SpringMvc源码-DispatcherServlet(一)

DispatcherServlet是springmvc最重要的组成部分，听名字就知道它负责一个请求到响应的流转，本篇主要讲DispatcherServlet的启动过程。
思考一个把问题？为什么DispatcherServlet初始化后，Spring的Ioc容器就会启动，容器中的Bean是什么时候注册到容器中的？

实际上整个SpringMvc的启动过程分为俩个阶段：

1. ContextLoaderListener初始化，实例化IOC容器，并将此容器注册到ServletContext中。

2. DispatcherServlet初始化，建立自己的上下文，也注册到ServletContext中。

   见下面这段web.xml的配置（springboot不在我们这篇文章讨论之列。）

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
      http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
      version="3.1">

    <display-name>ota</display-name>
    <description>ota web application</description>

    <!-- 系统组件加载顺序：context-param -> listener -> filter -> servlet -->
     <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:/applicationContext.xml</param-value>
    </context-param>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <!-- GZip -->
    <filter>
        <filter-name>gzipFilter</filter-name>
        <filter-class>com.travelsky.ibeplus.compress.Compress2WayFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>gzipFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <!--  Servlet that dispatches request to registered handlers (Controller implementations).  -->
    <servlet>
        <servlet-name>ota</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:spring/mvc-core-config.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <!-- 需要定义在对应的servlet之后 -->
    <servlet-mapping>
        <servlet-name>ota</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

</web-app>

ContextLoaderListener初始化

见web.xml这一段

 <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:/applicationContext.xml</param-value>
</context-param>

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

关键逻辑：
ContextLoaderListener采用了很典型的适配器模式，继承了ContextLoader,实现了ServletContextListener接口。
在contextInitialized()->initWebApplicationContext()

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
	......

	try {
		// Store context in local instance variable, to guarantee that
		// it is available on ServletContext shutdown. 初始化webApplication
     ......
		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);
				}
				// 该方法会调用setConfigLocation，将xml中的contextConfigLocation传递给WebApplicationContext,还会调用refresh方法执行容器的bean初始化等操作
				configureAndRefreshWebApplicationContext(cwac, servletContext);
			}
		}
		//将ConfigurableWebApplicationContext设置会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);
		}

		......
		return this.context;
	}	catch (RuntimeException | Error ex) {
		logger.error("Context initialization failed", ex);
		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
		throw ex;
	}
}

configureAndRefreshWebApplicationContext方法

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
	//关键1
	if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
		// The application context id is still set to its original default value
		// -> assign a more useful id based on available information
		String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
		if (idParam != null) {
			wac.setId(idParam);
		}
		else {
			// Generate default id...
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
					ObjectUtils.getDisplayString(sc.getContextPath()));
		}
	}

	//关键2 设置servletContext 和web.xml的configLocationParam 传给 ConfigurableWebApplicationContext
	wac.setServletContext(getServletContext());
	String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
	if (configLocationParam != null) {
		wac.setConfigLocation(configLocationParam);
	}

	// The wac environment's #initPropertySources will be called in any case when the context
	// is refreshed; do it eagerly here to ensure servlet property sources are in place for
	// use in any post-processing or initialization that occurs below prior to #refresh
	ConfigurableEnvironment env = wac.getEnvironment();
	if (env instanceof ConfigurableWebEnvironment) {
		((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
	}

	customizeContext(sc, wac);
	//关键3 见ApplicationContext.refresh方法 初始化容器的bean
	wac.refresh();
}

DispatcherServlet初始化

相关的xml配置

<!--  Servlet that dispatches request to registered handlers (Controller implementations).  -->
<servlet>
    <servlet-name>ota</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:spring/mvc-core-config.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>

<!-- 需要定义在对应的servlet之后 -->
<servlet-mapping>
    <servlet-name>ota</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

类的继承关系：DispatcherServlet–>FrameworkServlet–>HttpServletBean–>HttpServlet

HttpServletBean重写了Serlvet的init方法，封装了spring的web容器启动的过程。

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 {

	// Set bean properties from init parameters.
	//1. 获取web.xml中的getInitParameterNames,将servletConfig的initParam参数的key和value都保存到pvs中
	PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
	if (!pvs.isEmpty()) {
		try {
			//BeanWrapper实际是BeanWrapperImpl【DispatcherServlet】
			BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
			ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
			bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
			initBeanWrapper(bw);
			//通过BeanWrapper为bean【DispatcherServlet】赋值
			//BeanWrapper的target的属性的key要和web.xml中的<init-param>的param-name相等，这里就给DispatchServlet的contextConfigLocation赋值了
			bw.setPropertyValues(pvs, true);
		}
		catch (BeansException ex) {
			if (logger.isErrorEnabled()) {
				logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
			}
			throw ex;
		}
	}

	// Let subclasses do whatever initialization they like.
	// FrameworkServlet实现了initServletBean方法
	initServletBean();
}

FrameworkServlet

FrameworkServlet的Override了HttpServletBean的initServletBean方法，在该方法通过initWebApplicationContext初始化了webApplicationContext

/**
 * 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 " + getClass().getSimpleName() + " '" + getServletName() + "'");
	if (logger.isInfoEnabled()) {
		logger.info("Initializing Servlet '" + getServletName() + "'");
	}
	long startTime = System.currentTimeMillis();

	try {
		//关键方法 在这里初始化了 webApplicationContext
		this.webApplicationContext = initWebApplicationContext();
		initFrameworkServlet();
	}
	catch (ServletException | RuntimeException ex) {
		logger.error("Context initialization failed", ex);
		throw ex;
	}

	if (logger.isDebugEnabled()) {
		String value = this.enableLoggingRequestDetails ?
				"shown which may lead to unsafe logging of potentially sensitive data" :
				"masked to prevent unsafe logging of potentially sensitive data";
		logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
				"': request parameters and headers will be " + value);
	}

	if (logger.isInfoEnabled()) {
		logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
	}
}

FrameworkServlet.initWebApplicationContext()

protected WebApplicationContext initWebApplicationContext() {
	//ContextLoaderListerner中已经将context放在了ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
	WebApplicationContext rootContext =
			WebApplicationContextUtils.getWebApplicationContext(getServletContext());
	WebApplicationContext wac = null;

     //以下是初始化webApplicationContext的过程
	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);
				}
				//关键方法
				configureAndRefreshWebApplicationContext(cwac);
			}
		}
	}
	if (wac == null) {
		wac = findWebApplicationContext();
	}
	if (wac == null) {
		//关键方法 上面如果为空，createWebApplicationContext里会在执行configureAndRefreshWebApplicationContext
		wac = createWebApplicationContext(rootContext);
	}

	if (!this.refreshEventReceived) {
		synchronized (this.onRefreshMonitor) {
       		// DispatcherServlet实现了onRefresh-->即：initStrategies(context);
			onRefresh(wac);
		}
	}

	if (this.publishContext) {
		String attrName = getServletContextAttributeName();
		getServletContext().setAttribute(attrName, wac);
	}

	return wac;
}

重点方法，FrameworkServlet.configureAndRefreshWebApplicationContext()，初始化ConfigurableWebApplicationContext并且执行refresh。

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
	if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
		// The application context id is still set to its original default value
		// -> assign a more useful id based on available information
		if (this.contextId != null) {
			wac.setId(this.contextId);
		}
		else {
			// Generate default id...
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
					ObjectUtils.getDisplayString(getServletContext().getContextPath()) + '/' + getServletName());
		}
	}

	wac.setServletContext(getServletContext());
	wac.setServletConfig(getServletConfig());
	wac.setNamespace(getNamespace());
	//将ContextRefreshListener注册到ApplicationContext中
	//将ContextRefreshListener注册到ApplicationContext中
	//在调用wac.refresh时候会回触发spring框架中的的onApplicationEventContext
	// SourceFilteringListener.onApplicationEventContext-->RefreshListener.onApplicationEvent-->FrameworkServlet.this.onApplicationEvent-->DipatcherServlet.onRefresh
	wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));
	// The wac environment's #initPropertySources will be called in any case when the context
	// is refreshed; do it eagerly here to ensure servlet property sources are in place for
	// use in any post-processing or initialization that occurs below prior to #refresh
	ConfigurableEnvironment env = wac.getEnvironment();
	if (env instanceof ConfigurableWebEnvironment) {
		((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
	}

	postProcessWebApplicationContext(wac);
	applyInitializers(wac);

	//applicationContext.refresh()-->registerListener会触发ContextRefreshListener.onApplicationEvent
	wac.refresh();
}

DispatcheServlet

重点方法onRefresh：逻辑见下图，重点方法initStrategies。组件的优先级：自己注入的Bean>DespatchServlet.properties中的Bean

@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.
 * 初始化组件
 */
protected void initStrategies(ApplicationContext context) {
	//MultipartResolver,不注册不支持上传文件，默认是StandardServletMultipartResolver,需要自己注册
	initMultipartResolver(context);
	//i18n 默认调用getDefaultStrategy(),见：DispatchServlet.properties，org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver
	initLocaleResolver(context);
	//主题 默认调用getDefaultStrategy(),见：DispatchServlet.properties，org.springframework.web.servlet.theme.FixedThemeResolver
	initThemeResolver(context);
	//重要组件，detectAllHandlerMappings？从context包含父类取所有的HandlerMapping or 取一个，为空从DispatchServlet.properties
	initHandlerMappings(context);
	//重要组件，detectAllHandlerMappings？从context包含父类取所有的HandlerAdapter or 取一个，为空从DispatchServlet.properties
	initHandlerAdapters(context);
	//异常处理，同上类似的逻辑
	initHandlerExceptionResolvers(context);
	//Request->ViewName
	initRequestToViewNameTranslator(context);
	//ViewResolver
	initViewResolvers(context);
	//flashMapManager
	initFlashMapManager(context);
}

总结

这里的context默认是XmlWebApplicationContext，见contextLoader

//文件内容：org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";

//静态方法代码块
static {
	// Load default strategy implementations from properties file.
	// This is currently strictly internal and not meant to be customized
	// by application developers.
	try {
		ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
		defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
	}
	catch (IOException ex) {
		throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
	}
}

见DispatcherServlet

public static final Class<?> DEFAULT_CONTEXT_CLASS = XmlWebApplicationContext.class;

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
	//getContextClass返回值是DEFAULT_CONTEXT_CLASS
	Class<?> contextClass = getContextClass();
	......
	configureAndRefreshWebApplicationContext(wac);

	return wac;
}

启动时候如何加载的init-param，见HttpServletBean的代码，推荐下去试试BeanWrapper

//BeanWrapper实际是BeanWrapperImpl【DispatcherServlet】
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
initBeanWrapper(bw);
//为BeanWrapper赋值，init-Param的name对应的是bw的属性。。。这段不仔细看真看不出来 initParam怎么赋值的
bw.setPropertyValues(pvs, true);