SpringMVC父子容器加载与关系源码

我们都知道SpringMVC父子容器加载是通过dispatcherServlet与ContextLoaderListener类：他们的关系源码如下：

先说父容器加载：Context'Loader'Listener源码如下：

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

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);

加红这两行是重点，我们看到它把父容器加载的context放入了servletContext的一个属性中，这个属性的值如下：

String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";

后面我们看看DispatcherServlet的加载ApplicationContext的方式：

DispatcherServlet有一个父类是HttpServletBean进入它的init方法：

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, this.environment));
      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");
    }
  }

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();

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);
        }
      }
    }

我们看到子容器里面放了rootContext父容器，那么这个父容器是不是就是ContextLoaderListener加载的context的呢，我们看下第一行标红的代码：

WebApplicationContext rootContext =
        WebApplicationContextUtils.getWebApplicationContext(getServletContext());

public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
        return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
    }

public interface WebApplicationContext extends ApplicationContext {
    String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";

我们发现这个放入的值与我们ContextLoaderListener放入父容器中的值一样，说明子容器的rootContext就是我们之前放入的父容器。