描述

1. 对于一个WEB应用来说，它需要部署在WEB容器 中，且WEB容器 会提供一个全局的上下文环境ServletContext，也就是SpringIOC容器的宿主环境。

2. WEB容器启动时，会加载web.xml 中提供的 contextLoaderListener监听器，会触发容器初始化事件，contextLoaderListener 会监听到该事件，其 contextInitialized 方法会被调用。在该方法中，Spring 会初始化一个启动上下文，也被称为根上下文，即 WebApplicationContext ，确切说，其实际实现类是 XmlWebApplicaitonContext。其实它就是Spring IOC 容器，其对应的Bean定义配置，由web.xml中context-param 标签指定。在Spring IOC容器初始化完毕后，Spring以WebApplicationContext.ROOTWEBAPPLICATIONEXTATTRIBUTE 为属性key，将其存储到 ServletContext中servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);。

3.contextLoaderListener监听器初始化完毕后，开始初始化web.xml中配置的Servlet，且Servlet 可以配置多个，以最常见的DispatcherServlet为例，这个Servlet 实际上是一个标准的前端控制器，用以 转发 匹配 处理 每个servlet 请求。

4.DispatcherServlet上下文在初始化的时候会建立自己的 IOC容器上下文，用以持有Spring MVC 相关的Bean。在创建上下文时，会根据WebApplicationContext.ROOTWEBAPPLICATIONCONTEXTATTRIBUTE 该Key ，从ServletContext中获取之前的根上下文（WebApplicationContext），来作为自己上下文的 Parent 上下文 。有了该Parent 上下文后，再初始化自己持有的上下文。通过initStrategies 方法中可以看到，DispatcherServlet初始化自己上下文大概的工作，就是初始化处理器映射 视图解析等等 ，默认实现类也是 XmlWebApplicationContext。

5.DispatcherServlet上下文在初始化完成后，Spring会将与Servlet 的名字相关的属性，作为key，将其存到ServletContext中。这样每个 Servlet 都持有自己的上下文，即拥有自己独立的 Bean 空间，同时各个Servlet共享相同的Bean，即根上下文定义的Bean。

org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.MappingRegistry#register 控制台信息打印 [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容器中的创建及初始化

SpringIOC 是一个独立的模块，并不是直接在Web容器中发挥作用的，假若要在Web环境使用IOC容器的话，需要Spring 为IOC 设计一个启动过程，将IOC容器导入，并在Web容器 中建立起来。

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 对应的 value 是Spring配置文件 的绝对路径
ContextLoaderListener 监听器主要用来对Servlet容器(这里指 Tomcat )的行为进行监听

ContextLoaderListener 源码

/**
 * 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());
    }

}

由源码得出， ContextLoaderListener 继承自 ContextLoader，并且还实现了 ServletContextListener，而且它的构造函数中，需要传入了一个WebApplicationContext，它是继承自ApplicationContext 接口的 高级IOC容器

ServletContextListener 源码

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 中比较重要的一个接口，用于监听 Servlet 容器的启动 销毁 事件，所以在 ContextLoaderListener 中：

contextInitialized(ServletContextEvent sce)：参数为所要监听的ServletContextEvent，也就是Tomcat启动加载完web.xml会产生的事件，ServletContextEvent持有从web.xml 加载的初始化配置ServletContext 上下文
contextDestroyed(ServletContextEvent sce)：在Tomcat 关闭的时候执行该方法

启动时，ServletContextListener 的执行顺序与web.xml 中的配置顺序一致，停止时执行顺序正相反

流程梳理

当Servlet容器 启动事件发生时，将被ContextLoaderLister 监听，此时 ContextLoaderListener会调用contextInitialized(ServletContextEvent sce) 方法，该方法为ContextLoaderLister 实现ServletContextListener接口的方法，并将在web.xml 加载初始化完成后，获取的 ServletContext 传入initWebApplicationContext() 方法中，进行IoC容器的初始化

initWebApplicationContext() 方法从 ContextLoader 继承而来，进入ContextLoader 源码中查看

    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 类中的静态代码块

创建ClassPathResource 对象，同时把ContextLoader.properties 作为参数传入，易知 ContextLoader.properties 文件与 ContextLoader 类是在同一个目录下，ContextLoader.properties 文件内容如下: org.springframework.web.context.WebApplicationContext=org.springframework. web.context.support.XmlWebApplicationContex
得到一个 Properties 对象，后面将根据类名来创建对应的 ApplicationContext 容器
因此可知Spring默认初始化容器，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";
        //从servletContext中获取ApplicationContext容器，若容器存在，则抛处初始化失败异常
        //需要检查web.xml中是否定义了多个IOC容器的加载器
        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 {
            //将容器存储在本地变量，保证servletContext销毁之后，还能获取到它
            if (this.context == null) {
                this.context = createWebApplicationContext(servletContext);
            }
           //检查创建的ApplicationContext实例，是否实现了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";
            //把当前容器设为根容器，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.当调用ContextLoaderListener中的initWebApplicationContext()方法，并且将获取到的servletContext作为参数传入。
2.initWebApplicationContext() 首先会尝试从servletContext 中获取根容器，如果容器不为空，则容器初始化失败，因为web.xml 中可能定义了多个IOC容器 的加载器。
3.若此时容器还未初始化，则调用createWebApplicationContext() 方法创建一个容器。
4.创建完容器之后，将会调用一个非常重要的configureAndRefreshWebApplicationContext() 方法，在执行该方法的时候，会将从ApplicationContext.xml 配置文件中获取的内容，配置到已经创建好了的XmlWebApplicationContext 容器中去，并调用refresh() 方法来完成容器的初始化。
5.将已经完成初始化的XmlWebApplicationContext 容器注册到servletContext 中去

其实在Web容器中，ServletContext为Spring的IoC容器提供了宿主环境，对应的建立起一个IoC容器的体系。其中，首先需要建立的是根上下文，这个上下文持有的对象可以有业务对象、数据存取对象、资源、事务管理器等各种中间层对象。在这个上下文的基础上，与Web MVC相关还会有一个上下文来保持控制器之类的MVC对象，这样就构成了一个层次化的上下文结构。因为在initWebApplicationContext方法中我们可以看到其实创建ApplicationContext容器的工作是交由createWebApplicationContext方法来实现的，下面我们来看看这个方法

createWebApplicationContext源码

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
        //要创建的ApplicationContext的类型
        Class<?> contextClass = determineContextClass(sc);
        //如果获取到的ApplicationContext，没有实现ConfigurableWebApplicationContext接口，那么容器创建失败
        //因此要创建的ApplicationContext，必须实现ConfigurableWebApplicationContext接口
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        //实例化Spring容器
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

功能：
决定要创建的ApplicationContext类型
实例化一个ApplicationContext

那么它是如何决定要创建的ApplicationContext类型的呢？
由determineContextClass() 方法决定的

protected Class<?> determineContextClass(ServletContext servletContext) {
        //从web.xml获取要创建的IOC容器名称
        //public static final String CONTEXT_CLASS_PARAM = "contextClass";      
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
        //如果获取到的类名不为空，则创建该容器的class对象    
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load custom context class [" + contextClassName + "]", ex);
            }
        }
        //否则创建默认的容器Class对象，即org.springframework.web.context.support.XmlWebApplicationContext
        //在创建ContextLoader时，defaultStrategies = PropertiesLoaderUtils.loadProperties(resource); 
        //已经准备好默认的容器类
        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 的创建后，在initWebApplicationContext() 中将调用configureAndRefreshWebApplicationContext() 初始化该容器

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();
    }

为创建好的 IOC容器 ，设置 Web应用 的上下文，以便二者整合
为同一个IOC容器 ，设置配置文件的绝对路径
调用IOC容器 的 refresh() 函数对其进行初始化