Spring MVC的初始化

初始化Spring MVC

1. 首先到了GenericServlet#init(ServletConfig)中，而後調用this#init()方法，此處的this指向是DispatcherServlet對象

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

2. DispatcherServlet對象執行init()方法，DispatcherServlet對象中的這個方法是繼承自HttpServletBean，此時會進入到HttpServletBean#init中，這裏面的重點是initServletBean()方法，調用對應的子類進行初始化

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. 讓子類進行對應的初始化
	initServletBean();
}

3. 進入到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. 首先進入到了initWebApplicationContext方法，對web項目上下文進行初始化

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

	//創建父子容器關係
	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, 並建立好空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();
	}
	//給建好的IOC容器賦值
	if (wac == null) {
		// No context instance is defined for this servlet -> create a local one
		wac = createWebApplicationContext(rootContext);
	}
	//觸發刷新方法
	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容器初始化之後，調用了DispatcherServlet的onRefresh，在onRefresh中又調用了initStrategies方法，初始化SpringMVC的九大組件

@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) {
	//多文件上傳組件
	initMultipartResolver(context);
	//本地語言環境
	initLocaleResolver(context);
	//模板處理器
	initThemeResolver(context);
	//HandlerMapping
	initHandlerMappings(context);
	//適配器
	initHandlerAdapters(context);
	//異常攔截器
	initHandlerExceptionResolvers(context);
	//視圖預處理器
	initRequestToViewNameTranslator(context);
	//視圖轉換器
	initViewResolvers(context);
	//FlashMap
	initFlashMapManager(context);
}

附錄: 九大組件

1. HandlerMapping 根據request找到相應的處理器。由於Handler（Controller）有兩種形式，一種是基於類的Handler，另外一種是基於Method的Handler（也就是咱們經常使用的）

2. HandlerAdapter 調用Handler的適配器。若是把Handler（Controller）當作工具的話，那麼HandlerAdapter就至關於幹活的工人

3. HandlerExceptionResolver 對異常的處理

4. ViewResolver 用來將String類型的視圖名和Locale解析爲View類型的視圖

5. RequestToViewNameTranslator 有的Handler（Controller）處理完後沒有設置返回類型，好比是void方法，這是就須要從request中獲取viewName

6. LocaleResolver 從request中解析出Locale。Locale表示一個區域，好比zh-cn，對不一樣的區域的用戶，顯示不一樣的結果，這就是i18n（SpringMVC中有具體的攔截器LocaleChangeInterceptor）

7. ThemeResolver 主題解析，這種相似於咱們手機更換主題，不一樣的UI，css等

8. MultipartResolver 處理上傳請求，將普通的request封裝成MultipartHttpServletRequest

9. FlashMapManager 用於管理FlashMap，FlashMap用於在redirect重定向中傳遞參數