Spring MVC为何能准确的找到一个http请求对应controller的某个方法进行处理

迈不过友情╰ 2022-04-05 14:21 155阅读 0赞

问题已抛出，如标题。Spring版本基于**5.1.3**。

宏观而言，我们需要给一个类加注解@Controller，然后定义一个加了注解@RequestMapping的方法，这样Spring容器就可以准确找到对应的方法了。

其实要回答这个问题，可以从Spring源码去一步步分析。

在Spring MVC里，有一专门处理请求映射的接口**HandlerMapping**，查看此接口的实现类：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3Nvbmd6ZWhhbw_size_16_color_FFFFFF_t_70][]

其中，**RequestMappingHandlerMapping**是我们需要关注的。我们首先看一下**RequestMappingHandlerMapping**的抽象父类**AbstractHandlerMethodMapping**，省略其他方法，先关注这两个相关核心方法：

public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean {
     
        ...
        // Handler method detection
     
        /**
         * Detects handler methods at initialization.
         */
        @Override
        public void afterPropertiesSet() {
     
            initHandlerMethods();
     
            // Total includes detected mappings + explicit registrations via registerMapping..
            int total = this.getHandlerMethods().size();
     
            if ((logger.isTraceEnabled() && total == 0) || (logger.isDebugEnabled() && total > 0) ) {
                logger.debug(total + " mappings in " + formatMappingName());
            }
        }
     
        /**
         * Scan beans in the ApplicationContext, detect and register handler methods.
         * @see #isHandler(Class)
         * @see #getMappingForMethod(Method, Class)
         * @see #handlerMethodsInitialized(Map)
         */
        protected void initHandlerMethods() {
            String[] beanNames = obtainApplicationContext().getBeanNamesForType(Object.class);
     
            for (String beanName : beanNames) {
                if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                    Class<?> beanType = null;
                    try {
                        beanType = obtainApplicationContext().getType(beanName);
                    }
                    catch (Throwable ex) {
                        // An unresolvable bean type, probably from a lazy bean - let's ignore it.
                        if (logger.isTraceEnabled()) {
                            logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
                        }
                    }
                    if (beanType != null && isHandler(beanType)) {
                        detectHandlerMethods(beanName);
                    }
                }
            }
            handlerMethodsInitialized(getHandlerMethods());
        }
        ...
        
    }

可以看到**AbstractHandlerMethodMapping**实现了**InitializingBean**接口，在Spring初始化bean的时候，如果bean实现了**InitializingBean**接口，会自动调用**afterPropertiesSet**方法。**initHandlerMethods**方法，顾名思义是初始化HandlerMethods，查看它是被**afterPropertiesSet**方法调用，这个方法代表bean在容器中被初始化的时候，会去执行**initHandlerMethods**方法。

那**initHandlerMethods**方法具体做了什么事情？大概看一下方法内部的业务，首先拿到容器里的所有bean名称放进数组beanNames中；然后遍历数组，拿到每一个bean的类型beanType，对每一个beanType做了一个判断**isHandler**(beanType)，查看此方法的实现，即进入**RequestMappingHandlerMapping**中：

public class RequestMappingHandlerMapping extends RequestMappingInfoHandlerMapping
            implements EmbeddedValueResolverAware {
        ...
        /**
         * {@inheritDoc}
         * Expects a handler to have a type-level @{@link Controller} annotation.
         */
        @Override
        protected boolean isHandler(Class<?> beanType) {
            return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
                    AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
        }
        ...
    }

·很容易看出来，是去判断该类是否加了注解@Controller或@RequestMapping。点进去查看AnnotatedElementUtils.hasAnnotation方法的实现：

public static boolean hasAnnotation(AnnotatedElement element, Class<? extends Annotation> annotationType) {
    	// Shortcut: directly present on the element, with no processing needed?
    	if (element.isAnnotationPresent(annotationType)) {
    		return true;
    	}
    	return Boolean.TRUE.equals(searchWithFindSemantics(element, annotationType, null, alwaysTrueAnnotationProcessor));
    }

所以我们判断一个类上是否加了什么注解，可以这么写：

public class Test {
        public static void main(String[] args) {
            Test.class.isAnnotationPresent(Override.class);
        }
    }

判断完类上加了注解@Controller或者@RequestMapping后，看到继续执行了**detectHandlerMethods**方法：

/**
     * Look for handler methods in a handler.
     * @param handler the bean name of a handler or a handler instance
     */
    protected void detectHandlerMethods(final Object handler) {
    	Class<?> handlerType = (handler instanceof String ?
    			obtainApplicationContext().getType((String) handler) : handler.getClass());
    
    	if (handlerType != null) {
    		final Class<?> userType = ClassUtils.getUserClass(handlerType);
    		Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
    				(MethodIntrospector.MetadataLookup<T>) method -> getMappingForMethod(method, userType));
    		if (logger.isTraceEnabled()) {
    			logger.trace(formatMappings(userType, methods));
    		}
    		methods.forEach((key, mapping) -> {
    			Method invocableMethod = AopUtils.selectInvocableMethod(key, userType);
    			registerHandlerMethod(handler, invocableMethod, mapping);
    		});
    	}
    }
    
    /**
     * Register a handler method and its unique mapping. Invoked at startup for
     * each detected handler method.
     * @param handler the bean name of the handler or the handler instance
     * @param method the method to register
     * @param mapping the mapping conditions associated with the handler method
     * @throws IllegalStateException if another method was already registered
     * under the same mapping
     */
    protected void registerHandlerMethod(Object handler, Method method, T mapping) {
    	this.mappingRegistry.register(mapping, handler, method);
    }

这个方法筛选出了类中加了注解@RequestMapping的方法，放进Map集合methods中，紧接着去遍历每一个method，进入**registerHandlerMethod**方法，注册到映射注册表**mappingRegistry**中，其中就是一些Map。

至此，我们就明白了在Spring初始化bean的时候，就把所有的加了@Controller/@RequestMapping的类里面的加了@RequestMapping的方法放进了map中，当http请求来临时，直接去map中迅速拿到对应信息。

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