Spring MVC源碼——Root WebApplicationContext

　　打算開始讀一些框架的源碼,先拿 Spring MVC 練練手,歡迎點擊這里訪問我的源碼注釋, SpringMVC官方文檔一開始就給出了這樣的兩段示例:

WebApplicationInitializer示例:

public class MyWebApplicationInitializer implements WebApplicationInitializer {@Overridepublic void onStartup(ServletContext servletCxt) {// Load Spring web application configurationAnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();ac.register(AppConfig.class);ac.refresh();// Create and register the DispatcherServletDispatcherServlet servlet = new DispatcherServlet(ac);ServletRegistration.Dynamic registration = servletCxt.addServlet("app", servlet);registration.setLoadOnStartup(1);registration.addMapping("/app/*");}
}

web.xml 示例:

<web-app><listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener><context-param><param-name>contextConfigLocation</param-name><param-value>/WEB-INF/app-context.xml</param-value></context-param><servlet><servlet-name>app</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value></param-value></init-param><load-on-startup>1</load-on-startup></servlet><servlet-mapping><servlet-name>app</servlet-name><url-pattern>/app/*</url-pattern></servlet-mapping></web-app>

　　我們按照 web.xml 中的實例來看一下 Spring MVC 初始化過程.

上下文層次結構

　　Spring MVC 的上下文有如下這樣的層級:

　　圖中的?Servlet WebApplicationContext?是與?DispatcherServlet?綁定的上下文, 其中還有 controllers、ViewResolver、HandlerMapping 等組件.

Root WebApplicationContext?不是必須的上下文, 在需要時,可以用來在多個?DispatcherServlet?間共享一些 bean.

Root WebApplicationContext 初始化和銷毀

　　ContextLoaderListener

　　web.xml 中配置的?ContextLoaderListener?用于啟動和終止 Spring 的 root?WebApplicationContext.

　　 ContextLoaderListener

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {public ContextLoaderListener() {}public ContextLoaderListener(WebApplicationContext context) {super(context);}/*** Initialize the root web application context.* Servlet 上下文初始化,調用父類的方法初始化 WebApplicationContext*/@Overridepublic void contextInitialized(ServletContextEvent event) {initWebApplicationContext(event.getServletContext());}/*** Close the root web application context.* Servlet 上下文被銷毀,調用父類的方法銷毀 WebApplicationContext*/@Overridepublic void contextDestroyed(ServletContextEvent event) {closeWebApplicationContext(event.getServletContext());// 銷毀 ServletContext 上實現了 DisposableBean 的屬性并移除他們
        ContextCleanupListener.cleanupAttributes(event.getServletContext());}
}

ContextLoaderListener?直接調用了父類?ContextLoader?的方法來初始化和銷毀上下文.

ContextLoaderListener?在創建上下文時,會嘗試讀取?contextClass?context-param 來指定上下文的類型,被指定的類需要實現?ConfigurableWebApplicationContext?接口. 如果沒有獲取到,默認會使用?WebApplicationContext.

初始化上下文時,會嘗試讀取?contextConfigLocation?context-param, 作為 xml 文件的路徑.

　　初始化上下文

initWebApplicationContext()?方法如下:

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {// 檢查有沒有綁定上下文if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!");}servletContext.log("Initializing Spring root WebApplicationContext");Log logger = LogFactory.getLog(ContextLoader.class);if (logger.isInfoEnabled()) {logger.info("Root WebApplicationContext: initialization started");}// 初始化開始時間long startTime = System.currentTimeMillis();try {// Store context in local instance variable, to guarantee that// it is available on ServletContext shutdown.// 保存上下文到本地實例變量中，保證上細紋能在 ServletContext 關閉時訪問到if (this.context == null) {// 創建上下文this.context = createWebApplicationContext(servletContext);}if (this.context instanceof ConfigurableWebApplicationContext) {// 如果上下文實現了 ConfigurableWebApplicationContextConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;if (!cwac.isActive()) {// 上下文還沒有刷新，設置 父上下文（如果能找到），并且刷新// The context has not yet been refreshed -> provide services such as// setting the parent context, setting the application context id, etcif (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);}// 設置和刷新上下文
                configureAndRefreshWebApplicationContext(cwac, servletContext);}}// 將上下文綁定到 servletContext 的屬性上servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);// 獲取當前線程的上下文類加載器ClassLoader ccl = Thread.currentThread().getContextClassLoader();if (ccl == ContextLoader.class.getClassLoader()) {currentContext = this.context;}else if (ccl != null) {// 如果有線程上下文類加載器，而且不是 ContextLoader 本身的類加載器，放入到 currentContextPerThread 中。這是一個 static 的域currentContextPerThread.put(ccl, this.context);}if (logger.isInfoEnabled()) {long elapsedTime = System.currentTimeMillis() - startTime;logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");}return this.context;}catch (RuntimeException | Error ex) {// 發生異常, 把異常綁定到上下文對應的屬性上,之后不會再進行初始化logger.error("Context initialization failed", ex);servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);throw ex;}
}

initWebApplicationContext()?方法調用了?createWebApplicationContext()?方法來創建上下文;調用了?configureAndRefreshWebApplicationContext()?來對實現了?ConfigurableWebApplicationContext?接口的上下文做初始化.

createWebApplicationContext()?會調用?determineContextClass()?來獲取上下文類型的 Class 對象.

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {// 獲取上下文類型Class<?> contextClass = determineContextClass(sc);// 檢查是否實現了 ConfigurableWebApplicationContextif (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {throw new ApplicationContextException("Custom context class [" + contextClass.getName() +"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");}// 實例化return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}protected Class<?> determineContextClass(ServletContext servletContext) {// 獲取 serveltContext 的 'contextClass' 初始化參數。String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);if (contextClassName != null) {// 指定過上下文類型，加載類try {return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());}catch (ClassNotFoundException ex) {throw new ApplicationContextException("Failed to load custom context class [" + contextClassName + "]", ex);}}else {// 去默認策略里獲取默認的上下文類型名稱contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());try {// 加載類return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());}catch (ClassNotFoundException ex) {throw new ApplicationContextException("Failed to load default context class [" + contextClassName + "]", ex);}}
}

configureAndRefreshWebApplicationContext()?方法.

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {// 用可以獲取到的信息，獲取一個更有意義的上下文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// 獲取 ServletContext 的 'contextId' 初始化參數。String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);if (idParam != null) {wac.setId(idParam);}else {// Generate default id...// 生成默認 idwac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +ObjectUtils.getDisplayString(sc.getContextPath()));}}// 設置 servletContext 屬性
    wac.setServletContext(sc);// 設置配置文件路徑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// 初始化屬性源， 確保 servlet 屬性源到位并能夠在任何 refresh 之前的后期處理和初始化中使用ConfigurableEnvironment env = wac.getEnvironment();if (env instanceof ConfigurableWebEnvironment) {((ConfigurableWebEnvironment) env).initPropertySources(sc, null);}// 在設置了配置文件之后上下文刷新之前，自定義上下文
    customizeContext(sc, wac);wac.refresh();
}protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {// 根據 ServletContext 的 'contextInitializerClasses' 和 'globalInitializerClasses' 初始化參數 加載 ApplicationContextInitializer 的 classList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =determineContextInitializerClasses(sc);for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {// 獲取范型參數類型Class<?> initializerContextClass =GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);// 檢查 Initializer 是否適用于當前上下文對象if (initializerContextClass != null && !initializerContextClass.isInstance(wac)) {throw new ApplicationContextException(String.format("Could not apply context initializer [%s] since its generic parameter [%s] " +"is not assignable from the type of application context used by this " +"context loader: [%s]", initializerClass.getName(), initializerContextClass.getName(),wac.getClass().getName()));}// 創建 Initializer 實例，并添加到 contextInitializersthis.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));}// 根據 org.springframework.core.Ordered 和 org.springframework.core.annotation.Order 排序，如果沒有實現或注解，會被排到最后AnnotationAwareOrderComparator.sort(this.contextInitializers);// 執行每個 initializer 的 initialize() 方法for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {initializer.initialize(wac);}
}

　　銷毀上下文

closeWebApplicationContext()?方法如下:

public void closeWebApplicationContext(ServletContext servletContext) {servletContext.log("Closing Spring root WebApplicationContext");try {// 如果 context 是 ConfigurableWebApplicationContext 調用 close() 方法if (this.context instanceof ConfigurableWebApplicationContext) {((ConfigurableWebApplicationContext) this.context).close();}}finally {ClassLoader ccl = Thread.currentThread().getContextClassLoader();if (ccl == ContextLoader.class.getClassLoader()) {currentContext = null;}else if (ccl != null) {currentContextPerThread.remove(ccl);}// 移除 servletContext 中的 context 屬性
        servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);}
}

Servlet 3.0+ 中初始化

Servlet 3.0+ 中可以通過?ServletContext?的?addlistener()?方法來添加監聽器.因此可以先把 Spring 容器先創建好,再傳給?ContextLoaderListener?的構造器.這里就不自己寫例子了,選了單元測試中的?ContextLoaderTests.testContextLoaderListenerWithDefaultContext()?方法:

public void testContextLoaderListenerWithDefaultContext() {MockServletContext sc = new MockServletContext("");sc.addInitParameter(ContextLoader.CONFIG_LOCATION_PARAM,"/org/springframework/web/context/WEB-INF/applicationContext.xml " +"/org/springframework/web/context/WEB-INF/context-addition.xml");ServletContextListener listener = new ContextLoaderListener();ServletContextEvent event = new ServletContextEvent(sc);listener.contextInitialized(event);String contextAttr = WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE;WebApplicationContext context = (WebApplicationContext) sc.getAttribute(contextAttr);assertTrue("Correct WebApplicationContext exposed in ServletContext", context instanceof XmlWebApplicationContext);assertTrue(WebApplicationContextUtils.getRequiredWebApplicationContext(sc) instanceof XmlWebApplicationContext);LifecycleBean lb = (LifecycleBean) context.getBean("lifecycle");assertTrue("Has father", context.containsBean("father"));assertTrue("Has rod", context.containsBean("rod"));assertTrue("Has kerry", context.containsBean("kerry"));assertTrue("Not destroyed", !lb.isDestroyed());assertFalse(context.containsBean("beans1.bean1"));assertFalse(context.containsBean("beans1.bean2"));listener.contextDestroyed(event);assertTrue("Destroyed", lb.isDestroyed());assertNull(sc.getAttribute(contextAttr));assertNull(WebApplicationContextUtils.getWebApplicationContext(sc));
}