Spring5源码解析-论Spring DispatcherServlet的生命周期

Spring Web框架架构的主要部分是DispatcherServlet。也就是本文中重点介绍的对象。

在本文的第一部分中,我们将看到基于Spring的DispatcherServlet的主要概念:前端控制器模式。第二部分将专门介绍Spring应用程序中的执行链。接下来是DispatcherServlet类的解释。在最后一部分,我们将尝试开发一个自定义的dispatcher servlet。

请注意,本文分析的DispatcherServlet来自Spring的5.0.0.RC3版本。如果使用不同的版本,则可能需要进行几个调整,其实因为分析的都是比较固定的东西,很少有改的。

什么是前端控制器模式?

在进入DispatcherServlet之前,我们需要了解一些关于它的概念基础。DispatcherServlet所隐含的关键概念其实就是前端控制器模式。

此模式为Web应用程序提供了一个中心入口点。该集中入口点将系统组件的共同特征进行重新组合。我们可以在那里找到安全资源,语言切换,会话管理,缓存或输入过滤的处理程序。这样做的一个很大的好处是:这个共同的入口点有助于避免代码重复。

因此,从技术上讲,前端控制器模式由一个捕获所有传入请求的类组成。之后,分析每个请求以知道哪个控制器以及哪个方法应该来处理该请求。

前端控制器模式有助于对以下询问做出最佳响应:

  • 如何集中授权和认证?
  • 如何处理正确的视图渲染?
  • 如何使用URL重写映射将请求发送到适当的控制器?

这个前台控制器模式包含5名参与者:

  • 客户端:发送请求。
  • 控制器:应用程序的中心点,捕获所有请求。
  • 调度员:管理视图的选择,以呈现给客户端。
  • 视图:表示呈现给客户端的内容。
  • 帮助:帮助查看和/或控制器完成请求处理。

什么是DispatcherServlet的执行链?

由标题可以看到,前端控制器模式有自己的执行链。这意味着它有自己的逻辑来处理请求并将视图返回给客户端:

  1. 请求由客户端发送。它到达作为Spring的默认前端控制器的DispatcherServlet类。
  2. DispatcherServlet使用请求处理程序映射来发现将分析请求的控制器(controller)。接口org.springframework.web.servlet.HandlerMapping的实现返回一个包含org.springframework.web.servlet.HandlerExecutionChain类的实例。此实例包含可在控制器调用之前或之后调用的处理程序拦截器数组。你可以在Spring中有关于拦截器的文章中了解更多的信息。如果在所有定义的处理程序映射中找不到HandlerExecutionChain,这意味着Spring无法将URL与对应的控制器进行匹配。这样的话会抛出一个错误。
  3. 现在系统进行拦截器预处理并调用由映射处理器找到的相应的controller(其实就是在找到的controller之前进行一波拦截处理)。在controller处理请求后,DispatcherServlet开始拦截器的后置处理。在此步骤结束时,它从controller接收ModelAndView实例(整个过程其实就是 request请求->进入interceptors->controller->从interceptors出来->ModelAndView接收)。
  4. DispatcherServlet现在将使用的该视图的名称发送到视图解析器。这个解析器将决定前台的展现内容。接着,它将此视图返回给DispatcherServlet,其实也就是一个“视图生成后可调用”的拦截器。
  5. 最后一个操作是视图的渲染并作为对客户端request请求的响应。

什么是DispatcherServlet?

通过上面讲到的前端控制器模式,我们可以很轻易的知道DispatcherServlet是基于Spring的Web应用程序的中心点。它需要传入请求,并在处理程序映射,拦截器,控制器和视图解析器的帮助下,生成对客户端的响应。所以,我们可以分析这个类的细节,并总结出一些核心要点。

下面是处理一个请求时DispatcherServlet执行的步骤:

1. 策略初始化

DispatcherServlet是一个位于org.springframework.web.servlet包中的类,并扩展了同一个包中的抽象类FrameworkServlet。它包含一些解析器的私有静态字段(用于本地化,视图,异常或上传文件),映射处理器:handlerMapping和处理适配器:handlerAdapter(进入这个类的第一眼就能看到的)。DispatcherServlet非常重要的一个核心点就是是初始化策略的方法(protected void initStrategies (ApplicationContext context))。在调用onRefresh方法时调用此方法。最后一次调用是在FrameworkServlet中通过initServletBean和initWebApplicationContext方法进行的(initServletBean方法中调用initWebApplicationContext,后者调用onRefresh(wac))。initServletBean通过所提供的这些策略生成我们所需要的应用程序上下文。其中每个策略都会产生一类在DispatcherServlet中用来处理传入请求的对象。

基于篇幅,有些代码就不给贴示了,请在相应版本的源码中自行对照查找,此处只给一部分源码:

/**
 * This implementation calls {@link #initStrategies}.
 */
@Override
protected void onRefresh(ApplicationContext context) {
    initStrategies(context);
}
/**
 * Initialize the strategy objects that this servlet uses.
 * <p>May be overridden in subclasses in order to initialize further strategy objects.
 */
protected void initStrategies(ApplicationContext context) {
    initMultipartResolver(context);
    initLocaleResolver(context);
    initThemeResolver(context);
    initHandlerMappings(context);
    initHandlerAdapters(context);
    initHandlerExceptionResolvers(context);
    initRequestToViewNameTranslator(context);
    initViewResolvers(context);
    initFlashMapManager(context);
}

需要注意的是,如果找的结果不存在,则捕获异常NoSuchBeanDefinitionException(下面两段代码的第一段),并采用默认策略。如果在DispatcherServlet.properties文件中初始定义的默认策略不存在,则抛出BeanInitializationException异常(下面两段代码的第二段)。默认策略如下:

/**
     * Initialize the LocaleResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to AcceptHeaderLocaleResolver.
     */
    private void initLocaleResolver(ApplicationContext context) {
        try {
            this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using LocaleResolver [" + this.localeResolver + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate LocaleResolver with name '" + LOCALE_RESOLVER_BEAN_NAME +"': using default [" + this.localeResolver + "]");
            }
        }
    }

抛出异常后调用getDefaultStrategy(因为容器里都是单例的存在,所以只需要判断基于这个接口的默认实现实例size为1即可,两个以上还能叫默认么,都有选择了):

/**
 * Return the default strategy object for the given strategy interface.
 * The default implementation delegates to {@link #getDefaultStrategies},
 * expecting a single object in the list.
 * @param context the current WebApplicationContext
 * @param strategyInterface the strategy interface
 * @return the corresponding strategy object
 * @see #getDefaultStrategies
 */
protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
    List<T> strategies = getDefaultStrategies(context, strategyInterface);
    if (strategies.size() != 1) {
        throw new BeanInitializationException(
                "DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
    }
    return strategies.get(0);
}
    /**
     * Create a List of default strategy objects for the given strategy interface.
     * The default implementation uses the "DispatcherServlet.properties" file (in the same
     * package as the DispatcherServlet class) to determine the class names. It instantiates
     * the strategy objects through the context's BeanFactory.
     * @param context the current WebApplicationContext
     * @param strategyInterface the strategy interface
     * @return the List of corresponding strategy objects
     */
    @SuppressWarnings("unchecked")
    protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
        String key = strategyInterface.getName();
        String value = defaultStrategies.getProperty(key);
        if (value != null) {
            String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
            List<T> strategies = new ArrayList<>(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<>();
        }
    }

2.请求预处理

FrameworkServlet抽象类扩展了同一个包下的HttpServletBean,HttpServletBean扩展了javax.servlet.http.HttpServlet。点开这个类源码可以看到,HttpServlet是一个抽象类,其方法定义主要用来处理每种类型的HTTP请求:doGet(GET请求),doPost(POST),doPut(PUT),doDelete(DELETE),doTrace(TRACE),doHead(HEAD) ,doOptions(OPTIONS)。FrameworkServlet通过将每个传入的请求调度到processRequest(HttpServletRequest request,HttpServletResponse response)来覆盖它们。processRequest是一个protected和final的方法,它构造出LocaleContext和ServletRequestAttributes对象,两者都可以在initContextHolders(request, localeContext, requestAttributes)之后访问。所有这些操作的关键代码 请看:

@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    processRequest(request, response);
}
 ...
/**
 * Process this request, publishing an event regardless of the outcome.
 * The actual event handling is performed by the abstract
 * {@link #doService} template method.
 */
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    long startTime = System.currentTimeMillis();
    Throwable failureCause = null;

    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
    LocaleContext localeContext = buildLocaleContext(request);

    RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
    ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

    initContextHolders(request, localeContext, requestAttributes);

    try {
        doService(request, response);
    }
    catch (ServletException ex) {
        failureCause = ex;
        throw ex;
    }
    catch (IOException ex) {
        failureCause = ex;
        throw ex;
    }
    catch (Throwable ex) {
        failureCause = ex;
        throw new NestedServletException("Request processing failed", ex);
    }

    finally {
        resetContextHolders(request, previousLocaleContext, previousAttributes);
        if (requestAttributes != null) {
            requestAttributes.requestCompleted();
        }

        if (logger.isDebugEnabled()) {
            if (failureCause != null) {
                this.logger.debug("Could not complete request", failureCause);
            }
            else {
                if (asyncManager.isConcurrentHandlingStarted()) {
                    logger.debug("Leaving response open for concurrent processing");
                }
                else {
                    this.logger.debug("Successfully completed request");
                }
            }
        }

        publishRequestHandledEvent(request, startTime, failureCause);
    }
}
...

  private void initContextHolders(HttpServletRequest request,
            @Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) {
        if (localeContext != null) {
            LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
        }
        if (requestAttributes != null) {
            RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
        }
        if (logger.isTraceEnabled()) {
            logger.trace("Bound request context to thread: " + request);
        }
    }

3.请求处理

由上面所看到的,在processRequest的代码中,调用initContextHolders方法后,调用protected void doService(HttpServletRequest request,HttpServletResponse response)。doService将一些附加参数放入request(如Flash映射:request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap()),上下文信息:request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext())等)中,并调用protected void doDispatch(HttpServletRequest request,HttpServletResponse response)

doDispatch方法最重要的部分是处理(handler)的检索。doDispatch调用getHandler()方法来分析处理后的请求并返回HandlerExecutionChain实例。此实例包含handler mapping 和`interceptors(拦截器)。DispatcherServlet做的另一件事是应用预处理程序拦截器(applyPreHandle())。如果至少有一个返回false,则请求处理停止。否则,servlet使用与 handler adapter适配(其实理解成这也是个handler就对了)相应的handler mapping来生成视图对象。

doDispatch方法:

/**
 * Process the actual dispatching to the handler.
 * 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.
 * 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;
        boolean multipartRequestParsed = false;
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        try {
            ModelAndView mv = null;
            Exception dispatchException = null;
            try {
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);
                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }
                // Determine handler adapter for the current request.获取可处理request的Handler,适配器其实还                //是调用的相应的Handler,一样的功能,具体请参考本人的Spring设计模式中的适配器模式
                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;
                }
                // Actually invoke the handler.此处就会调用我们写的controller来执行咯
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                //视图解析
                applyDefaultViewName(processedRequest, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            catch (Throwable err) {
                // As of 4.3, we're processing Errors thrown from handler methods as well,
                // making them available for @ExceptionHandler methods and other scenarios.
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            //此处进行最后一步的视图渲染
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

4.视图解析

获取ModelAndView实例以查看呈现后,doDispatch方法调用private void applyDefaultViewName(HttpServletRequest request,ModelAndView mv)。默认视图名称根据定义的bean名称,即viewNameTranslator。默认情况下,它的实现是org.springframework.web.servlet.RequestToViewNameTranslator。这个默认实现只是简单的将URL转换为视图名称,例如(直接从RequestToViewNameTranslator获取):http:// localhost:8080/admin/index.html将生成视图admin / index。

代码如下:

下一步是调用后置拦截器(其实就是出拦截器)做的一些处理。

/** RequestToViewNameTranslator used by this servlet */
    @Nullable
    private RequestToViewNameTranslator viewNameTranslator;
...
  protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context); //看下面注释
        initViewResolvers(context);
        initFlashMapManager(context);
    }
...

  /**
     * Initialize the RequestToViewNameTranslator used by this servlet instance.
     * <p>If no implementation is configured then we default to DefaultRequestToViewNameTranslator.
     */
    private void initRequestToViewNameTranslator(ApplicationContext context) {
        try {
            this.viewNameTranslator =
                    context.getBean(REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using RequestToViewNameTranslator [" + this.viewNameTranslator + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate RequestToViewNameTranslator with name '" +
                        REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME + "': using default [" + this.viewNameTranslator +"]");
            }
        }
    }
....
  /**
     * Translate the supplied request into a default view name.
     * @param request current HTTP servlet request
     * @return the view name (or {@code null} if no default found)
     * @throws Exception if view name translation failed
     */
    @Nullable
    protected String getDefaultViewName(HttpServletRequest request) throws Exception {
        return (this.viewNameTranslator != null ? this.viewNameTranslator.getViewName(request) : null);

org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator实现的org.springframework.web.servlet.RequestToViewNameTranslator接口,其内对上段代码中getDefaultViewName的实现为:

public class DefaultRequestToViewNameTranslator implements RequestToViewNameTranslator {
    private static final String SLASH = "/";
    private String prefix = "";
    private String suffix = "";
    private String separator = SLASH;
    private boolean stripLeadingSlash = true;
    private boolean stripTrailingSlash = true;
    private boolean stripExtension = true;
    private UrlPathHelper urlPathHelper = new UrlPathHelper();
    /**
     * Set the prefix to prepend to generated view names.
     * @param prefix the prefix to prepend to generated view names
     */
    public void setPrefix(String prefix) {
        this.prefix = (prefix != null ? prefix : "");
    }
    /**
     * Set the suffix to append to generated view names.
     * @param suffix the suffix to append to generated view names
     */
    public void setSuffix(String suffix) {
        this.suffix = (suffix != null ? suffix : "");
    }
    /**
     * Set the value that will replace '{@code /}' as the separator
     * in the view name. The default behavior simply leaves '{@code /}'
     * as the separator.
     */
    public void setSeparator(String separator) {
        this.separator = separator;
    }
    ...
    /**
     * Translates the request URI of the incoming {@link HttpServletRequest}
     * into the view name based on the configured parameters.
     * @see org.springframework.web.util.UrlPathHelper#getLookupPathForRequest
     * @see #transformPath
     */
    @Override
    public String getViewName(HttpServletRequest request) {
        String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
        return (this.prefix + transformPath(lookupPath) + this.suffix);
    }
    /**
     * Transform the request URI (in the context of the webapp) stripping
     * slashes and extensions, and replacing the separator as required.
     * @param lookupPath the lookup path for the current request,
     * as determined by the UrlPathHelper
     * @return the transformed path, with slashes and extensions stripped
     * if desired
     */
    protected String transformPath(String lookupPath) {
        String path = lookupPath;
        if (this.stripLeadingSlash && path.startsWith(SLASH)) {
            path = path.substring(1);
        }
        if (this.stripTrailingSlash && path.endsWith(SLASH)) {
            path = path.substring(0, path.length() - 1);
        }
        if (this.stripExtension) {
            path = StringUtils.stripFilenameExtension(path);
        }
        if (!SLASH.equals(this.separator)) {
            path = StringUtils.replace(path, SLASH, this.separator);
        }
        return path;
    }
}

5.处理调度请求 - 视图渲染

现在,servlet知道应该是哪个视图被渲染。它通过private void processDispatchResult(HttpServletRequest request,HttpServletResponse response,@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,@Nullable Exception exception)方法来进行最后一步操作 - 视图渲染。

首先,processDispatchResult检查它们是否有参数传递异常。有一些异常的话,它定义了一个新的视图,专门用来定位错误页面。如果没有任何异常,该方法将检查ModelAndView实例,如果它不为null,则调用render方法。

渲染方法protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception。跳进此方法内部,根据定义的视图策略,它会查找得到一个View类实例。它将负责显示响应。如果没有找到View,则会抛出一个ServletException异常。有的话,DispatcherServlet会调用其render方法来显示结果。

其实可以说成是后置拦截器(进入拦截器前置拦截处理->controller处理->出拦截器之前的此拦截器的后置处理),也就是在请求处理的最后一个步骤中被调用。

下面是processDispatchResult和render(渲染)的相关代码:

/**
     * Handle the result of handler selection and handler invocation, which is
     * either a ModelAndView or an Exception to be resolved to a ModelAndView.
     */
    private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
            @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
            @Nullable Exception exception) throws Exception {
        boolean errorView = false;
        if (exception != null) {
            if (exception instanceof ModelAndViewDefiningException) {
                logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            }
            else {
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                mv = processHandlerException(request, response, handler, exception);
                errorView = (mv != null);
            }
        }
        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
          //开始渲染
            render(mv, request, response);
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        }
        else {
            if (logger.isDebugEnabled()) {
                logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                        "': assuming HandlerAdapter completed request handling");
            }
        }
        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Concurrent handling started during a forward
            return;
        }
        if (mappedHandler != null) {
            mappedHandler.triggerAfterCompletion(request, response, null);
        }
    }
...
  /**
     * Render the given ModelAndView.
     * <p>This is the last stage in handling a request. It may involve resolving the view by name.
     * @param mv the ModelAndView to render
     * @param request current HTTP servlet request
     * @param response current HTTP servlet response
     * @throws ServletException if view is missing or cannot be resolved
     * @throws Exception if there's a problem rendering the view
     */
    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        // Determine locale for request and apply it to the response.
        Locale locale =
                (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
        response.setLocale(locale);
        View view;
        String viewName = mv.getViewName();
        if (viewName != null) {
            // We need to resolve the view name.
            view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
            if (view == null) {
                throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
                        "' in servlet with name '" + getServletName() + "'");
            }
        }
        else {
            // No need to lookup: the ModelAndView object contains the actual View object.
            view = mv.getView();
            if (view == null) {
                throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +"View object in servlet with name '" + getServletName() + "'");
            }
        }
        // Delegate to the View object for rendering.
        if (logger.isDebugEnabled()) {
            logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
        }
        try {
            if (mv.getStatus() != null) {
                response.setStatus(mv.getStatus().value());
            }
            view.render(mv.getModelInternal(), request, response);
        }
        catch (Exception ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
                        getServletName() + "'", ex);
            }
            throw ex;
        }
    }
/**
 * Handle the result of handler selection and handler invocation, which is
 * either a ModelAndView or an Exception to be resolved to a ModelAndView.
 */
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
        HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

    boolean errorView = false;

    if (exception != null) {
        if (exception instanceof ModelAndViewDefiningException) {
            logger.debug("ModelAndViewDefiningException encountered", exception);
            mv = ((ModelAndViewDefiningException) exception).getModelAndView();
        }
        else {
            Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
            mv = processHandlerException(request, response, handler, exception);
            errorView = (mv != null);
        }
    }

    // Did the handler return a view to render?
    if (mv != null && !mv.wasCleared()) {
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    }
    else {
        if (logger.isDebugEnabled()) {
            logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                    "': assuming HandlerAdapter completed request handling");
        }
    }

    if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
        // Concurrent handling started during a forward
        return;
    }

    if (mappedHandler != null) {
        mappedHandler.triggerAfterCompletion(request, response, null);
    }
}

/**
 * Render the given ModelAndView.
 * This is the last stage in handling a request. It may involve resolving the view by name.
 * @param mv the ModelAndView to render
 * @param request current HTTP servlet request
 * @param response current HTTP servlet response
 * @throws ServletException if view is missing or cannot be resolved
 * @throws Exception if there's a problem rendering the view
 */
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
    // Determine locale for request and apply it to the response.
    Locale locale = this.localeResolver.resolveLocale(request);
    response.setLocale(locale);

    View view;
    if (mv.isReference()) {
        // We need to resolve the view name.
        view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
        if (view == null) {
            throw new ServletException(
                    "Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + getServletName() + "'");
        }
    }
    else {
        // No need to lookup: the ModelAndView object contains the actual View object.
        view = mv.getView();
        if (view == null) {
            throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " + "View object in servlet with name '" + getServletName() + "'");
        }
    }

    // Delegate to the View object for rendering.
    if (logger.isDebugEnabled()) {
        logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
    }
    try {
        view.render(mv.getModelInternal(), request, response);
    }
    catch (Exception ex) {
        if (logger.isDebugEnabled()) {
            logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'", ex);
        }
        throw ex;
    }
}

在这部分中,你需要记住的是我们定义了两个上下文:一个用于应用程序,另一个用于Web应用程序。他们有什么区别?应用程序上下文包含所有通用配置,比如service定义,数据库配置。Web应用程序上下文定义所有与Web相关的组件,比如controllers或视图解析器。

Custom DispatcherServlet

我们已经了解了DispatcherServlet的理论知识。通过文中的这些实用要点,我们可以编写自己的servlet来分派处理请求。同样的,我们也将按步进行,从捕获请求开始,以视图渲染结束。

通过上面的描述,为了捕获请求,我们需要覆盖doService方法:

public class CustomDispatcherServlet extends FrameworkServlet {
    private static final Logger LOGGER = LoggerFactory.getLogger(CustomDispatcherServlet.class);

    @Override
    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        LOGGER.debug("[CustomDispatcherServlet] I got the request !");
    }
}

这样,在我们的日志文件中,我们应该可以找到一条“[CustomDispatcherServlet]I got the request!”。接着,我们继续添加在DispatcherServlet中doDispatch方法所应该做的一些工作:

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    LOGGER.debug("[CustomDispatcherServlet] I got the request !");
    try {
        LOGGER.debug("[CustomDispatcherServlet] doService");
        LocaleContext localeContext = buildLocaleContext(request);

        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        initContextHolders(request, localeContext, requestAttributes);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

这个方法是做什么的?首先,它为构建一个Locale实例用来接收请求。第二步是初始化org.springframework.web.context.request.ServletRequestAttributes实例。它是RequestAttributes接口的实现,和本地化在同一级别。通过这个,我们可以访问servlet请求的对象和会话对象,而不必区分会话和全局会话。最后,我们调用初始化context holders的initContextHolders()方法,即从应用程序通过LocaleContextHolder和RequestContextHolder静态方法(分别为:getLocaleContext和getRequestAttributes)访问请求属性和区域设置的对象。

当请求被拦截,一些基本的设置就绪的时候。我们发现我们还没有执行链和处理器适配器。我们可以通过以下代码进行:

private HandlerExecutionChain getHandlerExecutionChain(HttpServletRequest request) throws Exception {
    for (HandlerMapping mapper : this.handlerMappings) {
        HandlerExecutionChain executionChain = mapper.getHandler(request);
        if (executionChain != null) {
            return executionChain;
        }
    }
    throw new Exception("Execution chain wasn't be found in provided handler mappings: "+this.handlerMappings);
}

通过执行链,我们可以通过 handler adapter将处理当前请求。看以下代码:

@Override
protected HandlerAdapter getHandlerAdapter(Object executionChain) throws ServletException {
    for (HandlerAdapter adapter : this.handlerAdapters) {
        LOGGER.debug("[CustomDispatcherServlet] "+adapter + " is instanceof HandlerMethod ? "+(adapter instanceof HandlerMethod));
        if (adapter.supports(executionChain)) {
            return adapter;
        }
    }
    throw new  ServletException("Handler adapter was not found from adapters list :"+this.handlerAdapters);
}

只有应用程序上下文中定义的适配器(this.handlerAdapter)支持适配所生成的执行链(adapter.supports)才可以返回我们想要的适配器。最后,我们可以返回到我们的doService方法并操作它们来渲染视图:

ModelAndView modelView = adapter.handle(request, response, executionChain.getHandler());
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale);

View view = null;
if (!modelView.isReference()) {
    throw new UnsupportedOperationException("Only view models defined as references can be used in this servlet");
}
for (ViewResolver viewResolver : this.viewResolvers) {
    view = viewResolver.resolveViewName(modelView.getViewName(), locale);
    if (view != null) {
        break;
    }
}
if (view == null) {
    throw new ServletException("Could not resolve view with name '" + modelView.getViewName() + "' in servlet with name '" + getServletName() + "'");
}
view.render(modelView.getModelMap(), request, response);

我们的servlet中简化了渲染。实际上,我们仅处理ModelAndView的引用对象。这意味着ModelAndView是一个String的实例,用来表示要解析的视图模型,例如:我们定义好几个模板解析器(比如freemaker,Thymeleaf),然后查看其配置。在这个检查之后,我们迭代当前视图解析器。能够生成View实例的第一个解析器被视为处理过的请求中使用的解析器。最后,我们检查视图是否正确生成。拿到view实例后,我们调用其render()方法来在屏幕中显示请求处理结果。

在这部分中,我们将描述和代码部分限制在最低限度。只是为了把Spring的整个过程给集中呈现以下,达到更好的理解,其实就是在Servlet中的service方法内做些对request和response的文章而已了。

本文介绍了Spring Web应用程序的中心点,一个调度器servlet。请记住,它是一个处理所有传入请求并将视图呈现给用户的类。在重写之前,你应该熟悉执行链,handler mapping 或handler adapter等概念。请记住,第一步要做的是定义在调度过程中我们要调用的所有元素。handler mapping 是将传入请求(也就是它的URL)映射到适当的controller。最后提到的元素,一个handler适配器,就是一个对象,它将通过其内包装的handler mapping将请求发送到controller。此调度产生的结果是ModelAndView类的一个实例,后面被用于生成和渲染视图。

原文:Spring5源码解析-论Spring DispatcherServlet的生命周期

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