEmpty()) {this.handlerMappings = new ArrayList<>(matchingBeans.values());// We keep HandlerMappings in sorted order.AnnotationAwareOrderComparator.sort(this.handlerMappings);}}else {try {HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);this.handlerMappings = Collections.singletonList(hm);}catch (NoSuchBeanDefinitionException ex) {// Ignore, we'll add a default HandlerMapping later.}}// Ensure we have at least one HandlerMapping, by registering// a default HandlerMapping if no other mappings are found.if (this.handlerMappings == null) {this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);if (logger.isTraceEnabled()) {logger.trace("No HandlerMappings declared for servlet '" + getServletName() +"': using default strategies from DispatcherServlet.properties");}}for (HandlerMapping mapping : this.handlerMappings) {if (mapping.usesPathPatterns()) {this.parseRequestPath = true;break;}}
}detectAllHandlerMappings 属性是否为 true （默认为true）
- true：获取 BeanFactory 中类型为 HandlerMapping 的 beans
- false : 获取 BeanFactory 中类型为 HandlerMapping，beanName为 handlerMapping 的 bean
如果 detectAllHandlerMappings 属性为 true，则会对查找到的 beans 进行排序，排序规则如下：
- HandlerMapping 是否继承 PriorityOrdered 接口，如果都继承 PriorityOrdered 接口，比较getOrder方法返回的值，值越小，优先级越高
- HandlerMapping 是否继承 Ordered 接口，如果都继承 Ordered接 口，比较 getOrder 方法返回的值，值越小，优先级越高
- HandlerMapping 所属class上是否存在 @Order 注解，如果存在，比较注解设置的值，值越小，优先级越高
- HandlerMapping 所属class上是否存在 @Priority 注解，如果存在，比较注解设置的值，值越小，优先级越高
如果从 BeanFactory 中未获取到相关 HandlerMapping，则使用默认 HandlerMapping
默认的 HandlerMapping 定义在 DispatcherServlet.properties 文件中
默认HandlerMapping为
- BeanNameUrlHandlerMapping
- RequestMappingHandlerMapping
- RouterFunctionMapping
请求映射
DispatcherServlet#doDispatch
DispatcherServlet#getHandler
一共有五个 HandlerMapping （SpringBoot 版本 2.6.13，最新版本貌似有6个，大家自行验证一下），即我们上文所分析的在类EnableWebMvcConfiguration、WebMvcConfigurationSupport 中定义的 HandlerMapping
如果某个 HandlerMapping 的 getHandler 方法返回了一个有效的 HandlerExecutionChain，我们就可以认为这个 HandlerMapping 可以处理这个请求。接下里以 RequestMappingHandlerMapping 为例进行分析，大部分请求也都是由这个 HandlerMapping 处理的
PS : WelcomePageHandlerMapping 就是处理欢迎页的
RequestMappingHandlerMapping的实例化
RequestMappingHandlerMapping的类继承关系
通过上图，我们知道其祖父类（AbstractHandlerMethodMapping） 继承 InitializingBean 接口，继承 InitializingBean 接口的类会在bean的实例化过程中执行 afterPropertiesSet 方法
AbstractHandlerMethodMapping#afterPropertiesSet
获取所有类型是 Object 的 beans，并且 beanName 不以 scopedTarget. 开头
AbstractHandlerMethodMapping#processCandidateBean
根据 beanName 获取 beanType
AbstractHandlerMethodMapping#isHandler
如果 beanType 上存在 @Controller 、@RequestMapping 注解则进行处理
AbstractHandlerMethodMapping#detectHandlerMethods
主要有三个方法 selectMethods、getMappingForMethod、registerHandlerMethod
- selectMethods ：遍历类中定义的方法以及接口实现方法（方法不能是合成的、桥接的，返回类型不能是Object ），如果某个方法执行接口函数（主体是 getMappingForMethod 方法）的返回值不为null，则将其放入一个类型为 Map<Method, T> 的 Map 中
- getMappingForMethod：如果方法存在 @RequestMapping （@GetMapping、@PostMapping、@PutMapping、@DeleteMapping 等）注解，则构建一个 RequestMappingInfo 对象
- registerHandlerMethod ： 遍历 selectMethods 方法的返回结果（Map<Method, T>），将其注册到 AbstractHandlerMethodMapping 的 mappingRegistry 属性中
PS : 经过测试，如果存在 @RequestMapping 注解，即使方法的描述符是 private，也可以接受请求
RequestMappingHandlerMapping的getHandler方法
AbstractHandlerMapping#getHandler
RequestMappingInfoHandlerMapping#getHandlerInternal
AbstractHandlerMethodMapping#getHandlerInternal
AbstractHandlerMethodMapping#lookupHandlerMethod
假设有一个UserController，明细由下方所示，我们来看一下这个 mappingRegistry 属性的结构
@RestController
public class UserController {@GetMapping("/user")public String getUser() {return "get user";}@PostMapping("/user")public String postUser() {return "post user";}@PutMapping("/user")public String putUser() {return "put user";}@DeleteMapping("/user")public String deleteUser() {return "delete user";}
}
URI 和具体方法的映射关系，都存储在 mappingRegistry 这个属性中
扩展：自定义HandlerMapping
创建 CustomHandlerMapping
public class CustomHandlerMapping implements HandlerMapping, PriorityOrdered {private ApplicationContext applicationContext;public CustomHandlerMapping(ApplicationContext applicationContext) {this.applicationContext = applicationContext;}@Overridepublic HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {String beanName = request.getParameter("beanName");String methodName = request.getParameter("method");if (StringUtils.isBlank(beanName) || StringUtils.isBlank(methodName) || !applicationContext.containsBean(beanName)) {return null;}Object object = applicationContext.getBean(beanName);Method method = null;Method[] declaredMethods = object.getClass().getDeclaredMethods();for (Method declaredMethod : declaredMethods) {if (declaredMethod.getName().equals(methodName)) {method = declaredMethod;break;}}if (method == null) {return null;}HandlerMethod handlerMethod = new HandlerMethod(object, method);return new HandlerExecutionChain(handlerMethod);}@Overridepublic int getOrder() {return 0;}
}继承 PriorityOrdered 接口，让我们自定义的 HandlerMapping 优先级最高
创建 HandlerMappingConfig
@Configuration
public class HandlerMappingConfig implements ApplicationContextAware {private ApplicationContext applicationContext;@Overridepublic void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}@Beanpublic CustomHandlerMapping customHandlerMapping() {return new CustomHandlerMapping(applicationContext);}
}创建 CustomHandlerMappingController
@Component("chmc")
public class CustomHandlerMappingController {@ResponseBodypublic String hello() {return "hello HandlerMapping! ";}
}发送请求 localhost:8080?beanName=chmc&method=hello
自定义 handlerMapping 生效