SpringMVC源码解析-DispatcherServlet启动流程和初始化

在使用springmvc框架，会在web.xml文件配置一个DispatcherServlet,这正是web容器开始初始化，同时会在建立自己的上下文来持有SpringMVC的bean对象。

先从DispatcherServlet入手，从名字来看，它是一个Servlet。它的定义如下：

public class DispatcherServlet extends FrameworkServlet {

它是继承FrameworkServlet,来看一下整个的继承关系。

从继承关系来看，DispatcherServlet继承FrameworkServlet和HttpServletBean而继承HttpServlet,通过使用Servlet API 来对HTTP请求进行响应，成为SpringMVC的前端处理器。

先看一个时序图

注：作为Servlet,DispatcherServlet的启动和Servlet的启动相关联的。在Servlet初始化过程中，Servlet的init方法会被调用，以进行初始化，然而DispatcherServlet的基类，所以从HttpServletBean中的初始化过程开始。

DispatcherServlet的工作分为2部分，一部分是初始化（也就是图的上半部分），有initServletBean()启动，通过initWebApplicationContext()方法最终调用DispatcherServlet中的initStrategies()方法。另一部分(也就是图的下半部分),是对HTTP请求进行响应，作为Servlet，Web容器会调用Servlet的doGet()和doPost()方法,在经过FrameworkServlet的processRequest()简单处理后，会调用DispatcherServlet的doService方法，在这个方法调用中封装了doDispatch(),继续调用processDispatchResult方法返回调用信息。

接下来看看初始化的源码的流程：

HttpServletBean.init方法

/**
	 * Map config parameters onto bean properties of this servlet, and
	 * invoke subclass initialization.
	 * @throws ServletException if bean properties are invalid (or required
	 * properties are missing), or if subclass initialization fails.
	 */
	@Override
	public final void init() throws ServletException {
		if (logger.isDebugEnabled()) {
			logger.debug("Initializing servlet '" + getServletName() + "'");
		}
　　　　　　　　　　//获取Servlet初始化参数
		// 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;
		}
                //调用子类的方法
		// Let subclasses do whatever initialization they like.
		initServletBean();

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

　　例如web.xml中配置参数：

<servlet>
    <servlet-name>TestServlet</servlet-name>
    <servlet-class>com.lzyer.TestServlet</servlet-class>
    <!--配置参数,可以通过ServletConfig获取参数-->
    <init-param>
        <param-name>servlet-name</param-name>
        <param-value>TestServlet</param-value>
    </init-param>
 </servlet>
 <servlet-mapping>
    <servlet-name>TestServlet</servlet-name>
    <url-pattern>/TestServlet</url-pattern>
  </servlet-mapping>

　　FrameworkServlet.initServletBean方法

/**
	 * Overridden method of {@link HttpServletBean}, invoked after any bean properties
	 * have been set. Creates this servlet's WebApplicationContext.
	 */
	@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 {
                         //创建Web应用上下文
			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");
		}
	}

　　FrameworkServlet.initWebApplicationContext()

/**
	 * 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 #FrameworkServlet(WebApplicationContext)
	 * @see #setContextClass
	 * @see #setContextConfigLocation
	 */
	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;
	}

　　先看WebApplicationContextUtils.getWebApplicationContext(getServletContext())，，主要是从ServletContext获取WebApplicationContext

public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
		Assert.notNull(sc, "ServletContext must not be null");
                //从ServletContext中获取
		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;
	}

　findWebApplicationContext和上面一样从ContextServlet中查找，如果不存在就调用下面的createWebApplicationContext方法。

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
　　　　　　　　　　//获取contextClass
		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");
		}
                //通过反射获取实例
		ConfigurableWebApplicationContext wac =
				(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

		wac.setEnvironment(getEnvironment());
                //设置双亲上下文
		wac.setParent(parent);
		wac.setConfigLocation(getContextConfigLocation());
                //配置和刷新应用
		configureAndRefreshWebApplicationContext(wac);

		return wac;
	}

　　看一下getContextClass()到底是哪个类？ XmlWebApplicationContext.class

最后configureAndRefreshWebApplicationContext调用refresh方法启动容器。

回到initWebApplicationContext方法中

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);
		}

　　这个会触发SpringMVC初始化策略

/**
	 * This implementation calls {@link #initStrategies}.
	 */
	@Override
	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);
                //映射关系
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}

　　到此，SpringMVC的初始化的流程大概就是这样，下篇就是SpringMVC请求流程。