SpringMVC源码总结(一)HandlerMapping和HandlerAdapter入门-阿里云开发者社区

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SpringMVC源码总结(一)HandlerMapping和HandlerAdapter入门

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刚接触SpringMVC,对它的xml文件配置一直比较模模糊糊,最近花了一点时间稍微看了下源代码,再加上调试,开始逐渐理解它,网上的类似的内容有很多,写本文主要是自己加深一下理解。本文适合用过SpringMVC的开发者,言归正传,首先搭建一个最简单的工程体验一下。 

该工程是基于maven的,pom配置不再说明,所使用的spring版本4.0.5。 
首先是web.xml文件配置,最简单的配置 

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<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 
<web-app>
  <display-name>Archetype Created Web Application</display-name>
  <servlet>
        <servlet-name>mvc</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
 
    <servlet-mapping>
        <servlet-name>mvc</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>
</web-app>

然后是mvc-servlet.xml文件的配置,上面配置DispatcherServlet会默认加载[servlet-name]-servlet.xml文件。对于我的配置,会去加载mvc-servlet.xml文件。 
mvc-servlet.xml文件的内容:
 
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<?xml version="1.0" encoding="UTF-8" ?>
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
    http://www.springframework.org/schema/mvc
    http://www.springframework.org/schema/mvc/spring-mvc-3.1.xsd
    http://www.springframework.org/schema/util
    http://www.springframework.org/schema/util/spring-util-2.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.2.xsd">
 
    <bean name="/index" class="com.lg.mvc.HomeAction"></bean>
    <bean class="org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer">
        <property name="templateLoaderPath" value="/WEB-INF/views" />
        <property name="defaultEncoding" value="utf-8" />
        <property name="freemarkerSettings">
            <props>
                <prop key="locale">zh_CN</prop>
            </props>
        </property>
    </bean>
    <bean class="org.springframework.web.servlet.view.freemarker.FreeMarkerViewResolver">
        <property name="suffix" value=".html" />
        <property name="contentType" value="text/html;charset=utf-8" />
        <property name="requestContextAttribute" value="request" />
        <property name="exposeRequestAttributes" value="true" />
        <property name="exposeSessionAttributes" value="true" />
    </bean>
</beans>

在该配置中定义了一个HomeAction的Bean。内容为: 
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package com.lg.mvc;
 
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");
    }
}

这是最原始的mvc做法,要继承Controller接口,先从原始的说起,最后再过渡到@Controller和@RequestMapping注解式的配置。它在mvc-serlet.xml文件中的配置有一个关键的属性name="/index"。 
WEB-INF/view目录下有一个简单的hello.html,内容为:
 
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<html>
    <head>
     
    </head>
    <body>
        hello lg !
    </body>
</html>

至此该工程就写完了,部署到tomcat中,项目路径为/,运行一下。 
访问 http://localhost:8080/index 
07101142_oQd9.png 
至此整个工程就算搭建成功了。 

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。 
07101143_4yTV.png 
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。 
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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: 
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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 
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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: 
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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
 
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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:
 
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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:
 
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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 
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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: 

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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: 
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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。 
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/** 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 
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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: 
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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: 

07101143_oPoi.png 
该默认的配置文件的内容如下: 
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# 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。

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