springmvc中dispatcherservlet初始化分析

首先看 DispatcherServlet 继承关系：
DispatcherServlet ——> FrameworkServlet ——-> HttpServletBean ——> HttpServlet

找到 HttpServletBean 的 init 方法：

@Override
public final void init() throws ServletException {
    // Set bean properties from init parameters.
    PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
    if (!pvs.isEmpty()) {
        BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
        ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
        bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
        initBeanWrapper(bw);
        bw.setPropertyValues(pvs, true);
    }
    // Let subclasses do whatever initialization they like.
    initServletBean();
}

跟到 FrameworkServlet 的 initServletBean 方法：

@Override
protected final void initServletBean() throws ServletException {
    this.webApplicationContext = initWebApplicationContext();
    initFrameworkServlet();    
}

继续跟到 initWebApplicationContext 方法：

protected WebApplicationContext initWebApplicationContext() {
    WebApplicationContext rootContext =
            WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    WebApplicationContext wac = null;
    if (this.webApplicationContext != null) {
        wac = this.webApplicationContext;
        if (wac instanceof ConfigurableWebApplicationContext) {
            ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
            if (!cwac.isActive()) {
                    if (cwac.getParent() == null) {
                    cwac.setParent(rootContext);
                }
                configureAndRefreshWebApplicationContext(cwac);
            }
        }
    }
    if (wac == null) {
        wac = findWebApplicationContext();
    }
    if (wac == null) {
        wac = createWebApplicationContext(rootContext);
    }
    if (!this.refreshEventReceived) {
        onRefresh(wac);
    }
    return wac;
}

一路跟到 FrameworkServlet 的 createWebApplicationContext 方法：

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
    ConfigurableWebApplicationContext wac =
            (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    wac.setEnvironment(getEnvironment());
    //设置 parent 上下文
    wac.setParent(parent);
    String configLocation = getContextConfigLocation();
    if (configLocation != null) {
        wac.setConfigLocation(configLocation);
    }
    configureAndRefreshWebApplicationContext(wac);
    return wac;
}

跟着看 configureAndRefreshWebApplicationContext 方法：

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
    wac.setServletContext(getServletContext());
    wac.setServletConfig(getServletConfig());
    wac.setNamespace(getNamespace());
    wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));
    ConfigurableEnvironment env = wac.getEnvironment();
    if (env instanceof ConfigurableWebEnvironment) {
        ((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
    }
    postProcessWebApplicationContext(wac);
    applyInitializers(wac);
    wac.refresh();
}

于是可以看出， DispatcherServlet 初始化了 ApplicationContext。

再看 DispatcherServlet 的 onRefresh 方法：

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);
}

到此， DispatcherServlet 的初始化流程大致清晰了。