Spring MVC源码分析（三）：SpringMVC的HandlerMapping和HandlerAdapter的体系结构设计与实现

概述
在我的上一篇文章：Spring源码分析（三）：DispatcherServlet的设计与实现中提到，DispatcherServlet在接收到客户端请求时，会遍历DispatcherServlet自身维护的一个HandlerMapping集合，来查找该请求对应的请求处理器，然后由该请求处理器来执行请求处理。

在SpringMVC中，DispatcherServlet通过HandlerMapping接口来定义请求和请求处理器之间的映射关系，通过HandlerAdapter接口来提供一个模板实现，即以统一的方式，调用不同handler来执行请求处理。
HandlerMapping和HandlerAdapter并不是跟名字所描述的这样，即每个HandlerMapping定义一个请求和请求处理器的映射对，这两个接口不是从请求处理方法的角度来定义的，而是从请求处理器的实现类型的角度来定义的。HandlerMapping包含Url和HandlerMethod两种实现，HandlerAdapter包含HttpMethod，Servlet，Controller，HttpRequestHandler四种实现。

 

HandlerMapping：请求与请求处理器映射

 

DispatcherServlet通过HandlerMapping获取处理某个请求的请求处理器，请求处理器由请求执行器handler和匹配的拦截器链interceptors两部分组成，具体由HandlerExecutionChain定义请求处理器实现。所以在HandlerMapping接口的实现当中，需要维护两个重要的组件，分别是请求和请求处理器handler的映射map，总拦截器链interceptors，拦截器链在HandlerMapping接口的最顶层抽象实现类

AbstractHandlerMapping定义。

 

其次在AbstractHandlerMapping中会定义Cors跨域访问的处理，通常也是在拦截器链添加一个跨域拦截器。然后通过请求的header的Origin的值和应用的跨域配置来执行请求拦截。

 

请求和请求处理器handler的映射map

 

1. 基于URL

主要在抽象类AbstractUrlHandlerMapping中定义，具体实现子类包含BeanNameUrlHandlerMapping和SimpleUrlHandlerMapping。其中BeanNameUrlHandlerMapping的映射map的key为请求URI，value是beanName，即处理匹配这个uri的请求的bean，在spring容器中的beanName，通常是以“/”开头，如beanName为“/foo”的bean处理URI为“/foo”的请求，同时可以指定beanName的别名alias，这样这些别名也作为匹配的请求URI。获取映射关系的源码如下：

public class BeanNameUrlHandlerMapping extends AbstractDetectingUrlHandlerMapping {

    /**
     * Checks name and aliases of the given bean for URLs, starting with "/".
     */
    @Override
    protected String[] determineUrlsForHandler(String beanName) {
        List<String> urls = new ArrayList<>();

        // urls为以这个beanName为value的多个key，即map的key和value
        // beanName以“/”开头
        if (beanName.startsWith("/")) {
            urls.add(beanName);
        }
        // 别名也作为value
        String[] aliases = obtainApplicationContext().getAliases(beanName);
        for (String alias : aliases) {
            if (alias.startsWith("/")) {
                urls.add(alias);
            }
        }
        return StringUtils.toStringArray(urls);
    }

}

SimpleUrlHandlerMapping与BeanNameUrlHandlerMapping类似，不过映射map的value的类型可以是beanName或者是bean实例。而这个value对应的请求执行器，在SimpleUrlHandlerMapping与BeanNameUrlHandlerMapping中，可以是Servlet，Controller（是Controller接口实现类，不是@Controller注解）HttpRequestHandler，这个是与HandlerAdapter的实现类对应的。

2. 基于HandlerMethod

 

我们在应用代码中通常使用@Controller和@RequestMapping来定义请求和请求处理方法直接的映射关系，这种方式的请求和请求处理方法映射map是基于HandlerMethod来实现的，即将每个处理方法在springMVC中都会抽象成一个HandlerMethod对象，而请求的匹配条件配置，则是通过RequestMappingInfo来定义的。而这个对应HandlerMapping体系结构的设计是RequestMappingHandlerMapping。

HandlerMethod：基于方法的请求执行器

HandlerMethod主要用于封装@Controller注解的类的使用@RequestMapping（或@RequestMapping的变体，如@GetMapping）注解的方法信息，类设计如下：

/**
 * Encapsulates information about a handler method consisting of a
 * {@linkplain #getMethod() method} and a {@linkplain #getBean() bean}.
 * Provides convenient access to method parameters, the method return value,
 * method annotations, etc.
 *
 * <p>The class may be created with a bean instance or with a bean name
 * (e.g. lazy-init bean, prototype bean). Use {@link #createWithResolvedBean()}
 * to obtain a {@code HandlerMethod} instance with a bean instance resolved
 * through the associated {@link BeanFactory}.
 * @since 3.1
 */
public class HandlerMethod {
    private final Object bean;
    @Nullable
    private final BeanFactory beanFactory;
    private final Class<?> beanType;
    private final Method method;
    private final Method bridgedMethod;
    private final MethodParameter[] parameters;
    @Nullable
    private HttpStatus responseStatus;
    @Nullable
    private String responseStatusReason;
    @Nullable
    private HandlerMethod resolvedFromHandlerMethod;
    @Nullable
    private volatile List<Annotation[][]> interfaceParameterAnnotations;

    ...

}

核心属性为bean，即@Controller注解类对象；method请求方法，主要用于反射调用；parameters方法参数，类型为MethodParameter，MethodParameter封装了参数的位置parameterIndex，参数类型parameterType，参数注解parameterAnnotations等信息。

 

RequestMappingInfo：请求的匹配条件

 

主要是对@RequestMapping注解的相关属性进行封装，然后作为请求和请求处理器映射map的key。通常@RequstMapping可以设置value，params，method，consumes，produces等参数来精确匹配请求。
value为匹配的路径；params是匹配的请求参数值；method为处理的http请求方法，consumers为请求的mediaType，produces为响应的mediaType。value，params，consumes，produces都可以设置多个，如下为一个用例：

@RestController
@RequestMapping("/home")
public class IndexController {
    @RequestMapping(value = {
     "/head",
     "/index"
    },
    method = RequestMethod.GET,
    headers = {
        "content-type=text/plain",
        "content-type=text/html"
    }, consumers = {
        "application/JSON",
        "application/XML"
    }, produces = {
        "application/JSON"
    }) String post() {
        return "Mapping applied along with headers";
    }
}  

RequestMappingInfo的类设计如下

public final class RequestMappingInfo implements RequestCondition<RequestMappingInfo> {
    @Nullable
    private final String name;
    private final PatternsRequestCondition patternsCondition;
    private final RequestMethodsRequestCondition methodsCondition;
    private final ParamsRequestCondition paramsCondition;
    private final HeadersRequestCondition headersCondition;
    private final ConsumesRequestCondition consumesCondition;
    private final ProducesRequestCondition producesCondition;
    private final RequestConditionHolder customConditionHolder;

    ...

        /**
     * Checks if all conditions in this request mapping info match the provided request and returns
     * a potentially new request mapping info with conditions tailored to the current request.
     * <p>For example the returned instance may contain the subset of URL patterns that match to
     * the current request, sorted with best matching patterns on top.
     * @return a new instance in case all conditions match; or {@code null} otherwise
     */
    @Override
    @Nullable
    public RequestMappingInfo getMatchingCondition(HttpServletRequest request) {
        RequestMethodsRequestCondition methods = this.methodsCondition.getMatchingCondition(request);
        ParamsRequestCondition params = this.paramsCondition.getMatchingCondition(request);
        HeadersRequestCondition headers = this.headersCondition.getMatchingCondition(request);
        ConsumesRequestCondition consumes = this.consumesCondition.getMatchingCondition(request);
        ProducesRequestCondition produces = this.producesCondition.getMatchingCondition(request);

        if (methods == null || params == null || headers == null || consumes == null || produces == null) {
            return null;
        }

        PatternsRequestCondition patterns = this.patternsCondition.getMatchingCondition(request);
        if (patterns == null) {
            return null;
        }

        RequestConditionHolder custom = this.customConditionHolder.getMatchingCondition(request);
        if (custom == null) {
            return null;
        }

        return new RequestMappingInfo(this.name, patterns,
                methods, params, headers, consumes, produces, custom.getCondition());
    }

    ...
}

RequestMappingHandlerMapping：基于HandlerMethod和RequestMappingInfo的HandlerMapping实现

 

RequestMappingHandlerMapping是HandlerMapping的一个实现，其请求和请求处理器的映射map是以RequestMappingInfo为key，HandlerMethod为value的。这样设计可以通过从客户端请求request对象中获取URI，http method，http header等信息，从而找到匹配的RequestMappingInfo。以该RequestMappingInfo作为key，在映射map中找到请求处理器或者说是请求处理方法。

 

RequestMappingHandlerMapping的类结构设计如下：

核心关注：RequestMappingHandlerMapping -> RequestMappingInfoHandlerMapping -> AbstractHandlerMethodMapping -> AbstractHandlerMapping，InitializingBean这个继承体系设计。同时从请求和请求处理器映射map的创建和初始化；给定请求，从请求和请求处理器映射map查找请求处理器两个角度来分析：

map的创建和初始化
由类的继承体系可知，实现了InitializingBean接口，故spring容器在创建这个bean时，填充好所有属性之后，会调用InitializingBean的afterPropertiesSet方法，以下为从RequestMappingHandlerMapping -> RequestMappingInfoHandlerMapping的afterPropertiesSet实现：
RequestMappingHandlerMapping的afterPropertiesSet方法定义：

    @Override
    public void afterPropertiesSet() {
        this.config = new RequestMappingInfo.BuilderConfiguration();
        this.config.setUrlPathHelper(getUrlPathHelper());
        this.config.setPathMatcher(getPathMatcher());
        this.config.setSuffixPatternMatch(this.useSuffixPatternMatch);
        this.config.setTrailingSlashMatch(this.useTrailingSlashMatch);
        this.config.setRegisteredSuffixPatternMatch(this.useRegisteredSuffixPatternMatch);
        this.config.setContentNegotiationManager(getContentNegotiationManager());

        // 调用RequestMappingInfoHandlerMapping的afterPropertiesSet方法

        super.afterPropertiesSet();
    }

RequestMappingInfoHandlerMapping的afterPropertiesSet方法：定义initHandlerMethods完成请求和请求处理器映射map的初始化，故map的初始化是定义在RequestMappingInfoHandlerMapping中的，其整体实现源码如下:

    // Handler method detection
    /**
     * Detects handler methods at initialization.
     * @see #initHandlerMethods
     */
    @Override
    public void afterPropertiesSet() {
        initHandlerMethods();
    }

    /**
     * Scan beans in the ApplicationContext, detect and register handler methods.
     * @see #getCandidateBeanNames()
     * @see #processCandidateBean
     * @see #handlerMethodsInitialized
     */
    protected void initHandlerMethods() {

        // 遍历所有bean

        for (String beanName : getCandidateBeanNames()) {
            if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {

                // 查找是handler的bean

                processCandidateBean(beanName);
            }
        }
        handlerMethodsInitialized(getHandlerMethods());
    }

获取所有Object类型的bean：getCandidateBeanNames

    // 获取所有Object类型的bean

    /**
     * Determine the names of candidate beans in the application context.
     * @since 5.1
     * @see #setDetectHandlerMethodsInAncestorContexts
     * @see BeanFactoryUtils#beanNamesForTypeIncludingAncestors
     */
    protected String[] getCandidateBeanNames() {
        return (this.detectHandlerMethodsInAncestorContexts ?
                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(obtainApplicationContext(), Object.class) :
                obtainApplicationContext().getBeanNamesForType(Object.class));
    }

processCandidateBean：通过isHandler方法，判断某个bean是否是handler，如果是则通过detectHandlerMethods方法，获取该handler的所有方法，然后保存注册到请求和请求处理器映射map。

    /**
     * Determine the type of the specified candidate bean and call
     * {@link #detectHandlerMethods} if identified as a handler type.
     * <p>This implementation avoids bean creation through checking
     * {@link org.springframework.beans.factory.BeanFactory#getType}
     * and calling {@link #detectHandlerMethods} with the bean name.
     * @param beanName the name of the candidate bean
     * @since 5.1
     * @see #isHandler
     * @see #detectHandlerMethods
     */
    protected void processCandidateBean(String beanName) {
        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);
            }
        }

        // 使用isHandler方法，判断bean是否是
        // 请求执行器handler
        // 这个是一个重要方法，在RequestMappingInfoHandlerMapping中实现

        if (beanType != null && isHandler(beanType)) {
            detectHandlerMethods(beanName);
        }
    }

    /**
     * Look for handler methods in the specified handler bean.
     * @param handler either a bean name or an actual handler instance
     * @see #getMappingForMethod
     */
    protected void detectHandlerMethods(Object handler) {
        Class<?> handlerType = (handler instanceof String ?
                obtainApplicationContext().getType((String) handler) : handler.getClass());

        if (handlerType != null) {
            Class<?> userType = ClassUtils.getUserClass(handlerType);
            Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                    (MethodIntrospector.MetadataLookup<T>) method -> {
                        try {
                            return getMappingForMethod(method, userType);
                        }
                        catch (Throwable ex) {
                            throw new IllegalStateException("Invalid mapping on handler class [" +
                                    userType.getName() + "]: " + method, ex);
                        }
                    });
            if (logger.isTraceEnabled()) {
                logger.trace(formatMappings(userType, methods));
            }
            methods.forEach((method, mapping) -> {
                Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
                registerHandlerMethod(handler, invocableMethod, mapping);
            });
        }
    }    

isHandler方法：判断bean是否是handler，这个是在主类RequestMappingHandlerMapping中定义的：判断bean是否使用@Controller或者@RequestMapping注解

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

从请求和请求处理器映射map查找请求处理器：主要是在DispatcherServlet中调用，对应RequestMappingHandlerMapping的底层实现如下：主要是在AbstractHandlerMethodMapping的getHandlerInternal方法定义，AbstractHandlerMethodMapping直接继承于AbstractHandlerMapping。getHandlerInternal的源码实现逻辑如下：

public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean {
    /**
     * Look up a handler method for the given request.
     */
    @Override
    protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
        String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
        this.mappingRegistry.acquireReadLock();
        try {
            HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
            return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
        }
        finally {
            this.mappingRegistry.releaseReadLock();
        }
    }

    /**
     * Look up the best-matching handler method for the current request.
     * If multiple matches are found, the best match is selected.
     * @param lookupPath mapping lookup path within the current servlet mapping
     * @param request the current request
     * @return the best-matching handler method, or {@code null} if no match
     * @see #handleMatch(Object, String, HttpServletRequest)
     * @see #handleNoMatch(Set, String, HttpServletRequest)
     */
    @Nullable
    protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
        List<Match> matches = new ArrayList<>();
        List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
        if (directPathMatches != null) {
            addMatchingMappings(directPathMatches, matches, request);
        }
        if (matches.isEmpty()) {
            // No choice but to go through all mappings...
            addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
        }

        if (!matches.isEmpty()) {
            Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
            matches.sort(comparator);
            Match bestMatch = matches.get(0);
            if (matches.size() > 1) {
                if (logger.isTraceEnabled()) {
                    logger.trace(matches.size() + " matching mappings: " + matches);
                }
                if (CorsUtils.isPreFlightRequest(request)) {
                    return PREFLIGHT_AMBIGUOUS_MATCH;
                }
                Match secondBestMatch = matches.get(1);
                if (comparator.compare(bestMatch, secondBestMatch) == 0) {
                    Method m1 = bestMatch.handlerMethod.getMethod();
                    Method m2 = secondBestMatch.handlerMethod.getMethod();
                    String uri = request.getRequestURI();
                    throw new IllegalStateException(
                            "Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");
                }
            }
            request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.handlerMethod);
            handleMatch(bestMatch.mapping, lookupPath, request);
            return bestMatch.handlerMethod;
        }
        else {
            return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
        }
    }

DispatcherServlet的handlerMappings集合
1. 初始化

DispatcherServlet可以包含多个HandlerMapping接口的实现类对象，其中RequestMappingHandlerMapping就是其中一个，用于获取使用@Controller和@RequestMapping注解的类和方法，RequestMappingHandlerMapping获取这些Controller类和其方法，从而生成请求和请求处理器映射map，供DispatcherServlet在接收到请求时查找并调用。

handlerMappings集合中的HandlerMapping对象的创建和初始化时机是在创建DispatcherServlet自身的bean容器WebApplicationContext的时候，具体为在解析WebApplicationContext的配置xml文件，如WEB-INF/myDispatcherServlet-servlet.xml时，通过spring-webmvc子项目的META-INF/spring.handlers获取命名空间处理器MvcNamespaceHandler，MvcNamespaceHandler注册并使用AnnotationDrivenBeanDefinitionParser来处理xml文件的annotation-driven标签：

/**
 * {@link NamespaceHandler} for Spring MVC configuration namespace.
 *
 * @author Keith Donald
 * @author Jeremy Grelle
 * @author Sebastien Deleuze
 * @since 3.0
 */
public class MvcNamespaceHandler extends NamespaceHandlerSupport {

    @Override
    public void init() {
        registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());

        ...

    }

}

RequestMappingHandlerMapping类型的HandlerMapping的创建：AnnotationDrivenBeanDefinitionParser在解析annotation-driven标签时，调用parse方法：此时创建RequestMappingHandlerMapping类型的bean：

    @Override
    @Nullable
    public BeanDefinition parse(Element element, ParserContext context) {

        ...

        // 创建RequestMappingHandlerMapping的BeanDefinition，
        // 之后创建bean实例

        RootBeanDefinition handlerMappingDef = new RootBeanDefinition(RequestMappingHandlerMapping.class);
        handlerMappingDef.setSource(source);
        handlerMappingDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        handlerMappingDef.getPropertyValues().add("order", 0);
        handlerMappingDef.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager);

        if (element.hasAttribute("enable-matrix-variables")) {
            Boolean enableMatrixVariables = Boolean.valueOf(element.getAttribute("enable-matrix-variables"));
            handlerMappingDef.getPropertyValues().add("removeSemicolonContent", !enableMatrixVariables);
        }

        configurePathMatchingProperties(handlerMappingDef, element, context);
        readerContext.getRegistry().registerBeanDefinition(HANDLER_MAPPING_BEAN_NAME , handlerMappingDef);

        RuntimeBeanReference corsRef = MvcNamespaceUtils.registerCorsConfigurations(null, context, source);
        handlerMappingDef.getPropertyValues().add("corsConfigurations", corsRef);

        ...

}

2. 请求处理

DispatcherServlet遍历自身的handlerMappings集合，从每个handlerMapping的map中找到处理这个请求的请求处理器：找到一个则返回，其中handlerMappings集合的实例类型，可以是AbstractUrlHandlerMapping的实现，如BeanNameUrlHandlerMapping和SimpleUrlHandlerMapping；或者是AbstractHandlerMethodMapping的实现，如RequestMappingHandlerMapping。

/**
 * Return the HandlerExecutionChain for this request.
 * <p>Tries all handler mappings in order.
 * @param request current HTTP request
 * @return the HandlerExecutionChain, or {@code null} if no handler could be found
 */
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    if (this.handlerMappings != null) {
        for (HandlerMapping mapping : this.handlerMappings) {

            // 找到一个则返回

            HandlerExecutionChain handler = mapping.getHandler(request);
            if (handler != null) {
                return handler;
            }
        }
    }
    return null;
}

调用HandlerMapping的getHandler方法获取一个请求处理器HandlerExecutionChain。

 

3. 请求执行

 

HandlerExecutionChain：由一个请求执行器handler和多个请求拦截器interceptors组成，handler在处理请求之前，需要先调用interceptors的preHandler方法来对请求进行拦截处理，只有通过了全部的拦截器，才能交给请求执行器handler完成真正的请求处理。源码设计如下：

/**
 * Handler execution chain, consisting of handler object and any handler interceptors.
 * Returned by HandlerMapping's {@link HandlerMapping#getHandler} method.
 */
public class HandlerExecutionChain {
    private final Object handler;

    @Nullable
    private HandlerInterceptor[] interceptors;

    @Nullable
    private List<HandlerInterceptor> interceptorList;

    private int interceptorIndex = -1;

    ...

}

完整请求处理过程

• DispatcherServlet中使用doDispatch方法完成整个请求处理，核心处理逻辑如下

    /**
     * Process the actual dispatching to the handler.
     * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
     * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
     * to find the first that supports the handler class.
     * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
     * themselves to decide which methods are acceptable.
     * @param request current HTTP request
     * @param response current HTTP response
     * @throws Exception in case of any kind of processing failure
     */
    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;

        ...
            // multipart请求加工
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // 查找请求处理器handler

            // Determine handler for the current request.
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // 获取请求执行器的执行适配器

            // Determine handler adapter for the current request.
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // Process last-modified header, if supported by the handler.
            String method = request.getMethod();
            boolean isGet = "GET".equals(method);
            if (isGet || "HEAD".equals(method)) {
                long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                    return;
                }
            }

            // 拦截器链前置处理applyPreHandle

            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }

            // 通过请求执行器适配器来调用请求执行器（如handlerMethod）处理请求

            // Actually invoke the handler.
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }

            // 拦截器链后置处理applyPostHandle

            applyDefaultViewName(processedRequest, mv);
            mappedHandler.applyPostHandle(processedRequest, response, mv);

        ...

        // 响应客户端
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}

 

HandlerAdapter：DispatcherServlet的请求执行适配器

 

DispatcherServlet通过HandlerAdapter来间接调用实际的请求执行器handler。由上面分析可知请求执行器包含：Servlet，Controller，HttpRequestHandler，HandlerMethod。
所以与上面这些请求执行器配套的HandlerAdapter分别为：SimpleServletHandlerAdapter，SimpleControllerHandlerAdapter，HttpRequestHandlerAdapter和RequestMappingHandlerAdapter。
如下为SimpleControllerHandlerAdapter的源码：其他类似

package org.springframework.web.servlet.mvc;
/**
 * Adapter to use the plain {@link Controller} workflow interface with
 * the generic {@link org.springframework.web.servlet.DispatcherServlet}.
 * Supports handlers that implement the {@link LastModified} interface.
 *
 * <p>This is an SPI class, not used directly by application code.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see org.springframework.web.servlet.DispatcherServlet
 * @see Controller
 * @see LastModified
 * @see HttpRequestHandlerAdapter
 */
public class SimpleControllerHandlerAdapter implements HandlerAdapter {
    @Override
    public boolean supports(Object handler) {
        return (handler instanceof Controller);
    }
    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {

        return ((Controller) handler).handleRequest(request, response);
    }
    @Override
    public long getLastModified(HttpServletRequest request, Object handler) {
        if (handler instanceof LastModified) {
            return ((LastModified) handler).getLastModified(request);
        }
        return -1L;
    }
}

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