描述

1. 對于一個WEB應用來說，它需要部署在WEB容器 中，且WEB容器 會提供一個全局的上下文環境ServletContext，也就是SpringIOC容器的宿主環境。

2. WEB容器啟動時，會加載web.xml 中提供的 contextLoaderListener監聽器，會觸發容器初始化事件，contextLoaderListener 會監聽到該事件，其 contextInitialized 方法會被調用。在該方法中，Spring 會初始化一個啟動上下文，也被稱為根上下文，即 WebApplicationContext ，確切說，其實際實現類是 XmlWebApplicaitonContext。其實它就是Spring IOC 容器，其對應的Bean定義配置，由web.xml中context-param 標簽指定。在Spring IOC容器初始化完畢后，Spring以WebApplicationContext.ROOTWEBAPPLICATIONEXTATTRIBUTE 為屬性key，將其存儲到 ServletContext中servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);。

3.contextLoaderListener監聽器初始化完畢后，開始初始化web.xml中配置的Servlet，且Servlet 可以配置多個，以最常見的DispatcherServlet為例，這個Servlet 實際上是一個標準的前端控制器，用以 轉發 匹配 處理 每個servlet 請求。

4.DispatcherServlet上下文在初始化的時候會建立自己的 IOC容器上下文，用以持有Spring MVC 相關的Bean。在創建上下文時，會根據WebApplicationContext.ROOTWEBAPPLICATIONCONTEXTATTRIBUTE 該Key ，從ServletContext中獲取之前的根上下文（WebApplicationContext），來作為自己上下文的 Parent 上下文 。有了該Parent 上下文后，再初始化自己持有的上下文。通過initStrategies 方法中可以看到，DispatcherServlet初始化自己上下文大概的工作，就是初始化處理器映射 視圖解析等等 ，默認實現類也是 XmlWebApplicationContext。

5.DispatcherServlet上下文在初始化完成后，Spring會將與Servlet 的名字相關的屬性，作為key，將其存到ServletContext中。這樣每個 Servlet 都持有自己的上下文，即擁有自己獨立的 Bean 空間，同時各個Servlet共享相同的Bean，即根上下文定義的Bean。

org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.MappingRegistry#register 控制臺信息打印 [org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping] :Mapped "{[/appEdition/list.do]}" onto public org.springframework.web.servlet.ModelAndView com.hengtn.jiang.controller.AppEditionController.list(com.hengtn.jiang.entity.query.AppEditionQuery,javax.servlet.http.HttpSession,java.lang.Integer)

Spring容器在Web容器中的創建及初始化

SpringIOC 是一個獨立的模塊，并不是直接在Web容器中發揮作用的，假若要在Web環境使用IOC容器的話，需要Spring 為IOC 設計一個啟動過程，將IOC容器導入，并在Web容器 中建立起來。

web.xml配置文件

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5">
  <filter>
    <filter-name>springSessionRepositoryFilter</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>springSessionRepositoryFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher>
    <dispatcher>ERROR</dispatcher>
  </filter-mapping>
  <filter>
    <filter-name>characterEncodingFilter</filter-name>
    <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
    <init-param>
      <param-name>encoding</param-name>
      <param-value>UTF-8</param-value>
    </init-param>
    <init-param>
      <param-name>forceEncoding</param-name>
      <param-value>true</param-value>
    </init-param>
  </filter>
  
  <filter-mapping>
    <filter-name>characterEncodingFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>

  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>/WEB-INF/spring/root-context.xml</param-value>
  </context-param>
  <listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>
  <servlet>
    <servlet-name>appServlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>/WEB-INF/spring/appServlet/servlet-context.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>
  <servlet-mapping>
    <servlet-name>appServlet</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>
  <session-config>
    <session-timeout>180</session-timeout>
  </session-config>
</web-app>

contextConfigLocation 對應的 value 是Spring配置文件 的絕對路徑
ContextLoaderListener 監聽器主要用來對Servlet容器(這里指 Tomcat )的行為進行監聽

ContextLoaderListener 源碼

/**
 * Bootstrap listener to start up and shut down Spring's root {@link WebApplicationContext}.
 * Simply delegates to {@link ContextLoader} as well as to {@link ContextCleanupListener}.
 *
 * <p>As of Spring 3.1, {@code ContextLoaderListener} supports injecting the root web
 * application context via the {@link #ContextLoaderListener(WebApplicationContext)}
 * constructor, allowing for programmatic configuration in Servlet 3.0+ environments.
 * See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
 *
 * @author Juergen Hoeller
 * @author Chris Beams
 * @since 17.02.2003
 * @see #setContextInitializers
 * @see org.springframework.web.WebApplicationInitializer
 */
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

    /**
     * Create a new {@code ContextLoaderListener} that will create a web application
     * context based on the "contextClass" and "contextConfigLocation" servlet
     * context-params. See {@link ContextLoader} superclass documentation for details on
     * default values for each.
     * <p>This constructor is typically used when declaring {@code ContextLoaderListener}
     * as a {@code <listener>} within {@code web.xml}, where a no-arg constructor is
     * required.
     * <p>The created application context will be registered into the ServletContext under
     * the attribute name {@link WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE}
     * and the Spring application context will be closed when the {@link #contextDestroyed}
     * lifecycle method is invoked on this listener.
     * @see ContextLoader
     * @see #ContextLoaderListener(WebApplicationContext)
     * @see #contextInitialized(ServletContextEvent)
     * @see #contextDestroyed(ServletContextEvent)
     */
    public ContextLoaderListener() {
    }

    /**
     * Create a new {@code ContextLoaderListener} with the given application context. This
     * constructor is useful in Servlet 3.0+ environments where instance-based
     * registration of listeners is possible through the {@link javax.servlet.ServletContext#addListener}
     * API.
     * <p>The context may or may not yet be {@linkplain
     * org.springframework.context.ConfigurableApplicationContext#refresh() refreshed}. If it
     * (a) is an implementation of {@link ConfigurableWebApplicationContext} and
     * (b) has <strong>not</strong> already been refreshed (the recommended approach),
     * then the following will occur:
     * <ul>
     * <li>If the given context has not already been assigned an {@linkplain
     * org.springframework.context.ConfigurableApplicationContext#setId id}, one will be assigned to it</li>
     * <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to
     * the application context</li>
     * <li>{@link #customizeContext} will be called</li>
     * <li>Any {@link org.springframework.context.ApplicationContextInitializer ApplicationContextInitializer}s
     * specified through the "contextInitializerClasses" init-param will be applied.</li>
     * <li>{@link org.springframework.context.ConfigurableApplicationContext#refresh refresh()} will be called</li>
     * </ul>
     * If the context has already been refreshed or does not implement
     * {@code ConfigurableWebApplicationContext}, none of the above will occur under the
     * assumption that the user has performed these actions (or not) per his or her
     * specific needs.
     * <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
     * <p>In any case, the given application context will be registered into the
     * ServletContext under the attribute name {@link
     * WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} and the Spring
     * application context will be closed when the {@link #contextDestroyed} lifecycle
     * method is invoked on this listener.
     * @param context the application context to manage
     * @see #contextInitialized(ServletContextEvent)
     * @see #contextDestroyed(ServletContextEvent)
     */
    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }


    /**
     * Initialize the root web application context.
     */
    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }


    /**
     * Close the root web application context.
     */
    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }

}

由源碼得出， ContextLoaderListener 繼承自 ContextLoader，并且還實現了 ServletContextListener，而且它的構造函數中，需要傳入了一個WebApplicationContext，它是繼承自ApplicationContext 接口的 高級IOC容器

ServletContextListener 源碼

public interface ServletContextListener extends EventListener {

    /**
     ** Notification that the web application initialization process is starting.
     * All ServletContextListeners are notified of context initialization before
     * any filter or servlet in the web application is initialized.
     * @param sce Information about the ServletContext that was initialized
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     ** Notification that the servlet context is about to be shut down. All
     * servlets and filters have been destroy()ed before any
     * ServletContextListeners are notified of context destruction.
     * @param sce Information about the ServletContext that was destroyed
     */
    public void contextDestroyed(ServletContextEvent sce);
}

ServletContextListener 是 Servlet 中比較重要的一個接口，用于監聽 Servlet 容器的啟動 銷毀 事件，所以在 ContextLoaderListener 中：

contextInitialized(ServletContextEvent sce)：參數為所要監聽的ServletContextEvent，也就是Tomcat啟動加載完web.xml會產生的事件，ServletContextEvent持有從web.xml 加載的初始化配置ServletContext 上下文
contextDestroyed(ServletContextEvent sce)：在Tomcat 關閉的時候執行該方法

啟動時，ServletContextListener 的執行順序與web.xml 中的配置順序一致，停止時執行順序正相反

流程梳理

當Servlet容器 啟動事件發生時，將被ContextLoaderLister 監聽，此時 ContextLoaderListener會調用contextInitialized(ServletContextEvent sce) 方法，該方法為ContextLoaderLister 實現ServletContextListener接口的方法，并將在web.xml 加載初始化完成后，獲取的 ServletContext 傳入initWebApplicationContext() 方法中，進行IoC容器的初始化

initWebApplicationContext() 方法從 ContextLoader 繼承而來，進入ContextLoader 源碼中查看

    static {
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
            //private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
            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());
        }
    }

ContextLoader 類中的靜態代碼塊

創建ClassPathResource 對象，同時把ContextLoader.properties 作為參數傳入，易知 ContextLoader.properties 文件與 ContextLoader 類是在同一個目錄下，ContextLoader.properties 文件內容如下: org.springframework.web.context.WebApplicationContext=org.springframework. web.context.support.XmlWebApplicationContex
得到一個 Properties 對象，后面將根據類名來創建對應的 ApplicationContext 容器
因此可知Spring默認初始化容器，是 XmlWebApplicationContext容器

initiWebApplicationContext() 方法

/**
     * Initialize Spring's web application context for the given servlet context,
     * using the application context provided at construction time, or creating a new one
     * according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and
     * "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.
     * @param servletContext current servlet context
     * @return the new WebApplicationContext
     * @see #ContextLoader(WebApplicationContext)
     * @see #CONTEXT_CLASS_PARAM
     * @see #CONFIG_LOCATION_PARAM
     */
    public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
        //String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
        //從servletContext中獲取ApplicationContext容器，若容器存在，則拋處初始化失敗異常
        //需要檢查web.xml中是否定義了多個IOC容器的加載器
        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!");
        }

        Log logger = LogFactory.getLog(ContextLoader.class);
        servletContext.log("Initializing Spring root WebApplicationContext");
        if (logger.isInfoEnabled()) {
            logger.info("Root WebApplicationContext: initialization started");
        }
        long startTime = System.currentTimeMillis();

        try {
            //將容器存儲在本地變量，保證servletContext銷毀之后，還能獲取到它
            if (this.context == null) {
                this.context = createWebApplicationContext(servletContext);
            }
           //檢查創建的ApplicationContext實例，是否實現了ConfigurableWebApplicationContext接口
            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);
                    }
                    configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }
            //String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
            //把當前容器設為根容器，并存放在servletContext中
            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);
            }

            if (logger.isDebugEnabled()) {
                logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
                        WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
            }
            if (logger.isInfoEnabled()) {
                long elapsedTime = System.currentTimeMillis() - startTime;
                logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
            }

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

initWebApplicationContext()流程梳理

1.當調用ContextLoaderListener中的initWebApplicationContext()方法，并且將獲取到的servletContext作為參數傳入。
2.initWebApplicationContext() 首先會嘗試從servletContext 中獲取根容器，如果容器不為空，則容器初始化失敗，因為web.xml 中可能定義了多個IOC容器 的加載器。
3.若此時容器還未初始化，則調用createWebApplicationContext() 方法創建一個容器。
4.創建完容器之后，將會調用一個非常重要的configureAndRefreshWebApplicationContext() 方法，在執行該方法的時候，會將從ApplicationContext.xml 配置文件中獲取的內容，配置到已經創建好了的XmlWebApplicationContext 容器中去，并調用refresh() 方法來完成容器的初始化。
5.將已經完成初始化的XmlWebApplicationContext 容器注冊到servletContext 中去

其實在Web容器中，ServletContext為Spring的IoC容器提供了宿主環境，對應的建立起一個IoC容器的體系。其中，首先需要建立的是根上下文，這個上下文持有的對象可以有業務對象、數據存取對象、資源、事務管理器等各種中間層對象。在這個上下文的基礎上，與Web MVC相關還會有一個上下文來保持控制器之類的MVC對象，這樣就構成了一個層次化的上下文結構。因為在initWebApplicationContext方法中我們可以看到其實創建ApplicationContext容器的工作是交由createWebApplicationContext方法來實現的，下面我們來看看這個方法

createWebApplicationContext源碼

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
        //要創建的ApplicationContext的類型
        Class<?> contextClass = determineContextClass(sc);
        //如果獲取到的ApplicationContext，沒有實現ConfigurableWebApplicationContext接口，那么容器創建失敗
        //因此要創建的ApplicationContext，必須實現ConfigurableWebApplicationContext接口
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        //實例化Spring容器
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

功能：
決定要創建的ApplicationContext類型
實例化一個ApplicationContext

那么它是如何決定要創建的ApplicationContext類型的呢？
由determineContextClass() 方法決定的

protected Class<?> determineContextClass(ServletContext servletContext) {
        //從web.xml獲取要創建的IOC容器名稱
        //public static final String CONTEXT_CLASS_PARAM = "contextClass";      
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
        //如果獲取到的類名不為空，則創建該容器的class對象    
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load custom context class [" + contextClassName + "]", ex);
            }
        }
        //否則創建默認的容器Class對象，即org.springframework.web.context.support.XmlWebApplicationContext
        //在創建ContextLoader時，defaultStrategies = PropertiesLoaderUtils.loadProperties(resource); 
        //已經準備好默認的容器類
        else {
            contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
            try {
                return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load default context class [" + contextClassName + "]", ex);
            }
        }
    }

完成IOC容器 的創建后，在initWebApplicationContext() 中將調用configureAndRefreshWebApplicationContext() 初始化該容器

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
        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()));
            }
        }

        wac.setServletContext(sc);
        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);
        wac.refresh();
    }

為創建好的 IOC容器 ，設置 Web應用 的上下文，以便二者整合
為同一個IOC容器 ，設置配置文件的絕對路徑
調用IOC容器 的 refresh() 函數對其進行初始化