Spring HandlerMapping ，Handleradapter ，ViewResolver

HandlerMapping是用来找到url对应的处理handler对象(beans)，而不是找到url对应的处理函数。Handleradapter则是用来匹配到handler的某个具体的处理函数上，然后调度执行这个函数。

ViewResolver:这个是用来解析视图，并返回资源到前端用到。

 

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.Controller;

public class HomeAction implements Controller{

    @Override
    public ModelAndView handleRequest(HttpServletRequest request,
            HttpServletResponse response) throws Exception {
        return new ModelAndView("hello");
    }
}

 

 

下面就要说说原理了。 
用过python Django框架的都知道Django对于访问方式的配置就是，一个url路径和一个函数配对，你访问这个url，就会直接调用这个函数，简单明了。对于java的面向对象来说，就要分两步走。第一步首先要找到是哪个对象，即handler，本工程的handler则是HomeAction对象。第二步要找到访问的函数，即HomeAction的handleRequest方法。所以就出现了两个源码接口 HandlerMapping和HandlerAdapter，前者负责第一步，后者负责第二步。借用网上的SpringMVC架构图。 

HandlerMapping接口的实现（只举了我认识的几个） ： 

• 
  BeanNameUrlHandlerMapping ：通过对比url和bean的name找到对应的对象 
  SimpleUrlHandlerMapping ：也是直接配置url和对应bean,比BeanNameUrlHandlerMapping功能更多 
  DefaultAnnotationHandlerMapping : 主要是针对注解配置@RequestMapping的，已过时 
  RequestMappingHandlerMapping ：取代了上面一个 

HandlerAdapter 接口实现： 

• 
  HttpRequestHandlerAdapter ： 要求handler实现HttpRequestHandler接口，该接口的方法为                                                             void handleRequest(HttpServletRequest request, HttpServletResponse response)也就是  handler必须有一个handleRequest方法 
  
  SimpleControllerHandlerAdapter：要求handler实现Controller接口，该接口的方法为ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response)，也就是本工程采用的 
  
  AnnotationMethodHandlerAdapter ：和上面的DefaultAnnotationHandlerMapping配对使用的，也已过时 
  
  RequestMappingHandlerAdapter : 和上面的RequestMappingHandlerMapping配对使用，针对@RequestMapping 

先简单的说下这个工程的流程，访问http://localhost:8080/index首先由DispatcherServlet进行转发，通过BeanNameUrlHandlerMapping(含有 /index->HomeAction的配置)，找到了HomeAction,然后再拿HomeAction和每个adapter进行适配，由于HomeAction实现了Controller接口，所以最终会有SimpleControllerHandlerAdapter来完成对HomeAction的handleRequest方法的调度。然后就顺利的执行了我们想要的方法，后面的内容不在本节中说明。 

了解了大概流程，然后就需要看源代码了。 
首先就是SpringMVC的入口类，DispatcherServlet，它实现了Servlet接口，不再详细说DispatcherServlet的细节，不然又是一大堆的内容。每次请求都会调用它的doService->doDispatch，我们关注的重点就在doDispatch方法中。 

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);
                      //这个是重点，第一步由HandlerMapping找到对应的handler
                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                       //这是第二步，找到合适的HandlerAdapter，然后由它来调度执行handler的方法
                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 (logger.isDebugEnabled()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

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

                try {
                    // Actually invoke the handler.
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                }
                finally {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
                }

                applyDefaultViewName(request, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                return;
            }
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }

第一步详细查看： 

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        for (HandlerMapping hm : this.handlerMappings) {
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
            }
            HandlerExecutionChain handler = hm.getHandler(request);
            if (handler != null) {
                return handler;
            }
        }
        return null;
    }

可以看到就是通过遍历所有已注册的HandlerMapping来找到对应的handler,然后构建出一个HandlerExecutionChain，它包含了handler和HandlerMapping本身的一些拦截器，如下 

public class HandlerExecutionChain {

    private final Object handler;

    private HandlerInterceptor[] interceptors;

    private List<HandlerInterceptor> interceptorList;

        //其他代码省略
}

其中HandlerMapping的getHandler实现： 

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        Object handler = getHandlerInternal(request);
        if (handler == null) {
            handler = getDefaultHandler();
        }
        if (handler == null) {
            return null;
        }
        // Bean name or resolved handler?
        if (handler instanceof String) {
            String handlerName = (String) handler;
            handler = getApplicationContext().getBean(handlerName);
        }
        return getHandlerExecutionChain(handler, request);
    }

这里的getHandlerInternal(request)是个抽象方法，由具体的HandlerMapping来实现，获取到的handler如果为空,则获取默认配置的handler，如果handler为String类型，则表示这个则会去Spring容器里面去找这样名字的bean。 
再看下BeanNameUrlHandlerMapping的getHandlerInternal（request）的具体实现（通过一系列的接口设计，之后再好好看看这个设计，到BeanNameUrlHandlerMapping这只用实现该方法中的一部分），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<String>();
        if (beanName.startsWith("/")) {
            urls.add(beanName);
        }
        String[] aliases = getApplicationContext().getAliases(beanName);
        for (String alias : aliases) {
            if (alias.startsWith("/")) {
                urls.add(alias);
            }
        }
        return StringUtils.toStringArray(urls);
    }

}

这里面注释说，bean的name必须以/开头，它才处理，将信息存储在Map<String, Object> handlerMap中，对于本工程来说就是{'/index':HomeAction对象}。 
至此这里完成了第一步，下面开始第二步，即方法HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());的具体实现： 

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        for (HandlerAdapter ha : this.handlerAdapters) {
            if (logger.isTraceEnabled()) {
                logger.trace("Testing handler adapter [" + ha + "]");
            }
            if (ha.supports(handler)) {
                return ha;
            }
        }
        throw new ServletException("No adapter for handler [" + handler +
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }

遍历所有的HandlerAdapter,判断他们是否支持这个handler。 
我们来看下HttpRequestHandlerAdapter的supports(handler)方法： 

public class HttpRequestHandlerAdapter implements HandlerAdapter {

    @Override
    public boolean supports(Object handler) {
          //就是判断handler是否实现了HttpRequestHandler接口
        return (handler instanceof HttpRequestHandler);
    }

    @Override
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
           //若handler实现了HttpRequestHandler接口，则调用该接口的方法，执行我们在该方法中写的业务逻辑
        ((HttpRequestHandler) handler).handleRequest(request, response);
        return null;
    }

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

}

同理SimpleControllerHandlerAdapter也是这样类似的逻辑 

public class SimpleControllerHandlerAdapter implements HandlerAdapter {

    @Override
    public boolean supports(Object handler) {
        return (handler instanceof Controller);
    }

    @Override
    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;
    }

}

剩余两个AnnotationMethodHandlerAdapter和RequestMappingHandlerAdapter就比较复杂，我也没看。 
按照本工程的配置，则SimpleControllerHandlerAdapter是支持HomeAction的，然后就会执行SimpleControllerHandlerAdapter的handle(processedRequest, response, mappedHandler.getHandler())方法。本质上就会调用HomeAction实现Controller接口的方法。至此就分析完了。 
了解过程了之后，然后就是最重要的也是经常配置出问题的地方。DispatcherServlet的handlerMappings和handlerAdapters的来源问题。 

DispatcherServlet初始化的时候，会调用一个方法如下： 

protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
//初始化一些HandlerMapping
        initHandlerMappings(context);
//初始化一些HandlerAdapter
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }

这里可以看到，它会初始化一些HandlerMapping和HandlerAdapter，这两个方法非常重要，理解了这两个方法你就会知道，配置不对问题出在哪里，下面具体看下这两个方法： 

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<HandlerMapping>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                OrderComparator.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.isDebugEnabled()) {
                logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
            }
        }
    }

detectAllHandlerMappings是DispatcherServlet的一个属性，你是可以在web.xml中配置的，默认是true，如果为true,则会去从本工程mvc-servlet.xml文件中去探测所有实现了HandlerMapping的bean，如果有，则加入DispatcherServlet的handlerMappings中。如果detectAllHandlerMappings为false,则直接去容器中找id="handlerMapping"且实现了HandlerMapping的bean.如果以上都没找到，则会去加载默认的HandlerMapping。 

/** Detect all HandlerMappings or just expect "handlerMapping" bean? */
    private boolean detectAllHandlerMappings = true;

本工程由于没有配置HandlerMapping,所以它会去加载默认的，下面看看默认的配置是什么 

protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
        String key = strategyInterface.getName();
//defaultStrategies存储了默认的配置
        String value = defaultStrategies.getProperty(key);
        if (value != null) {
            String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
            List<T> strategies = new ArrayList<T>(classNames.length);
            for (String className : classNames) {
                try {
                    Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
                    Object strategy = createDefaultStrategy(context, clazz);
                    strategies.add((T) strategy);
                }
                catch (ClassNotFoundException ex) {
                    throw new BeanInitializationException(
                            "Could not find DispatcherServlet's default strategy class [" + className +
                                    "] for interface [" + key + "]", ex);
                }
                catch (LinkageError err) {
                    throw new BeanInitializationException(
                            "Error loading DispatcherServlet's default strategy class [" + className +
                                    "] for interface [" + key + "]: problem with class file or dependent class", err);
                }
            }
            return strategies;
        }
        else {
            return new LinkedList<T>();
        }
    }

继续看看defaultStrategies是如何初始化的： 

private static final Properties defaultStrategies;

    static {
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
//这里的DEFAULT_STRATEGIES_PATH就是DispatcherServlet.properties
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
        }
    }

这里使用静态代码块来加载配置文件DispatcherServlet.properties，它所在位置就是和DispatcherServlet同一目录下面的，如下图所示： 


该默认的配置文件的内容如下： 

# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver

#这里就是默认的HandlerMapping的配置
org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
    org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping
#这里就是默认的HandlerAdapter的配置
org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
    org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
    org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
    org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator

org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver

org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

也就是说，当你什么都没有配置时，默认会加载以上的配置。正是由于有了上述默认配置的BeanNameUrlHandlerMapping（它要求name必须是以/开头的），它才会存储我们在mvc-servlet.xml中配置的<bean name="/index" class="com.lg.mvc.HomeAction"></bean>,同样正是由于有了SimpleControllerHandlerAdapter（由于handler实现了Controller接口，所以它的support方法支持我们的handler），才会调度执行HomeAction的handleRequest方法。