Spring MVC中的IoC容器初始化

Spring Framework本身没有Web功能,Spring MVC使用WebApplicationContext类扩展ApplicationContext,使得拥有web功能。那么,Spring MVC是如何在web环境中创建IoC容器呢?web环境中的IoC容器的结构又是什么结构呢?web环境中,spring IoC容器是怎么启动呢?

      先看一下WebApplicationContext是如何扩展ApplicationContext来添加对Web环境的支持的。WebApplicationContext接口定义如下:

public interface WebApplicationContext extends ApplicationContext {  
    //根上下文在ServletContext中的名称  
        String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";  
        //取得web容器的ServletContext  
    ServletContext getServletContext();  
}

      对于web容器中创建IoC容器的过程,我们从web.xml配置文件讲起。看一下Spring MVC的web.xml中的相关配置:

    

<context-param>  
        <param-name>contextConfigLocation</param-name>  
        <param-value>/WEB-INF/applicationContext.xml</param-value>  
    </context-param>  
      
    <listener>  
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>  
    </listener>  
  
    <!-- Handles all requests into the application -->  
    <servlet>  
        <servlet-name>Spring MVC Dispatcher Servlet</servlet-name>  
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>  
        <init-param>  
            <param-name>contextConfigLocation</param-name>  
            <param-value>  
                /WEB-INF/spring/*.xml  
            </param-value>  
        </init-param>  
        <load-on-startup>1</load-on-startup>  
    </servlet>  
          
    <!-- Maps all /app requests to the DispatcherServlet for handling -->  
    <servlet-mapping>  
        <servlet-name>Spring MVC Dispatcher Servlet</servlet-name>  
        <url-pattern>/app/*</url-pattern>  
    </servlet-mapping>

      在web.xml配置文件中,有两个主要的配置:ContextLoaderListener和DispatcherServlet。同样的关于spring配置文件的相关配置也有两部分:context-param和DispatcherServlet中的init-param。那么,这两部分的配置有什么区别呢?它们都担任什么样的职责呢?

      在Spring MVC中,Spring Context是以父子的继承结构存在的。Web环境中存在一个ROOT Context,这个Context是整个应用的根上下文,是其他context的双亲Context。同时Spring MVC也对应的持有一个独立的Context,它是ROOT Context的子上下文。

      对于这样的Context结构在Spring MVC中是如何实现的呢?下面就先从ROOT Context入手,ROOT Context是在ContextLoaderListener中配置的,ContextLoaderListener读取context-param中的contextConfigLocation指定的配置文件,创建ROOT Context。下面看一下ContextLoaderListener中创建context的源码:

/** 
     * Initialize Spring's web application context for the given servlet context, 
     * according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and 
     * "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params. 
     * @param servletContext current servlet context 
     * @return the new WebApplicationContext 
     * @see #CONTEXT_CLASS_PARAM 
     * @see #CONFIG_LOCATION_PARAM 
     */  
    public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {  
                //PS : ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE=WebApplicationContext.class.getName() + ".ROOT" 根上下文的名称  
                  //PS : 默认情况下,配置文件的位置和名称是: DEFAULT_CONFIG_LOCATION = "/WEB-INF/applicationContext.xml"  
                  //在整个web应用中,只能有一个根上下文  
        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!");  
        }  
  
        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 {  
            // Determine parent for root web application context, if any.  
            ApplicationContext parent = loadParentContext(servletContext);  
  
            // Store context in local instance variable, to guarantee that  
            // it is available on ServletContext shutdown.  
                        // 在这里执行了创建WebApplicationContext的操作  
            this.context = createWebApplicationContext(servletContext, parent);  
  
                        //PS: 将根上下文放置在servletContext中  
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);  
  
            ClassLoader ccl = Thread.currentThread().getContextClassLoader();  
            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 ex) {  
            logger.error("Context initialization failed", ex);  
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);  
            throw ex;  
        }  
        catch (Error err) {  
            logger.error("Context initialization failed", err);  
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);  
            throw err;  
        }  
    }

    再看一下WebApplicationContext对象是如何创建的:

protected WebApplicationContext createWebApplicationContext(ServletContext sc, ApplicationContext parent) {  
                //根据web.xml中的配置决定使用何种WebApplicationContext。默认情况下使用XmlWebApplicationContext  
                //web.xml中相关的配置context-param的名称“contextClass”  
        Class<?> contextClass = determineContextClass(sc);  
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {  
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +  
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");  
        }  
  
                //实例化WebApplicationContext的实现类  
        ConfigurableWebApplicationContext wac =  
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);  
  
        // Assign the best possible id value.  
        if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {  
            // Servlet <= 2.4: resort to name specified in web.xml, if any.  
            String servletContextName = sc.getServletContextName();  
            if (servletContextName != null) {  
                wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + servletContextName);  
            }  
            else {  
                wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX);  
            }  
        }  
        else {  
            // Servlet 2.5's getContextPath available!  
            wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + sc.getContextPath());  
        }  
  
        wac.setParent(parent);  
  
        wac.setServletContext(sc);  
                //设置spring的配置文件  
        wac.setConfigLocation(sc.getInitParameter(CONFIG_LOCATION_PARAM));  
        customizeContext(sc, wac);  
                //spring容器初始化  
        wac.refresh();  
        return wac;  
    }

       以上是web容器中根上下文的加载与初始化,下面介绍一下Spring MVC对应的上下文是如何加载的。

     

       Spring MVC中核心的类是DispatcherServlet,在这个类中完成Spring context的加载与创建,并且能够根据Spring Context的内容将请求分发给各个Controller类。DispatcherServlet继承自HttpServlet,关于Spring Context的配置文件加载和创建是在init()方法中进行的,主要的调用顺序是init-->initServletBean-->initWebApplicationContext。

       先来看一下initWebApplicationContext的实现

/** 
     * Initialize and publish the WebApplicationContext for this servlet. 
     * <p>Delegates to {@link #createWebApplicationContext} for actual creation 
     * of the context. Can be overridden in subclasses. 
     * @return the WebApplicationContext instance 
     * @see #setContextClass 
     * @see #setContextConfigLocation 
     */  
    protected WebApplicationContext initWebApplicationContext() {  
                //先从web容器的ServletContext中查找WebApplicationContext  
        WebApplicationContext wac = findWebApplicationContext();  
        if (wac == null) {  
            // No fixed context defined for this servlet - create a local one.  
                        //从ServletContext中取得根上下文  
            WebApplicationContext parent =  
                    WebApplicationContextUtils.getWebApplicationContext(getServletContext());  
                        //创建Spring MVC的上下文,并将根上下文作为起双亲上下文  
            wac = createWebApplicationContext(parent);  
        }  
  
        if (!this.refreshEventReceived) {  
            // Apparently not a ConfigurableApplicationContext with refresh support:  
            // triggering initial onRefresh manually here.  
            onRefresh(wac);  
        }  
  
        if (this.publishContext) {  
            // Publish the context as a servlet context attribute.  
                        // 取得context在ServletContext中的名称  
            String attrName = getServletContextAttributeName();  
                        //将Spring MVC的Context放置到ServletContext中  
            getServletContext().setAttribute(attrName, wac);  
            if (this.logger.isDebugEnabled()) {  
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +  
                        "' as ServletContext attribute with name [" + attrName + "]");  
            }  
        }  
  
        return wac;  
    }

        通过initWebApplicationContext方法的调用,创建了DispatcherServlet对应的context,并将其放置到ServletContext中,这样就完成了在web容器中构建Spring IoC容器的过程。

        最后,在分别给出ContextLoaderListener和DispatcherServlet构建context的时序。

        ContextLoaderListener构建Root Context时序图:

 Spring MVC中的IoC容器初始化

         DispatcherServlet创建context时序图:

 Spring MVC中的IoC容器初始化

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