AbstractHandlerMethodMapping 是什么

官方解释是这样的。

/**
 * Abstract base class for {@link HandlerMapping} implementations that define
 * a mapping between a request and a {@link HandlerMethod}.
 *
 * <p>For each registered handler method, a unique mapping is maintained with
 * subclasses defining the details of the mapping type {@code <T>}.
 * @param <T> The mapping for a {@link HandlerMethod} containing the conditions
 * needed to match the handler method to incoming request.
 */
public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean 

我的理解为AbstractHandlerMethodMapping为每个注册的handler method，对于每个子类映射类型都维护着其唯一的一个映射，就是维护handler method 和URL的关系。主要用于@Controller,@RequestMapping 等注解

AbstractHandlerMethodMapping 实现了InitializingBean接口，InitializingBean是在程序启动的时候执行其唯一的afterPropertiesSet()方法，那我们就先看一下启动时候的要做哪些操作。

initHandlerMethods()

/**
      在初始化的时候发现 handler methods
    * Detects handler methods at initialization.
     */
    @Override
    public void afterPropertiesSet() {
        initHandlerMethods();
    }

在项目启动时会执行initHandlerMethods方法，它的主要功能是扫描应用上下文，发现并注册handler methods。

/**
 * Scan beans in the ApplicationContext, detect and register handler methods.
 * @see #isHandler(Class)
 * @see #getMappingForMethod(Method, Class)
 * @see #handlerMethodsInitialized(Map)
 */
protected void initHandlerMethods() {
        if (logger.isDebugEnabled()) {
                logger.debug("Looking for request mappings in application context: " + getApplicationContext());
        }
        // 获取IOC容器中所有bean
        String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
                        BeanFactoryUtils.beanNamesForTypeIncludingAncestors(obtainApplicationContext(), Object.class) :
                        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.isDebugEnabled()) {
                                        logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
                                }
                        }
                        // 判断bean 是否有Controller 注解或者RequestMapping 注解
                        if (beanType != null && isHandler(beanType)) {
                                detectHandlerMethods(beanName);
                        }
                }
        }
        handlerMethodsInitialized(getHandlerMethods());
}

isHandler()

AbstractHandlerMethodMapping#isHandler() RequestMappingHandlerMapping 是目前为止AbstractHandlerMethodMapping的子类中唯一实现了isHandler()方法的子类，看下实现

protected boolean isHandler(Class<?> beanType) {
        return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
                AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
    }

isHandler()方法在这里的作用就是检查beanType是否有@Controller或@RequestMapping注解，这两个注解经常使用，应该都很熟悉了。

detectHandlerMethods()

AbstractHandlerMethodMapping#detectHandlerMethods() 这个方法的作用是从handler中获取handler method并注册

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

        if (handlerType != null) {
                //为给定的类返回用户定义的类:通常只是给定的类，如果是cglib生成的子类，则返回原始的类。
                final Class<?> userType = ClassUtils.getUserClass(handlerType);
                Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                                (MethodIntrospector.MetadataLookup<T>) method -> {
                                        try {
                                                // 为处理程序方法提供映射
                                                return getMappingForMethod(method, userType);
                                        }
                                        catch (Throwable ex) {
                                                throw new IllegalStateException("Invalid mapping on handler class [" +
                                                                userType.getName() + "]: " + method, ex);
                                        }
                                });
                if (logger.isDebugEnabled()) {
                        logger.debug(methods.size() + " request handler methods found on " + userType + ": " + methods);
                }

                // 为查找到的handler method 进行注册
                methods.forEach((method, mapping) -> {
                        Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
                        registerHandlerMethod(handler, invocableMethod, mapping);
                });
        }
    }

registerHandlerMethod()

AbstractHandlerMethodMapping#registerHandlerMethod() 的作用是为每个handler method注册它们唯一的映射路径，源码如下：

/**
 * 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);
}

register方法是AbstractHandlerMethodMapping 内部类MappingRegistry里的方法，MappingRegistry是一个注册表