DispatcherServlet的启动和初始化

在使用SpringMVC时会在Web.xml配置如下代码：

<!-- 加入SpringMVC配置 begin-->
    <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:/dispatcher-servlet.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
    <!-- end -->

先看看DispatcherServlet类的继承关系

DispatcherServlet类的继承关系

作为Servlet，DispatcherServlet的启动与Servlet的启动过程是想联系的，在Servlet启动的过程中,Servlet的init()方法会被调用，以进行初始化，首先会调用HttpServletBean的init（）方法，因为HttpServletBean继承自HttpServlet

• 运用了依赖注入思想完成了<init-param>配置元素的读取

public void setContextConfigLocation(String contextConfigLocation) {
        this.contextConfigLocation = contextConfigLocation;
    }

读取在web.xml配置的 classpath:/dispatcher-servlet.xml

• HttpServletBean类init（）方法

@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;
        }

        // 让子类去初始化
        initServletBean();

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

• initServletBean的初始化过程在FrameworkServlet中完成

    @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 {
            //初始化WebApplicationContext上下文
            this.webApplicationContext = initWebApplicationContext();
            //空实现
            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");
        }
    }

protected WebApplicationContext initWebApplicationContext() {
        //调用WebApplicationContextUtils静态类得到跟上下文，这个跟上下文是保存在ServletContext中的
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;
    //如果webApplicationContext已经不为空，表示这个Servlet类是通过编程式注册到容器中的（Servlet 3.0+中的ServletContext.addServlet() ），上下文也由编程式传入。
  //若这个传入的上下文还没被初始化，将rootContext上下文设置为它的父上下文，然后将其初始化，否则直接使用
        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) {
// 此时以contextAttribute属性的值为键，在ServletContext中查找上下文，查找得到，说明上下文已经以别的方式初始化并注册在contextAttribute下，直接使用
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            //没有上下文实例定义servlet ->创建一个
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            //刷新上下文
            onRefresh(wac);
        }
        //把当前创建的上下文保存到ServletContext中去，使用的属性名是和当前Servlet名相关的
        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 + "]");
            }
        }
        //返回当前创建的上下文
        return wac;
    }

那么这个MVC的上下文就建立起来了，具体取得跟上下文的过程在WebApplicationContextUtils中实现的这个根上下文是在ContextLoader设置到ServletContext中去的,使用的属性是ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE，这个根上下文是DispatherServlet建立的上下文的父上下文，所以根上下管理的Bean也是可以被DispatherServlet的上下文使用的。通过getBean向Ioc容器获取Bean时，容器会先到他的父Ioc容器中获得getBean

• WebApplicationContextUtils.getWebApplicationContext

public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
        return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
    }
public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
        Assert.notNull(sc, "ServletContext must not be null");
        Object attr = sc.getAttribute(attrName);
        if (attr == null) {
            return null;
        }
        if (attr instanceof RuntimeException) {
            throw (RuntimeException) attr;
        }
        if (attr instanceof Error) {
            throw (Error) attr;
        }
        if (attr instanceof Exception) {
            throw new IllegalStateException((Exception) attr);
        }
        if (!(attr instanceof WebApplicationContext)) {
            throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr);
        }
        return (WebApplicationContext) attr;
    }

• FrameworkServlet的createWebApplicationContext()方法

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
        // getContextClass()--> XmlWebApplicationContext.class
        Class<?> contextClass = getContextClass();
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Servlet with name '" + getServletName() +
                    "' will try to create custom WebApplicationContext context of class '" +
                    contextClass.getName() + "'" + ", using parent context [" + parent + "]");
        }
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException(
                    "Fatal initialization error in servlet with name '" + getServletName() +
                    "': custom WebApplicationContext class [" + contextClass.getName() +
                    "] is not of type ConfigurableWebApplicationContext");
        }
        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

        wac.setEnvironment(getEnvironment());
        //设置父上下文
        wac.setParent(parent);
        wac.setConfigLocation(getContextConfigLocation());
        //刷新上下文
        configureAndRefreshWebApplicationContext(wac);

        return wac;
    }