spring mvc DispatcherServlet詳解之前傳---FrameworkServlet

做項目時碰到Controller不能使用aop進行攔截，從網上搜索得知：使用spring mvc 啟動了兩個context：applicationContext 和WebapplicationContext。

首先我們來了解applicationContext 和WebapplicationContext區別和聯系吧

1. ApplicationContext和WebApplicationContext是繼承關系

/**
 * Interface to provide configuration for a web application. This is read-only while
 * the application is running, but may be reloaded if the implementation supports this.
 *
 * <p>This interface adds a {@code getServletContext()} method to the generic
 * ApplicationContext interface, and defines a well-known application attribute name
 * that the root context must be bound to in the bootstrap process.
 *
 * <p>Like generic application contexts, web application contexts are hierarchical.
 * There is a single root context per application, while each servlet in the application * (including a dispatcher servlet in the MVC framework) has its own child context.
 *
 * <p>In addition to standard application context lifecycle capabilities,
 * WebApplicationContext implementations need to detect {@link ServletContextAware}
 * beans and invoke the {@code setServletContext} method accordingly.*/
public interface WebApplicationContext extends ApplicationContext {

2. ContextLoaderListener 創建基於web的應用根 applicationContext 並將它放入到ServletContext. applicationContext加載或者卸載spring管理的beans。在structs和spring mvc的控制層都是這樣使用的。

3. DispatcherServlet創建自己的WebApplicationContext並管理這個WebApplicationContext里面的 handlers/controllers/view-resolvers. 

4. 當ContextLoaderListener和DispatcherServlet一起使用時, ContextLoaderListener 先創建一個根applicationContext，然后DispatcherSerlvet創建一個子applicationContext並且綁定到根applicationContext。

首先來看看web.xml的定義：

一般的web應用，通過ContextLoaderListener監聽，ContextLoaderListener中加載的context成功后，spring 將 applicationContext存放在ServletContext中key值為"org.springframework.web.context.WebApplicationContext.ROOT"的attribute中。

<listener>  
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>  
</listener>  
<context-param>  
<param-name>contextConfigLocation</param-name>  
<param-value>classpath*:conf/applicationContext*.xml</param-value>  
</context-param>  

DispatcherServlet加載的context成功后，會將applicationContext存放在org.springframework.web.servlet.FrameworkServlet.CONTEXT. + (servletName)的attribute中。 

<servlet>
      <servlet-name>mvcServlet</servlet-name>
      <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
      <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:conf/spring-dispatcher-servlet.xml</param-value>
      </init-param>
      <load-on-startup>1</load-on-startup>
</servlet>

當然，如果沒有指定*servlet.xml 配置，則默認使用DispatcherServlet的默認配置DispatcherServlet.properties

# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver

org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\ org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\ org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\ org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\ org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\ org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator

org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver

org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

簡單的來說：spring bean的管理在applicationContext中，ContextLoaderListener的作用：

1. 將applicationContext的生命周期和servletContext的生命周期聯系到一起。

2. 自動管理applicationContext的創建，ContextLoaderListener 給我們提供了一個便利，不用顯式的去創建applicationContext。

DispatcherServlet 相關bean的管理在WebApplicationContext，ServletContextListener創建WebApplicationContext，WebApplicationContext可以訪問ServletContext/ServletContextAware這些bean，還可以訪問getServletContext方法。

正式的官方文檔：

A web application can define any number of DispatcherServlets. Each servlet will operate in its own namespace, loading its own application context with mappings, handlers, etc. Only the root application context as loaded by ContextLoaderListener, if any, will be shared. 

As of Spring 3.1, DispatcherServlet may now be injected with a web application context, rather than creating its own internally. This is useful in Servlet 3.0+ environments, which support programmatic registration of servlet instances. 

在一個web應用中使用多個DispatcherServlet，每個servlet通過自己的命名空間來獲取自己的webapplicationContext，然后加載此applicationContext里面的hangdlerMapping，hangdlerAdapter等等。ContextLoaderListener加載根application，所有子applicationContext共享根applicationContext。

從版本3.1 后，spring 支持將DispatcherServlet注入到根applicationContext，而不用創建自己的webapplicationContext，這主要為支持servlet 3.0 以上版本環境的要求，因為servlet 3.0 以上版本支持使用編程的方式來注冊servlet實例。

spring支持使用多層層次applicationContext，通常我們使用兩層結構就夠了。

 

接下來，通過深入源代碼層來探究WebApplicationContext是如何創建的？

1. DispatcherServlet的初始化過程使用到applicationContext。

    我們知道DispatcherServlet直接繼承自FrameworkServlet，而FrameworkServlet又繼承了HttpServletBean和 ApplicationContextAware。

2. FrameworkServlet實現了ApplicationContextAware接口的setApplicationContext()方法，可知DispatcherServlet的applicationContext來自FrameworkServlet。

    /**
     * Called by Spring via {@link ApplicationContextAware} to inject the current
     * application context. This method allows FrameworkServlets to be registered as
     * Spring beans inside an existing {@link WebApplicationContext} rather than
     * {@link #findWebApplicationContext() finding} a
     * {@link org.springframework.web.context.ContextLoaderListener bootstrapped} context.
     * <p>Primarily added to support use in embedded servlet containers.
     * @since 4.0
     */
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) {
        if (this.webApplicationContext == null && applicationContext instanceof WebApplicationContext) {
            this.webApplicationContext = (WebApplicationContext) applicationContext;
            this.webApplicationContextInjected = true;
        }
    }

3. FrameworkServlet的setApplicationContext()方法中WebApplicationContext是如何實例化的呢？

   FrameworkServlet繼承自HttpServletBean，HttpServletBean的初始化方法：

/**
     * Map config parameters onto bean properties of this servlet, and
     * invoke subclass initialization.
     * @throws ServletException if bean properties are invalid (or required
     * properties are missing), or if subclass initialization fails.
     */
    @Override
    public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
            logger.debug("Initializing servlet '" + getServletName() + "'");
        }

        // Set bean properties from init parameters.
        try {
            PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            initBeanWrapper(bw);
            bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
            throw ex;
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();

        if (logger.isDebugEnabled()) {
            logger.debug("Servlet '" + getServletName() + "' configured successfully");
        }
    }

FrameworkServlet 實現了initServletBean(); 

    /**
     * Overridden method of {@link HttpServletBean}, invoked after any bean properties
     * have been set. Creates this servlet's WebApplicationContext.
     */
    @Override
    protected final void initServletBean() throws ServletException {
        getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
        if (this.logger.isInfoEnabled()) {
            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
        }
        long startTime = System.currentTimeMillis();

        try {
            this.webApplicationContext = initWebApplicationContext();
            initFrameworkServlet();
        }
        catch (ServletException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }
        catch (RuntimeException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }

        if (this.logger.isInfoEnabled()) {
            long elapsedTime = System.currentTimeMillis() - startTime;
            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
                    elapsedTime + " ms");
        }
    }

最終追蹤到FrameworkServlet 的initWebApplicationContext()方法

/**
     * Initialize and publish the WebApplicationContext for this servlet.
     * <p>Delegates to {@link #createWebApplicationContext} for actual creation
     * of the context. Can be overridden in subclasses.
     * @return the WebApplicationContext instance
     * @see #FrameworkServlet(WebApplicationContext)
     * @see #setContextClass
     * @see #setContextConfigLocation
     */
    protected WebApplicationContext initWebApplicationContext() {
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        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);
                }
            }
        }
        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();
        }
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        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.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

        return wac;
    }

我們來分析整個流程吧：

1. 獲取根applicationContext。

        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());

    /**
     * Find the root {@link WebApplicationContext} for this web app, typically
     * loaded via {@link org.springframework.web.context.ContextLoaderListener}.
     * <p>Will rethrow an exception that happened on root context startup,
     * to differentiate between a failed context startup and no context at all.
     * @param sc ServletContext to find the web application context for
     * @return the root WebApplicationContext for this web app, or {@code null} if none
     * @see org.springframework.web.context.WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
     */
    public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
        return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
    }
    /**
     * Find a custom {@link WebApplicationContext} for this web app.
     * @param sc ServletContext to find the web application context for
     * @param attrName the name of the ServletContext attribute to look for
     * @return the desired WebApplicationContext for this web app, or {@code null} if none
     */
    public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
        Assert.notNull(sc, "ServletContext must not be null");
        Object attr = sc.getAttribute(attrName);
        if (attr == null) {
            return null;
        }
        if (attr instanceof RuntimeException) {
            throw (RuntimeException) attr;
        }
        if (attr instanceof Error) {
            throw (Error) attr;
        }
        if (attr instanceof Exception) {
            throw new IllegalStateException((Exception) attr);
        }
        if (!(attr instanceof WebApplicationContext)) {
            throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr);
        }
        return (WebApplicationContext) attr;
    }

2. 判斷webapplicationContext是否存在？存在的話就重利用該webapplicationContext

    protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
        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
            if (this.contextId != null) {
                wac.setId(this.contextId);
            }
            else {
                // Generate default id...
                wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                        ObjectUtils.getDisplayString(getServletContext().getContextPath()) + "/" + getServletName());
            }
        }

        wac.setServletContext(getServletContext());
        wac.setServletConfig(getServletConfig());
        wac.setNamespace(getNamespace());
        wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

        // 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(getServletContext(), getServletConfig());
        }

        postProcessWebApplicationContext(wac);
        applyInitializers(wac);
        wac.refresh();
    }

不存在的話，根據配置的屬性名去查詢：

    /**
     * Retrieve a {@code WebApplicationContext} from the {@code ServletContext}
     * attribute with the {@link #setContextAttribute configured name}. The
     * {@code WebApplicationContext} must have already been loaded and stored in the
     * {@code ServletContext} before this servlet gets initialized (or invoked).
     * <p>Subclasses may override this method to provide a different
     * {@code WebApplicationContext} retrieval strategy.
     * @return the WebApplicationContext for this servlet, or {@code null} if not found
     * @see #getContextAttribute()
     */
    protected WebApplicationContext findWebApplicationContext() {
        String attrName = getContextAttribute();
        if (attrName == null) {
            return null;
        }
        WebApplicationContext wac =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);
        if (wac == null) {
            throw new IllegalStateException("No WebApplicationContext found: initializer not registered?");
        }
        return wac;
    }

3. 如果還查詢不到WebapplicationContext，那么就創建一個新的WebapplicationContext，並綁定到root WebapplicationContext上：

/**
     * Instantiate the WebApplicationContext for this servlet, either a default
     * {@link org.springframework.web.context.support.XmlWebApplicationContext}
     * or a {@link #setContextClass custom context class}, if set.
     * <p>This implementation expects custom contexts to implement the
     * {@link org.springframework.web.context.ConfigurableWebApplicationContext}
     * interface. Can be overridden in subclasses.
     * <p>Do not forget to register this servlet instance as application listener on the
     * created context (for triggering its {@link #onRefresh callback}, and to call
     * {@link org.springframework.context.ConfigurableApplicationContext#refresh()}
     * before returning the context instance.
     * @param parent the parent ApplicationContext to use, or {@code null} if none
     * @return the WebApplicationContext for this servlet
     * @see org.springframework.web.context.support.XmlWebApplicationContext
     */
    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());
        wac.setParent(parent);
        wac.setConfigLocation(getContextConfigLocation());

        configureAndRefreshWebApplicationContext(wac);

        return wac;
    }

4. 將子applicationContext發布到servlet context上。

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

/**
* Return the ServletContext attribute name for this servlet's WebApplicationContext.
* <p>The default implementation returns
* {@code SERVLET_CONTEXT_PREFIX + servlet name}.
* @see #SERVLET_CONTEXT_PREFIX
* @see #getServletName
*/
public String getServletContextAttributeName() {
return SERVLET_CONTEXT_PREFIX + getServletName();
}

 

/**
* Prefix for the ServletContext attribute for the WebApplicationContext.
* The completion is the servlet name.
*/
public static final String SERVLET_CONTEXT_PREFIX = FrameworkServlet.class.getName() + ".CONTEXT.";

最后，ContextLoaderListener啟動時如何產生applicationContext呢？

見參考我的這篇文章：http://www.cnblogs.com/davidwang456/archive/2013/03/12/2956125.html

小結：

我們就以FrameworkServlet的官方說明來結束吧。沒有比它更合適的！希望你喜歡。

public abstract class FrameworkServlet
extends HttpServletBean
implements ApplicationContextAwareBase servlet for Spring's web framework. Provides integration with a Spring application context, in a JavaBean-based overall solution. 
This class offers the following functionality: 

Manages a WebApplicationContext instance per servlet. The servlet's configuration is determined by beans in the servlet's namespace. 
Publishes events on request processing, whether or not a request is successfully handled. 
Subclasses must implement doService(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse) to handle requests. Because this extends HttpServletBean rather than HttpServlet directly, bean properties are automatically mapped onto it. Subclasses can override initFrameworkServlet() for custom initialization. 

Detects a "contextClass" parameter at the servlet init-param level, falling back to the default context class, XmlWebApplicationContext, if not found. Note that, with the default FrameworkServlet, a custom context class needs to implement the ConfigurableWebApplicationContext SPI. 

Accepts an optional "contextInitializerClasses" servlet init-param that specifies one or more ApplicationContextInitializer classes. The managed web application context will be delegated to these initializers, allowing for additional programmatic configuration, e.g. adding property sources or activating profiles against the context's environment. See also ContextLoader which supports a "contextInitializerClasses" context-param with identical semantics for the "root" web application context. 

Passes a "contextConfigLocation" servlet init-param to the context instance, parsing it into potentially multiple file paths which can be separated by any number of commas and spaces, like "test-servlet.xml, myServlet.xml". If not explicitly specified, the context implementation is supposed to build a default location from the namespace of the servlet. 

Note: In case of multiple config locations, later bean definitions will override ones defined in earlier loaded files, at least when using Spring's default ApplicationContext implementation. This can be leveraged to deliberately override certain bean definitions via an extra XML file. 

The default namespace is "'servlet-name'-servlet", e.g. "test-servlet" for a servlet-name "test" (leading to a "/WEB-INF/test-servlet.xml" default location with XmlWebApplicationContext). The namespace can also be set explicitly via the "namespace" servlet init-param. 

As of Spring 3.1, FrameworkServlet may now be injected with a web application context, rather than creating its own internally. This is useful in Servlet 3.0+ environments, which support programmatic registration of servlet instances. See FrameworkServlet(WebApplicationContext) Javadoc for details.

 

參考資料：

http://starscream.iteye.com/blog/1107036

http://www.softwarevol.com/en/tutorial/Spring-ContextLoaderListener-And-DispatcherServlet-Concepts