Spring Boot处理@RequestMapping注解

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要了解Spring MVC是怎么把@RequestMapping注解的方法以及类关联到相应的url上，首先需要看下AbstractHandlerMethodMapping：

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public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean {

　　AbstractHandlerMethodMapping是一个InitializingBean，因此来看afterPropertiesSet()方法：

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@Override
public void afterPropertiesSet() {
	initHandlerMethods();
}
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protected void initHandlerMethods() {
       // …略
       // 读取Spring容器中的所有beanNames
	String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
			BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
			getApplicationContext().getBeanNamesForType(Object.class));

	for (String beanName : beanNames) {
		if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
			Class<?> beanType = null;
			try {
				beanType = getApplicationContext().getType(beanName);
			}
			catch (Throwable ex) {
				// …略
			}
               // 判断这个bean是不是处理类
               // 根据这个类是否使用了@Controller或@RequestMapping
			if (beanType != null && isHandler(beanType)) {
				detectHandlerMethods(beanName);
			}
		}
	}
	handlerMethodsInitialized(getHandlerMethods());
}

　　关注点就需要放在detectHandlerMethods这个方法上:

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protected void detectHandlerMethods(final Object handler) {
	Class<?> handlerType = (handler instanceof String ?
			getApplicationContext().getType((String) handler) : handler.getClass());
	final Class<?> userType = ClassUtils.getUserClass(handlerType);

       // 从处理类(@Controller)中获取处理方法(@RequestMapping)
	Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
			new MethodIntrospector.MetadataLookup<T>() {
				@Override
				public T inspect(Method method) {
					try {
                           // 返回RequestMappingInfo，包含@RequestMapping的标签信息
						return getMappingForMethod(method, userType);
					}
					catch (Throwable ex) {
						throw new IllegalStateException("Invalid mapping on handler class [" +
								userType.getName() + "]: " + method, ex);
					}
				}
			});
	// …略
       // 注册方法
	for (Map.Entry<Method, T> entry : methods.entrySet()) {
		Method invocableMethod = AopUtils.selectInvocableMethod(entry.getKey(), userType);
		T mapping = entry.getValue();
		registerHandlerMethod(handler, invocableMethod, mapping);
	}
}

　　先调用getMappingForMethod获取RequestMappingInfo，然后调用registerHandlerMethod注册到MappingRegistry中，在MappingRegistry中关联了url与HandlerMethod、url与RequestMappingInfo等。

　　到这里，知道了AbstractHandlerMethodMapping怎么处理了@RequestMapping，那么AbstractHandlerMethodMapping是什么时候加入到了Spring容器中的呢？答案是实例化RequestMappingHandlerMapping。
　　Spring Boot的自动注解中，由ConfigurationClassPostProcessor把在配置项中的WebMvcAutoConfiguration进行处理，会把WebMvcAutoConfiguration$EnableWebMvcConfiguration注册到beanFactory中， 在EnableWebMvcConfiguration中：

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@Bean
@Primary
@Override
public RequestMappingHandlerMapping requestMappingHandlerMapping() {
	// Must be @Primary for MvcUriComponentsBuilder to work
	return super.requestMappingHandlerMapping();
}

　　定义一个requestMappingHandlerMapping的bean：

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public RequestMappingHandlerMapping requestMappingHandlerMapping() {
	RequestMappingHandlerMapping mapping = createRequestMappingHandlerMapping();
	// …略
	return mapping;
}

　　在RequestMappingHandlerMapping定义如下：

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public class RequestMappingHandlerMapping extends RequestMappingInfoHandlerMapping
		implements MatchableHandlerMapping, EmbeddedValueResolverAware {
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public abstract class RequestMappingInfoHandlerMapping extends AbstractHandlerMethodMapping<RequestMappingInfo> {

　　在RequestMappingHandlerMapping继承了AbstractHandlerMethodMapping，就把AbstractHandlerMethodMapping注册到了Spring容器中。