【Spring】浅谈ContextLoaderListener及其上下文与DispatcherServlet的区别

一般在使用SpingMVC开发的项目中，一般都会在web.xml文件中配置ContextLoaderListener监听器，如下：

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

在开始讲解这个之前先讲讲web工程的上下文，对于一个web容器，web容器提供了一个全局的上下文环境，这个上下文就是ServletContext，其为后面Spring IOC容器提供宿主环境。

在web容器启动时会触发容器初始化事件，contextLoaderListener监听到这个事件后其contextInitialized方法就会被调用，在这个方法中，spring会初始化一个启动上下文，这个上下文就是根上下文，也就是WebApplicationContext，实际实现类一般是XmlWebApplicationContext，这个其实就是spring的IoC容器，这个IoC容器初始化完后，Spring会将它存储到ServletContext，可供后面获取到该IOC容器中的bean。

下面一步步来跟进，看下ContextLoaderListener源码：

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

    public ContextLoaderListener() {
    }

    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }

    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}

从上面可以看出ContextLoaderListener继承ContextLoader类并实现了ServletContextListener接口，ServletContextListener接口中只有初始化和销毁的两个方法，如下：

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.
     */
    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.
     */
    public void contextDestroyed ( ServletContextEvent sce );
}

ContextLoaderListener主要的功能还是在继承的ContextLoader类中实现，接下来看看ContextLoaderListener中上下文初始化的方法，也就是：

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

跟进initWebApplicationContext()方法，其调用的实现就在ContextLoader类中：

/**
 * 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) {
    // 先判断ServletContext中是否已存在上下文，有的话说明已加载或配置信息有误（看下面抛出的异常信息）
    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 {
        // Store context in local instance variable, to guarantee that
        // it is available on ServletContext shutdown.
        if (this.context == null) {
            // 创建WebApplicationContext上下文
            this.context = createWebApplicationContext(servletContext);
        }
        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.
                    // 加载父上下文
                    ApplicationContext parent = loadParentContext(servletContext);
                    cwac.setParent(parent);
                }
                // 对WebApplicationContext进行初始化，初始化参数从web.xml中取
                configureAndRefreshWebApplicationContext(cwac, servletContext);
            }
        }
        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);
        }

        /* 省略部分代码 */
}

上面initWebApplicationContext()方法中，通过createWebApplicationContext(servletContext)创建root上下文（即IOC容器），之后Spring会以WebApplicationContext.ROOTWEBAPPLICATIONCONTEXTATTRIBUTE属性为Key，将该root上下文存储到ServletContext中，下面看看createWebApplicationContext(servletContext)源码：

/**
 * Instantiate the root WebApplicationContext for this loader, either the
 * default context class or a custom context class if specified.
 * <p>This implementation expects custom contexts to implement the
 * {@link ConfigurableWebApplicationContext} interface.
 * Can be overridden in subclasses.
 * <p>In addition, {@link #customizeContext} gets called prior to refreshing the
 * context, allowing subclasses to perform custom modifications to the context.
 * @param sc current servlet context
 * @return the root WebApplicationContext
 * @see ConfigurableWebApplicationContext
 */
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    // 确定载入的上下文的类型，参数是在web.xml中配置的contextClass（没有则使用默认的）
    Class<?> contextClass = determineContextClass(sc);
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
            "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    }
    // 初始化WebApplicationContext并强转为ConfigurableWebApplicationContext类型
    return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}

从上面源码也可以看出使用createWebApplicationContext方法创建的上下文肯定是实现了ConfigurableWebApplicationContext接口，否则抛出异常。上面createWebApplicationContext(servletContext)方法里的determineContextClass方法用于查找root上下文的Class类型，看源码：

/**
 * Return the WebApplicationContext implementation class to use, either the
 * default XmlWebApplicationContext or a custom context class if specified.
 * @param servletContext current servlet context
 * @return the WebApplicationContext implementation class to use
 * @see #CONTEXT_CLASS_PARAM
 * @see org.springframework.web.context.support.XmlWebApplicationContext
 */
protected Class<?> determineContextClass(ServletContext servletContext) {
    String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
    if (contextClassName != null) {
        try {
            return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
        }
        catch (ClassNotFoundException ex) {
            throw new ApplicationContextException(
                    "Failed to load custom context class [" + contextClassName + "]", ex);
        }
    }
    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);
        }
    }
}

从以上可以看到如果web.xml中配置了实现ConfigurableWebApplicationContext的contextClass类型就用那个参数，否则使用默认的XmlWebApplicationContext。

上面ContextLoader类的initWebApplicationContext()方法里还有个加载父上下文的方法loadParentContext(ServletContext servletContext)，也来看看其源码：

/**
 * Template method with default implementation (which may be overridden by a
 * subclass), to load or obtain an ApplicationContext instance which will be
 * used as the parent context of the root WebApplicationContext. If the
 * return value from the method is null, no parent context is set.
 * <p>The main reason to load a parent context here is to allow multiple root
 * web application contexts to all be children of a shared EAR context, or
 * alternately to also share the same parent context that is visible to
 * EJBs. For pure web applications, there is usually no need to worry about
 * having a parent context to the root web application context.
 * <p>The default implementation uses
 * {@link org.springframework.context.access.ContextSingletonBeanFactoryLocator},
 * configured via {@link #LOCATOR_FACTORY_SELECTOR_PARAM} and
 * {@link #LOCATOR_FACTORY_KEY_PARAM}, to load a parent context
 * which will be shared by all other users of ContextsingletonBeanFactoryLocator
 * which also use the same configuration parameters.
 * @param servletContext current servlet context
 * @return the parent application context, or {@code null} if none
 * @see org.springframework.context.access.ContextSingletonBeanFactoryLocator
 */
protected ApplicationContext loadParentContext(ServletContext servletContext) {
    ApplicationContext parentContext = null;
    String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);
    String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);

    if (parentContextKey != null) {
        // locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"
        BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
        Log logger = LogFactory.getLog(ContextLoader.class);
        if (logger.isDebugEnabled()) {
            logger.debug("Getting parent context definition: using parent context key of '" +
                    parentContextKey + "' with BeanFactoryLocator");
        }
        this.parentContextRef = locator.useBeanFactory(parentContextKey);
        parentContext = (ApplicationContext) this.parentContextRef.getFactory();
    }

    return parentContext;
}

上面源码就是实现根据locatorFactorySelector和parentContextKey来给上下文设置父上下文，前提是我们在web.xml中配置了这两个参数，不过一般开发中很少会设置这两个参数，从上面源码的大段注释也可以看出如果没有的话父上下文就为空。

在contextLoaderListener监听器初始化完毕后，开始初始化web.xml中配置的Servlet，这个servlet可以配置多个，以DispatcherServlet为例，这个servlet实际上是一个标准的前端控制器，用以转发、处理每个servlet请求。DispatcherServlet上下文在初始化的时候会建立自己的IoC上下文，用以持有spring mvc相关的bean。在建立DispatcherServlet自己的IoC上下文时，会利用WebApplicationContext.ROOTWEBAPPLICATIONCONTEXTATTRIBUTE先从ServletContext中获取之前的根上下文(即WebApplicationContext)作为自己上下文的parent上下文。有了这个parent上下文之后，再初始化自己持有的上下文。这个DispatcherServlet初始化自己上下文的工作在其initStrategies方法中实现的，基本工作就是初始化处理器映射、视图解析等。这个servlet自己持有的上下文默认实现类也是XmlWebApplicationContext。初始化完毕后，spring以与servlet的名字相关的属性为Key，也将其存到ServletContext中。这样每个servlet就持有自己的上下文，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(WebApplicationContext)。

最后讲讲ContextLoaderListener与DispatcherServlet所创建的上下文ApplicationContext的区别：

1.ContextLoaderListener中创建ApplicationContext主要用于整个Web应用程序需要共享的一些组件，比如DAO，数据库的ConnectionFactory等。而由DispatcherServlet创建的ApplicationContext主要用于和该Servlet相关的一些组件，比如Controller、ViewResovler等。
2.对于作用范围而言，在DispatcherServlet中可以引用由ContextLoaderListener所创建的ApplicationContext，而反过来不行。

这两个ApplicationContext都是通过ServletContext的setAttribute方法放到ServletContext中的。从web.xml的配置可知ContextLoaderListener会先于DispatcherServlet创建ApplicationContext，DispatcherServlet在创建ApplicationContext时会先找到由ContextLoaderListener所创建的ApplicationContext，再将后者的ApplicationContext作为参数传给DispatcherServlet的ApplicationContext的setParent()方法，作为它的父上下文，在Spring源代可以看出：

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
    Class<?> contextClass = getContextClass();
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Servlet with name '" + getServletName() +
                "' will try to create custom WebApplicationContext context of class '" +
                contextClass.getName() + "'" + ", using parent context [" + parent + "]");
    }
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException(
                "Fatal initialization error in servlet with name '" + getServletName() +
                "': custom WebApplicationContext class [" + contextClass.getName() +
                "] is not of type ConfigurableWebApplicationContext");
    }
    ConfigurableWebApplicationContext wac =
            (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

    wac.setEnvironment(getEnvironment());

    // 设置父ApplicationContext
    wac.setParent(parent);
    wac.setConfigLocation(getContextConfigLocation());

    configureAndRefreshWebApplicationContext(wac);

    return wac;
}

这里wac即为由DisptcherServlet创建的ApplicationContext，而parent则为有ContextLoaderListener创建的ApplicationContext。

当Spring在执行ApplicationContext的getBean时，如果在自己context中找不到对应的bean，则会在父ApplicationContext中去找。这也解释了为什么我们可以在DispatcherServlet中获取到由ContextLoaderListener对应的ApplicationContext中的bean。