Spring MVC的初始化

初始化Spring MVC

1. 首先到了GenericServlet#init(ServletConfig)中，而后调用this#init()方法，此处的this指向是DispatcherServlet对象

public void init(ServletConfig config) throws ServletException {
	this.config = config;
	this.init();
}

2. DispatcherServlet对象执行init()方法，DispatcherServlet对象中的这个方法是继承自HttpServletBean，此时会进入到HttpServletBean#init中，这里面的重点是initServletBean()方法，调用对应的子类进行初始化

public final void init() throws ServletException {
	// Set bean properties from init parameters.
	PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
	if (!pvs.isEmpty()) {
		try {
			BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
			ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
			bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
			initBeanWrapper(bw);
			bw.setPropertyValues(pvs, true);
		}
		//...忽略
	}
	// Let subclasses do whatever initialization they like. 让子类进行对应的初始化
	initServletBean();
}

3. 进入到FrameworkServlet#initServletBean中，核心是initWebApplicationContext和initFrameworkServlet方法

protected final void initServletBean() throws ServletException {
	getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
	try {
		//
		this.webApplicationContext = initWebApplicationContext();
		//
		initFrameworkServlet();
	}
	catch (ServletException | RuntimeException ex) {
		logger.error("Context initialization failed", ex);
		throw ex;
	}
	//log..
}

4. 首先进入到了initWebApplicationContext方法，对web项目上下文进行初始化

protected WebApplicationContext initWebApplicationContext() {
	//首先得到父容器
	WebApplicationContext rootContext =
		WebApplicationContextUtils.getWebApplicationContext(getServletContext());
	//什么子容器
	WebApplicationContext wac = null;

	//创建父子容器关系
	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);
			}
		}
	}
	//查找web容器的引用是否存在, 并建立好空ioc容器
	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();
	}
	//给建好的IOC容器赋值
	if (wac == null) {
		// No context instance is defined for this servlet -> create a local one
		wac = createWebApplicationContext(rootContext);
	}
	//触发刷新方法
	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.
		synchronized (this.onRefreshMonitor) {
			onRefresh(wac);
		}
	}

	if (this.publishContext) {
		// Publish the context as a servlet context attribute.
		String attrName = getServletContextAttributeName();
		getServletContext().setAttribute(attrName, wac);
	}

	return wac;
}

5. IOC容器初始化之后，调用了DispatcherServlet的onRefresh，在onRefresh中又调用了initStrategies方法，初始化SpringMVC的九大组件

@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) {
	//多文件上传组件
	initMultipartResolver(context);
	//本地语言环境
	initLocaleResolver(context);
	//模板处理器
	initThemeResolver(context);
	//HandlerMapping
	initHandlerMappings(context);
	//适配器
	initHandlerAdapters(context);
	//异常拦截器
	initHandlerExceptionResolvers(context);
	//视图预处理器
	initRequestToViewNameTranslator(context);
	//视图转换器
	initViewResolvers(context);
	//FlashMap
	initFlashMapManager(context);
}

附录: 九大组件

1. HandlerMapping 根据request找到相应的处理器。由于Handler（Controller）有两种形式，一种是基于类的Handler，另外一种是基于Method的Handler（也就是咱们经常使用的）

2. HandlerAdapter 调用Handler的适配器。若是把Handler（Controller）当作工具的话，那么HandlerAdapter就至关于干活的工人

3. HandlerExceptionResolver 对异常的处理

4. ViewResolver 用来将String类型的视图名和Locale解析为View类型的视图

5. RequestToViewNameTranslator 有的Handler（Controller）处理完后没有设置返回类型，好比是void方法，这是就须要从request中获取viewName

6. LocaleResolver 从request中解析出Locale。Locale表示一个区域，好比zh-cn，对不一样的区域的用户，显示不一样的结果，这就是i18n（SpringMVC中有具体的拦截器LocaleChangeInterceptor）

7. ThemeResolver 主题解析，这种相似于咱们手机更换主题，不一样的UI，css等

8. MultipartResolver 处理上传请求，将普通的request封装成MultipartHttpServletRequest

9. FlashMapManager 用于管理FlashMap，FlashMap用于在redirect重定向中传递参数