Web容器初始化过程

2018-09-18 06:37:37来源:博客园 阅读 ()

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一、SpringMVC启动过程

Spring的MVC是基于Servlet功能实现的,每个web工程中都有一个web.xml文件,web容器在启动的时候会加载这个配置文件,当一个web应用加载到web容器中后,在web应用开始响应客户端的请求之前,要按照顺序执行下面几个步骤:

1、实例化部署描述符中的<listener>元素标识的时间监听器的实例;

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

2、在实现了ServletContextListener接口的监听器实例中,调用contextInitialized()方法。ContextLoaderListener实现了ServletContextListener接口,ServletContextListener作为Servlet的监听者,会在ServletContext创建、销毁过程中监听ServletContextEvent事件,然后进行响应的处理。换句话说,就是在启动web容器时,ContextLoaderListener类会自动装配ApplicationContext的配置信息。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    public ContextLoaderListener() {
    }

    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }
  // 这里调用其父类ContextLoader的initWebApplicationContext进行初始化工作   
    public void contextInitialized(ServletContextEvent event) {
        this.initWebApplicationContext(event.getServletContext());
    }
  
    public void contextDestroyed(ServletContextEvent event) {
        this.closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}

  ContextLoaderListener类继承了ContextLoader,后者是实际的ServletContextListener接口实现者,这里用到了代理模式,利用ContextLoader类创建Spring application context,并将其注册到ServletContext中去。ContextLoaderListener就是调用ContextLoader类的initWebApplicationContext()方法和closeWebApplicationContext()方法来实现ServletContext的创建和销毁工作的。

  

   其中,initWebApplicationContext()方法主要完成了两个部分的工作:一是查看是否已经指定父容器,如果不存在则设置其父容器。二是将Spring WebApplicationContext放置到线程Map中。

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
        if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
            throw new IllegalStateException("Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!");
        } else {
            Log logger = LogFactory.getLog(ContextLoader.class);
            servletContext.log("Initializing Spring root WebApplicationContext");
            if (logger.isInfoEnabled()) {
                logger.info("Root WebApplicationContext: initialization started");
            }

            long startTime = System.currentTimeMillis();

            try {
                if (this.context == null) {
                    this.context = this.createWebApplicationContext(servletContext);
                }

                if (this.context instanceof ConfigurableWebApplicationContext) {
                    ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)this.context;
                    if (!cwac.isActive()) {
              //查看是否指定父容器
                        if (cwac.getParent() == null) {
                            ApplicationContext parent = this.loadParentContext(servletContext);
                            cwac.setParent(parent);
                        }

                        this.configureAndRefreshWebApplicationContext(cwac, servletContext);
                    }
                }

                servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
                ClassLoader ccl = Thread.currentThread().getContextClassLoader();
                //将WebApplicationContext放置到线程Map中
                if (ccl == ContextLoader.class.getClassLoader()) {
                    currentContext = this.context;
                } else if (ccl != null) {
                    currentContextPerThread.put(ccl, this.context);
                }

                if (logger.isDebugEnabled()) {
                    logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" + WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
                }

                if (logger.isInfoEnabled()) {
                    long elapsedTime = System.currentTimeMillis() - startTime;
                    logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
                }

                return this.context;
            } catch (RuntimeException var8) {
                logger.error("Context initialization failed", var8);
                servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, var8);
                throw var8;
            } catch (Error var9) {
                logger.error("Context initialization failed", var9);
                servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, var9);
                throw var9;
            }
        }
    }    

在初始化方法执行后,将WebApplicationContext存到了两个地方,分别是ServletContext和currentContextPerThread中,我们可以通过这两个地方来获取WebApplicationContext:1.从servletContext中去拿;2.从当前的线程Map中去拿。

3、实例化部署符中<filter>元素标识的过滤器实例,并调用每个过滤器实例的init()方法。

4、实例化部署符中<servlet>元素标识的Servlet实例,按照该元素包含的load-on-startup顺序,调用每个Servlet实例的init()方法。

  <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>

在SpringMVC的架构中,DispatcherServlet主要负责客户端请求的分发,起到控制器的作用。DispatcherServlet是实现Servlet接口的实现类,其结构如下图所示。Servlet的生命周期由Servlet的容器来控制,主要分为初始化、运行和销毁三个阶段,分别对应三个方法:init()、service()和destory()。

  在DispatcherServlet的父类HTTPServletBean中,重写了init()方法,主要实现将当前的Servlet类实例转换为BeanWrapper类型实例,以便使用Spring中提供的注入功能进行相应属性的注入。

  在DispatcherServlet的父类FameworkServelet中,重写了service()方法,对于不同的方法,Spring统一将程序引导至processRequest(request,response)中,processRequest方法再对处理请求进行了一些准备工作后,又将细节实现转移到了doService方法中。

  protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String method = request.getMethod();
        if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {
            this.processRequest(request, response);
        } else {
            super.service(request, response);
        }

   }
  protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        long startTime = System.currentTimeMillis();
        Throwable failureCause = null;
        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        LocaleContext localeContext = this.buildLocaleContext(request);
        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = this.buildRequestAttributes(request, response, previousAttributes);
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new FrameworkServlet.RequestBindingInterceptor());
        this.initContextHolders(request, localeContext, requestAttributes);
        try {
            //调用DispatcherServlet的doService()方法
            this.doService(request, response);
        } catch (ServletException var17) {
            failureCause = var17;
            throw var17;
        } catch (IOException var18) {
            failureCause = var18;
            throw var18;
        } catch (Throwable var19) {
            failureCause = var19;
            throw new NestedServletException("Request processing failed", var19);
        } finally {
            this.resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }
            if (this.logger.isDebugEnabled()) {
                if (failureCause != null) {
                    this.logger.debug("Could not complete request", (Throwable)failureCause);
                } else if (asyncManager.isConcurrentHandlingStarted()) {
                    this.logger.debug("Leaving response open for concurrent processing");
                } else {
                    this.logger.debug("Successfully completed request");
                }
            }
            this.publishRequestHandledEvent(request, response, startTime, (Throwable)failureCause);
        }
    }        
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (this.logger.isDebugEnabled()) {
            String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
            this.logger.debug("DispatcherServlet with name '" + this.getServletName() + "'" + resumed + " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
        }

        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap();
            Enumeration attrNames = request.getAttributeNames();
            label108:
            while(true) {
                String attrName;
                do {
                    if (!attrNames.hasMoreElements()) {
                        break label108;
                    }

                    attrName = (String)attrNames.nextElement();
                } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));

                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }
        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource());
        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if (inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }

        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

        try {
           // 最终的请求处理函数
            this.doDispatch(request, response);
        } finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot);
            }

        }

    }

  经过层层的准备工作,最终在doDispatch()方法中实现了完整的请求处理过程:

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

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

                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                //根据request信息寻找对应的处理器
                    mappedHandler = this.getHandler(processedRequest);
                    if (mappedHandler == null || mappedHandler.getHandler() == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }
                //根据处理器寻找对应的处理器适配器
                    HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
                    String method = request.getMethod();
                    boolean isGet = "GET".equals(method);
                    if (isGet || "HEAD".equals(method)) {
                        long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                        if (this.logger.isDebugEnabled()) {
                            this.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;
                    }
                   //返回视图
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }

                    this.applyDefaultViewName(request, mv);
                    mappedHandler.applyPostHandle(processedRequest, response, mv);
                } catch (Exception var19) {
                    dispatchException = var19;
                }

                this.processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
            } catch (Exception var20) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var20);
            } catch (Error var21) {
                this.triggerAfterCompletionWithError(processedRequest, response, mappedHandler, var21);
            }

        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else if (multipartRequestParsed) {
                this.cleanupMultipart(processedRequest);
            }

        }
    }    

二、ServletContext、ApplicationContext和WebApplicationContext区别?

  1、ServletContext:是一个Web应用的上下文,是一个全局信息的存储空间,代表当前的web应用。Servlet容器(如Tomcat)在启动一个Webapp时,会为它创建一个ServletContext对象,即servlet上下文环境。每个webapp都有唯一的ServletContext对象。同一个webapp的所有servlet对象共享一个ServeltContext,servlet对象可以通过ServletContext来访问容器中的各种资源。ServletContext在web应用启动时创建,在web应用服务关闭时释放。

   2、ApplicationContext:它是建立在BeanFactory基础之上的,除了包含BeanFactory的所有功能之外,在国际化支持、资源访问、事件传播等方面进行了良好的支持。其体系结构如下图。

  ApplicationContext接口有三个常用的实现类,如下:

类名称 功能描述
ClassPathXmlApplicationContext
从类路径ClassPath中寻找指定的XML配置文件,找到并装载
完成ApplicationContext的实例化工作
FileSystemXmlApplicationContext
从指定的文件系统路径中寻找指定的XML配置文件,找到并装载
完成ApplicationContext的实例化工作
XmlWebApplicationContext 从Web应用中寻找指定的XML配置文件,找到并装载完成ApplicationContext的实例化工作

  与BeanFactory不同的是,ApplicationContext容器实例化后会自动对所有的单实例Bean进行实例化与依赖关系的装配,使之处于待用状态。而BeanFactory容器实例化后并不会自动实例化Bean,只有当Bean被使用时BeanFactory容器才会对该Bean进行实例化与依赖关系的装配。

  3、WebApplicationContext:它是专门为web应用提供的,它允许从相对于web根目录路径中装载配置文件完成初始化;从WebApplicationContext中可以获得ServletContext的引用,同时为了方便web应用访问Spring应用上下文,WebApplicationContext也将作为一个属性放到ServletContext中,可以通过WebApplicationContextUtils的getWebApplicationContext(ServletContext sc)方法获取。

三、总结

  ApplicationContext是Spring的核心,Context为上下文环境,在Web应用中,会用到WebApplicationContext,它继承自ApplicationContext。在SpringMVC启动过程中,ContextLoaderListener类起着至关重要的作用。它读取web.xml中配置的context-param中的配置文件,提前在web容器初始化前准备业务对应的Application context,将创建好的WebApplicationContext放置于ServletContext属性中,这样我们只要得到Servlet就可以得到WebApplicationContext对象,并利用这个对象访问spring容器管理的bean。

四、参考资料

  1、https://www.cnblogs.com/RunForLove/p/5688731.html

  2、spring源码深度解析(郝佳)

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