spring上下文和springMVC上下文的关系

在tomcat部署一个用SSM搭起来的项目，一般会有两个上下文容器，分别是spring和SpringMVC的容器，在spring中一个容器是可以有父容器的，那么spring的容器和SpringMVC的容器关系是怎么样的。 先看tomcat中的web.xml配置文件

spring的上下文：

<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0">

    <context-param>
        <param-name>contextConfigLocationparam-name>
        <param-value>classpath:com/fancy/resource/spring-mybatis.xmlparam-value>
    context-param>


    
    <filter>
        <filter-name>encodingFilterfilter-name>
        <filter-class>org.springframework.web.filter.CharacterEncodingFilterfilter-class>
        <init-param>
            <param-name>encodingparam-name>
            <param-value>UTF-8param-value>
        init-param>
    filter>
    <filter-mapping>
        <filter-name>encodingFilterfilter-name>
        <url-pattern>/*url-pattern>
    filter-mapping>

    
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListenerlistener-class>
    listener>

    
    <listener>
        <listener-class>org.springframework.web.util.IntrospectorCleanupListenerlistener-class>
    listener>

    
    <servlet>
        <servlet-name>SpringMVCservlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServletservlet-class>
        <init-param>
            <param-name>contextConfigLocationparam-name>
            <param-value>classpath:com/fancy/resource/spring-mvc.xmlparam-value>
        init-param>
        <load-on-startup>1load-on-startup>
        <async-supported>trueasync-supported>
    servlet>
    <servlet-mapping>
        <servlet-name>SpringMVCservlet-name>
        
        <url-pattern>*.dourl-pattern>
    servlet-mapping>

web-app>

    <listener>
                      <listenerclass>org.springframework.web.context.ContextLoaderListenerlistener-class>
    listener>

其中ContextLoadListener监听器在tomcat容器初始化的时候监听tomcat的servlet上下文，这个上下文给spring的上下文提供了寄生环境，在这个监听器中，spring会初始一个上下文，并将此上下文以key为ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE这个常量，value当然是自己的上下文了。看下面代码就可以看出来

依次调用顺序为：
1.ContextLoaderListener中的

@Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }

2.ContextLoad中的

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!");
        }

        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 {
            // Store context in local instance variable, to guarantee that
            // it is available on ServletContext shutdown.
            if (this.context == null) {
                this.context = createWebApplicationContext(servletContext);
            }
            if (this.context instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent ->
                        // determine parent for root web application context, if any.
                        ApplicationContext parent = loadParentContext(servletContext);
                        cwac.setParent(parent);
                    }
                    configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }
            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;
        }
    }

其中比较重要的代码如下：

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

将spring的上下文以key为ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE存储在servlet根上下中。

SpringMVC的上下文：

spring的上下文是通过监听器来初始化，SpringMVC的上下文是通过DispatchServelet来加载，因为DispatchServelet会拦截所有我们感兴趣的请求来分发，DispatchServelet初始的时候会创建自己的上下文，并从servlet根上下文中取出我们上面说的spring上下文作为自己上下文的父容器，为什么这么做呢，因为spring中容器存在父子关系，父容器不能访问子容器的资源，而子容器可以访问父容器的资源。这样springMVC的上下文就可以获取父容器的Bean来初始化自己的Bean。
分析DispatchServelet初始化SpringMVC容器的上下文之前需要知道servlet生命周期：

• 调用 init() 方法初始化
• 调用 service() 方法来处理客户端的请求
• 调用 destroy() 方法释放资源，标记自身为可回收
• 被垃圾回收器回收

其中在init方法中进行初始化工作，来看看DispatchServelet的继承关系

其中主要的父类：FrameworkServle，HttpServletBean
FrameworkServle主要就是创建上下文，HttpServletBean中

@Override
    public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
            logger.debug("Initializing servlet '" + getServletName() + "'");
        }

        // Set bean properties from init parameters.
        try {
            PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            initBeanWrapper(bw);
            bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
            throw ex;
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();

        if (logger.isDebugEnabled()) {
            logger.debug("Servlet '" + getServletName() + "' configured successfully");
        }
    }

定义了init方法，这个方法中主要调用了initServletBean()方法，这个方法是个抽象方法，是由子类FrameworkServle实现，那么看看FrameworkServle中是如何重写这个方法的：

@Override
    protected final void initServletBean() throws ServletException {
        getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
        if (this.logger.isInfoEnabled()) {
            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
        }
        long startTime = System.currentTimeMillis();

        try {
            this.webApplicationContext = initWebApplicationContext();
            initFrameworkServlet();
        }
        catch (ServletException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }
        catch (RuntimeException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }

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

其中重要的两行代码：

this.webApplicationContext = initWebApplicationContext(); initFrameworkServlet();

初始化web上下文，这个上下文就是SpringMVC的上下文，那么继续看这个方法的具体实现：

protected WebApplicationContext initWebApplicationContext() {
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        if (wac == null) {
            // No context instance was injected at construction time -> see if one
            // has been registered in the servlet context. If one exists, it is assumed
            // that the parent context (if any) has already been set and that the
            // user has performed any initialization such as setting the context id
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            // Either the context is not a ConfigurableApplicationContext with refresh
            // support or the context injected at construction time had already been
            // refreshed -> trigger initial onRefresh manually here.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

        return wac;
    }

其中重要的部分：

if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }

首先从servlet根上下文中获取rootContext，这个上下文就是上面所说的spring上下文，然后将springMVC 上下文中的父容器设置为rootContext，这样父子容器关系就建立起来了。

为什么要缕清这样的关系

知道了这样的关系就可以避免这样的情况：spring容器初始化时，需要依赖的Bean配置在SpringMVC中，因为spring容器是SpringMVC容器的父容器，这样是不能正常初始化spring上下文的。