Spring 源码分析（十）--SpringMVC（上篇）

/** 
 * Interface for receiving notification events about ServletContext
 * lifecycle changes.
 *
 * <p>In order to receive these notification events, the implementation
 * class must be either declared in the deployment descriptor of the web
 * application, annotated with {@link javax.servlet.annotation.WebListener},
 * or registered via one of the addListener methods defined on
 * {@link ServletContext}.
 *
 * <p>Implementations of this interface are invoked at their
 * {@link #contextInitialized} method in the order in which they have been
 * declared, and at their {@link #contextDestroyed} method in reverse
 * order.
 *
 * @see ServletContextEvent
 *
 * @since Servlet 2.3
 */
public interface ServletContextListener extends EventListener {

    /**
     * Receives notification that the web application initialization
     * process is starting.
     *
     * <p>All ServletContextListeners are notified of context
     * initialization before any filters or servlets in the web
     * application are initialized.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being initialized
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     * Receives notification that the ServletContext is about to be
     * shut down.
     *
     * <p>All servlets and filters will have been destroyed before any
     * ServletContextListeners are notified of context
     * destruction.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being destroyed
     */
    public void contextDestroyed(ServletContextEvent sce);
}

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());
    }

}

public class ContextLoader {

    /**
     * Config param for the root WebApplicationContext id,
     * to be used as serialization id for the underlying BeanFactory: {@value}
     */
    public static final String CONTEXT_ID_PARAM = "contextId";

    /**
     * Name of servlet context parameter (i.e., {@value}) that can specify the
     * config location for the root context, falling back to the implementation‘s
     * default otherwise.
     * @see org.springframework.web.context.support.XmlWebApplicationContext#DEFAULT_CONFIG_LOCATION
     */
    public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";

    /**
     * Config param for the root WebApplicationContext implementation class to use: {@value}
     * @see #determineContextClass(ServletContext)
     */
    public static final String CONTEXT_CLASS_PARAM = "contextClass";

    /**
     * Config param for {@link ApplicationContextInitializer} classes to use
     * for initializing the root web application context: {@value}
     * @see #customizeContext(ServletContext, ConfigurableWebApplicationContext)
     */
    public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";

    /**
     * Config param for global {@link ApplicationContextInitializer} classes to use
     * for initializing all web application contexts in the current application: {@value}
     * @see #customizeContext(ServletContext, ConfigurableWebApplicationContext)
     */
    public static final String GLOBAL_INITIALIZER_CLASSES_PARAM = "globalInitializerClasses";

    /**
     * Optional servlet context parameter (i.e., "{@code locatorFactorySelector}")
     * used only when obtaining a parent context using the default implementation
     * of {@link #loadParentContext(ServletContext servletContext)}.
     * Specifies the ‘selector‘ used in the
     * {@link ContextSingletonBeanFactoryLocator#getInstance(String selector)}
     * method call, which is used to obtain the BeanFactoryLocator instance from
     * which the parent context is obtained.
     * <p>The default is {@code classpath*:beanRefContext.xml},
     * matching the default applied for the
     * {@link ContextSingletonBeanFactoryLocator#getInstance()} method.
     * Supplying the "parentContextKey" parameter is sufficient in this case.
     */
    public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector";

    /**
     * Optional servlet context parameter (i.e., "{@code parentContextKey}")
     * used only when obtaining a parent context using the default implementation
     * of {@link #loadParentContext(ServletContext servletContext)}.
     * Specifies the ‘factoryKey‘ used in the
     * {@link BeanFactoryLocator#useBeanFactory(String factoryKey)} method call,
     * obtaining the parent application context from the BeanFactoryLocator instance.
     * <p>Supplying this "parentContextKey" parameter is sufficient when relying
     * on the default {@code classpath*:beanRefContext.xml} selector for
     * candidate factory references.
     */
    public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey";

    /**
     * Any number of these characters are considered delimiters between
     * multiple values in a single init-param String value.
     */
    private static final String INIT_PARAM_DELIMITERS = ",; \t\n";

    /**
     * Name of the class path resource (relative to the ContextLoader class)
     * that defines ContextLoader‘s default strategy names.
     */
    private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";


    private static final Properties defaultStrategies;

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


    /**
     * Map from (thread context) ClassLoader to corresponding ‘current‘ WebApplicationContext.
     */
    private static final Map<ClassLoader, WebApplicationContext> currentContextPerThread =
            new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1);

    /**
     * The ‘current‘ WebApplicationContext, if the ContextLoader class is
     * deployed in the web app ClassLoader itself.
     */
    private static volatile WebApplicationContext currentContext;


    /**
     * The root WebApplicationContext instance that this loader manages.
     */
    private WebApplicationContext context;

    /**
     * Holds BeanFactoryReference when loading parent factory via
     * ContextSingletonBeanFactoryLocator.
     */
    private BeanFactoryReference parentContextRef;

    /** Actual ApplicationContextInitializer instances to apply to the context */
    private final List<ApplicationContextInitializer<ConfigurableApplicationContext>> contextInitializers =
            new ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>>();


    /**
     * Create a new {@code ContextLoader} that will create a web application context
     * based on the "contextClass" and "contextConfigLocation" servlet context-params.
     * See class-level documentation for details on default values for each.
     * <p>This constructor is typically used when declaring the {@code
     * ContextLoaderListener} subclass as a {@code <listener>} within {@code web.xml}, as
     * 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 subclasses are free to call the {@link #closeWebApplicationContext} method on
     * container shutdown to close the application context.
     * @see #ContextLoader(WebApplicationContext)
     * @see #initWebApplicationContext(ServletContext)
     * @see #closeWebApplicationContext(ServletContext)
     */
    public ContextLoader() {
    }

    /**
     * Create a new {@code ContextLoader} 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 ServletContext#addListener}
     * API.
     * <p>The context may or may not yet be {@linkplain
     * 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
     * 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 ApplicationContextInitializer}s specified through the
     * "contextInitializerClasses" init-param will be applied.</li>
     * <li>{@link 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 subclasses are
     * free to call the {@link #closeWebApplicationContext} method on container shutdown
     * to close the application context.
     * @param context the application context to manage
     * @see #initWebApplicationContext(ServletContext)
     * @see #closeWebApplicationContext(ServletContext)
     */
    public ContextLoader(WebApplicationContext context) {
        this.context = context;
    }


    /**
     * Specify which {@link ApplicationContextInitializer} instances should be used
     * to initialize the application context used by this {@code ContextLoader}.
     * @since 4.2
     * @see #configureAndRefreshWebApplicationContext
     * @see #customizeContext
     */
    @SuppressWarnings("unchecked")
    public void setContextInitializers(ApplicationContextInitializer<?>... initializers) {
        if (initializers != null) {
            for (ApplicationContextInitializer<?> initializer : initializers) {
                this.contextInitializers.add((ApplicationContextInitializer<ConfigurableApplicationContext>) initializer);
            }
        }
    }


    /**
     * 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) {
        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) {
                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.
                        ApplicationContext parent = loadParentContext(servletContext);
                        cwac.setParent(parent);
                    }
                    configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }
            //ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
            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;
        }
    }

    /**
     * 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) {
        Class<?> contextClass = determineContextClass(sc);
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

    /**
     * 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) {
        //CONTEXT_CLASS_PARAM = "contextClass"
        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);
            }
        }
    }

    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();
    }

    /**
     * Customize the {@link ConfigurableWebApplicationContext} created by this
     * ContextLoader after config locations have been supplied to the context
     * but before the context is <em>refreshed</em>.
     * <p>The default implementation {@linkplain #determineContextInitializerClasses(ServletContext)
     * determines} what (if any) context initializer classes have been specified through
     * {@linkplain #CONTEXT_INITIALIZER_CLASSES_PARAM context init parameters} and
     * {@linkplain ApplicationContextInitializer#initialize invokes each} with the
     * given web application context.
     * <p>Any {@code ApplicationContextInitializers} implementing
     * {@link org.springframework.core.Ordered Ordered} or marked with @{@link
     * org.springframework.core.annotation.Order Order} will be sorted appropriately.
     * @param sc the current servlet context
     * @param wac the newly created application context
     * @see #CONTEXT_INITIALIZER_CLASSES_PARAM
     * @see ApplicationContextInitializer#initialize(ConfigurableApplicationContext)
     */
    protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
        List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
                determineContextInitializerClasses(sc);

        for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
            Class<?> initializerContextClass =
                    GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
            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()));
            }
            this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
        }

        AnnotationAwareOrderComparator.sort(this.contextInitializers);
        for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
            initializer.initialize(wac);
        }
    }

    /**
     * Return the {@link ApplicationContextInitializer} implementation classes to use
     * if any have been specified by {@link #CONTEXT_INITIALIZER_CLASSES_PARAM}.
     * @param servletContext current servlet context
     * @see #CONTEXT_INITIALIZER_CLASSES_PARAM
     */
    protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>
            determineContextInitializerClasses(ServletContext servletContext) {

        List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes =
                new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>();

        String globalClassNames = servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM);
        if (globalClassNames != null) {
            for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
                classes.add(loadInitializerClass(className));
            }
        }

        String localClassNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM);
        if (localClassNames != null) {
            for (String className : StringUtils.tokenizeToStringArray(localClassNames, INIT_PARAM_DELIMITERS)) {
                classes.add(loadInitializerClass(className));
            }
        }

        return classes;
    }

    @SuppressWarnings("unchecked")
    private Class<ApplicationContextInitializer<ConfigurableApplicationContext>> loadInitializerClass(String className) {
        try {
            Class<?> clazz = ClassUtils.forName(className, ClassUtils.getDefaultClassLoader());
            if (!ApplicationContextInitializer.class.isAssignableFrom(clazz)) {
                throw new ApplicationContextException(
                        "Initializer class does not implement ApplicationContextInitializer interface: " + clazz);
            }
            return (Class<ApplicationContextInitializer<ConfigurableApplicationContext>>) clazz;
        }
        catch (ClassNotFoundException ex) {
            throw new ApplicationContextException("Failed to load context initializer class [" + className + "]", ex);
        }
    }

    /**
     * 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;
    }

    /**
     * Close Spring‘s web application context for the given servlet context. If
     * the default {@link #loadParentContext(ServletContext)} implementation,
     * which uses ContextSingletonBeanFactoryLocator, has loaded any shared
     * parent context, release one reference to that shared parent context.
     * <p>If overriding {@link #loadParentContext(ServletContext)}, you may have
     * to override this method as well.
     * @param servletContext the ServletContext that the WebApplicationContext runs in
     */
    public void closeWebApplicationContext(ServletContext servletContext) {
        servletContext.log("Closing Spring root WebApplicationContext");
        try {
            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);
            }
            servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
            if (this.parentContextRef != null) {
                this.parentContextRef.release();
            }
        }
    }


    /**
     * Obtain the Spring root web application context for the current thread
     * (i.e. for the current thread‘s context ClassLoader, which needs to be
     * the web application‘s ClassLoader).
     * @return the current root web application context, or {@code null}
     * if none found
     * @see org.springframework.web.context.support.SpringBeanAutowiringSupport
     */
    public static WebApplicationContext getCurrentWebApplicationContext() {
        ClassLoader ccl = Thread.currentThread().getContextClassLoader();
        if (ccl != null) {
            WebApplicationContext ccpt = currentContextPerThread.get(ccl);
            if (ccpt != null) {
                return ccpt;
            }
        }
        return currentContext;
    }

}

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

# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.

org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

/**
     * 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 {
            //解析init-param并封装至pvs中
            PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
            //将当前的Servlet类转化为一个BeanWrapper，从而能够以Spring的方式来对init-param的值进行注入
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            //空实现，留给子类覆盖
            initBeanWrapper(bw);
            bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
            if (logger.isErrorEnabled()) {
                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");
        }
    }

/**
     * PropertyValues implementation created from ServletConfig init parameters.
     */
    private static class ServletConfigPropertyValues extends MutablePropertyValues {

        /**
         * Create new ServletConfigPropertyValues.
         * @param config ServletConfig we‘ll use to take PropertyValues from
         * @param requiredProperties set of property names we need, where
         * we can‘t accept default values
         * @throws ServletException if any required properties are missing
         */
        public ServletConfigPropertyValues(ServletConfig config, Set<String> requiredProperties)
            throws ServletException {

            Set<String> missingProps = (requiredProperties != null && !requiredProperties.isEmpty() ?
                    new HashSet<String>(requiredProperties) : null);

            Enumeration<String> paramNames = config.getInitParameterNames();
            while (paramNames.hasMoreElements()) {
                String property = paramNames.nextElement();
                Object value = config.getInitParameter(property);
                addPropertyValue(new PropertyValue(property, value));
                if (missingProps != null) {
                    missingProps.remove(property);
                }
            }

            // Fail if we are still missing properties.
            if (!CollectionUtils.isEmpty(missingProps)) {
                throw new ServletException(
                    "Initialization from ServletConfig for servlet ‘" + config.getServletName() +
                    "‘ failed; the following required properties were missing: " +
                    StringUtils.collectionToDelimitedString(missingProps, ", "));
            }
        }
    }

/**
     * 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();
            //设计为子类覆盖
            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");
        }
    }

/**
     * 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() {
        //通过属性WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE属性在servletContext中获取
        //一般是ContextLoaderListener的初始化函数initWebApplicationContext中设置的
        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;
    }

/**
     * 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.
     * Delegates to #createWebApplicationContext(ApplicationContext).
     * @param parent the parent WebApplicationContext to use, or {@code null} if none
     * @return the WebApplicationContext for this servlet
     * @see org.springframework.web.context.support.XmlWebApplicationContext
     * @see #createWebApplicationContext(ApplicationContext)
     */
    protected WebApplicationContext createWebApplicationContext(WebApplicationContext parent) {
        return createWebApplicationContext((ApplicationContext) parent);
    }

/**
     * 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) {
        //获取servlet的初始化参数contextClass，如果没有配置默认为XmlWebApplicationContext.class
        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");
        }
        //通过反射方式实例化contextClass
        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

        wac.setEnvironment(getEnvironment());
        //parent为在ContextLoaderListener中创建的实例
        //在ContextLoaderListener加载的时候初始化的WebApplicationContext类型实例
        wac.setParent(parent);
        wac.setConfigLocation(getContextConfigLocation());

        //初始化Spring环境包括加载配置文件等
        configureAndRefreshWebApplicationContext(wac);

        return wac;
    }

    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);
        //加载配置文件及整合parent到wac
        wac.refresh();
    }

/**
     * ApplicationListener endpoint that receives events from this servlet‘s WebApplicationContext
     * only, delegating to {@code onApplicationEvent} on the FrameworkServlet instance.
     */
    private class ContextRefreshListener implements ApplicationListener<ContextRefreshedEvent> {

        @Override
        public void onApplicationEvent(ContextRefreshedEvent event) {
            FrameworkServlet.this.onApplicationEvent(event);
        }
    }

/**
     * Callback that receives refresh events from this servlet‘s WebApplicationContext.
     * <p>The default implementation calls {@link #onRefresh},
     * triggering a refresh of this servlet‘s context-dependent state.
     * @param event the incoming ApplicationContext event
     */
    public void onApplicationEvent(ContextRefreshedEvent event) {
        this.refreshEventReceived = true;
        onRefresh(event.getApplicationContext());
    }

/**
     * This implementation calls {@link #initStrategies}.
     */
    @Override
    protected void onRefresh(ApplicationContext context) {
        initStrategies(context);
    }

    /**
     * Initialize the strategy objects that this servlet uses.
     * <p>May be overridden in subclasses in order to initialize further strategy objects.
     */
    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }

/**
     * Initialize the MultipartResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * no multipart handling is provided.
     */
    private void initMultipartResolver(ApplicationContext context) {
        try {
            //MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver"
            this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using MultipartResolver [" + this.multipartResolver + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // Default is no multipart resolver.
            this.multipartResolver = null;
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate MultipartResolver with name ‘" + MULTIPART_RESOLVER_BEAN_NAME +
                        "‘: no multipart request handling provided");
            }
        }
    }

/**
     * Initialize the HandlerMappings used by this class.
     * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
     * we default to BeanNameUrlHandlerMapping.
     */
    private void initHandlerMappings(ApplicationContext context) {
        this.handlerMappings = null;

        if (this.detectAllHandlerMappings) { //默认为true
            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerMapping> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerMappings);
            }
        }
        else {
            try {
                //HANDLER_MAPPING_BEAN_NAME = "handlerMapping"
                HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
                this.handlerMappings = Collections.singletonList(hm);
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we‘ll add a default HandlerMapping later.
            }
        }

        // Ensure we have at least one HandlerMapping, by registering
        // a default HandlerMapping if no other mappings are found.
        if (this.handlerMappings == null) {
            this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
            if (logger.isDebugEnabled()) {
                logger.debug("No HandlerMappings found in servlet ‘" + getServletName() + "‘: using default");
            }
        }
    }

/**
     * Initialize the HandlerAdapters used by this class.
     * <p>If no HandlerAdapter beans are defined in the BeanFactory for this namespace,
     * we default to SimpleControllerHandlerAdapter.
     */
    private void initHandlerAdapters(ApplicationContext context) {
        this.handlerAdapters = null;

        if (this.detectAllHandlerAdapters) {
            // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerAdapter> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerAdapters = new ArrayList<HandlerAdapter>(matchingBeans.values());
                // We keep HandlerAdapters in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerAdapters);
            }
        }
        else {
            try {
                HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
                this.handlerAdapters = Collections.singletonList(ha);
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we‘ll add a default HandlerAdapter later.
            }
        }

        // Ensure we have at least some HandlerAdapters, by registering
        // default HandlerAdapters if no other adapters are found.
        if (this.handlerAdapters == null) {
            this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
            if (logger.isDebugEnabled()) {
                logger.debug("No HandlerAdapters found in servlet ‘" + getServletName() + "‘: using default");
            }
        }
    }

/**
     * Create a List of default strategy objects for the given strategy interface.
     * <p>The default implementation uses the "DispatcherServlet.properties" file (in the same
     * package as the DispatcherServlet class) to determine the class names. It instantiates
     * the strategy objects through the context‘s BeanFactory.
     * @param context the current WebApplicationContext
     * @param strategyInterface the strategy interface
     * @return the List of corresponding strategy objects
     */
    @SuppressWarnings("unchecked")
    protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
        String key = strategyInterface.getName();
        String value = defaultStrategies.getProperty(key);
        if (value != null) {
            String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
            List<T> strategies = new ArrayList<T>(classNames.length);
            for (String className : classNames) {
                try {
                    Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
                    Object strategy = createDefaultStrategy(context, clazz);
                    strategies.add((T) strategy);
                }
                catch (ClassNotFoundException ex) {
                    throw new BeanInitializationException(
                            "Could not find DispatcherServlet‘s default strategy class [" + className +
                                    "] for interface [" + key + "]", ex);
                }
                catch (LinkageError err) {
                    throw new BeanInitializationException(
                            "Error loading DispatcherServlet‘s default strategy class [" + className +
                                    "] for interface [" + key + "]: problem with class file or dependent class", err);
                }
            }
            return strategies;
        }
        else {
            return new LinkedList<T>();
        }
    }

static {
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
            //DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties"
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load ‘DispatcherServlet.properties‘: " + ex.getMessage());
        }
    }

# 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

/**
     * Initialize the HandlerExceptionResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to no exception resolver.
     */
    private void initHandlerExceptionResolvers(ApplicationContext context) {
        this.handlerExceptionResolvers = null;

        if (this.detectAllHandlerExceptionResolvers) {
            // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
                    .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerExceptionResolvers = new ArrayList<HandlerExceptionResolver>(matchingBeans.values());
                // We keep HandlerExceptionResolvers in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
            }
        }
        else {
            try {
                HandlerExceptionResolver her =
                        context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
                this.handlerExceptionResolvers = Collections.singletonList(her);
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Ignore, no HandlerExceptionResolver is fine too.
            }
        }

        // Ensure we have at least some HandlerExceptionResolvers, by registering
        // default HandlerExceptionResolvers if no other resolvers are found.
        if (this.handlerExceptionResolvers == null) {
            this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("No HandlerExceptionResolvers found in servlet ‘" + getServletName() + "‘: using default");
            }
        }
    }

/**
     * Initialize the ViewResolvers used by this class.
     * <p>If no ViewResolver beans are defined in the BeanFactory for this
     * namespace, we default to InternalResourceViewResolver.
     */
    private void initViewResolvers(ApplicationContext context) {
        this.viewResolvers = null;

        if (this.detectAllViewResolvers) {
            // Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
            Map<String, ViewResolver> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.viewResolvers = new ArrayList<ViewResolver>(matchingBeans.values());
                // We keep ViewResolvers in sorted order.
                AnnotationAwareOrderComparator.sort(this.viewResolvers);
            }
        }
        else {
            try {
                //VIEW_RESOLVER_BEAN_NAME = "viewResolver"
                ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class);
                this.viewResolvers = Collections.singletonList(vr);
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we‘ll add a default ViewResolver later.
            }
        }

        // Ensure we have at least one ViewResolver, by registering
        // a default ViewResolver if no other resolvers are found.
        if (this.viewResolvers == null) {
            this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("No ViewResolvers found in servlet ‘" + getServletName() + "‘: using default");
            }
        }
    }

public interface ViewResolver {

    /**
     * Resolve the given view by name.
     * <p>Note: To allow for ViewResolver chaining, a ViewResolver should
     * return {@code null} if a view with the given name is not defined in it.
     * However, this is not required: Some ViewResolvers will always attempt
     * to build View objects with the given name, unable to return {@code null}
     * (rather throwing an exception when View creation failed).
     * @param viewName name of the view to resolve
     * @param locale Locale in which to resolve the view.
     * ViewResolvers that support internationalization should respect this.
     * @return the View object, or {@code null} if not found
     * (optional, to allow for ViewResolver chaining)
     * @throws Exception if the view cannot be resolved
     * (typically in case of problems creating an actual View object)
     */
    View resolveViewName(String viewName, Locale locale) throws Exception;

}

public abstract class AbstractCachingViewResolver extends WebApplicationObjectSupport implements ViewResolver {

    /** Default maximum number of entries for the view cache: 1024 */
    public static final int DEFAULT_CACHE_LIMIT = 1024;

    /** Dummy marker object for unresolved views in the cache Maps */
    private static final View UNRESOLVED_VIEW = new View() {
        @Override
        public String getContentType() {
            return null;
        }
        @Override
        public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) {
        }
    };


    /** The maximum number of entries in the cache */
    private volatile int cacheLimit = DEFAULT_CACHE_LIMIT;

    /** Whether we should refrain from resolving views again if unresolved once */
    private boolean cacheUnresolved = true;

    /** Fast access cache for Views, returning already cached instances without a global lock */
    private final Map<Object, View> viewAccessCache = new ConcurrentHashMap<Object, View>(DEFAULT_CACHE_LIMIT);

    /** Map from view key to View instance, synchronized for View creation */
    @SuppressWarnings("serial")
    private final Map<Object, View> viewCreationCache =
            new LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT, 0.75f, true) {
                @Override
                protected boolean removeEldestEntry(Map.Entry<Object, View> eldest) {
                    if (size() > getCacheLimit()) {
                        viewAccessCache.remove(eldest.getKey());
                        return true;
                    }
                    else {
                        return false;
                    }
                }
            };


    /**
     * Specify the maximum number of entries for the view cache.
     * Default is 1024.
     */
    public void setCacheLimit(int cacheLimit) {
        this.cacheLimit = cacheLimit;
    }

    /**
     * Return the maximum number of entries for the view cache.
     */
    public int getCacheLimit() {
        return this.cacheLimit;
    }

    /**
     * Enable or disable caching.
     * <p>This is equivalent to setting the {@link #setCacheLimit "cacheLimit"}
     * property to the default limit (1024) or to 0, respectively.
     * <p>Default is "true": caching is enabled.
     * Disable this only for debugging and development.
     */
    public void setCache(boolean cache) {
        this.cacheLimit = (cache ? DEFAULT_CACHE_LIMIT : 0);
    }

    /**
     * Return if caching is enabled.
     */
    public boolean isCache() {
        return (this.cacheLimit > 0);
    }

    /**
     * Whether a view name once resolved to {@code null} should be cached and
     * automatically resolved to {@code null} subsequently.
     * <p>Default is "true": unresolved view names are being cached, as of Spring 3.1.
     * Note that this flag only applies if the general {@link #setCache "cache"}
     * flag is kept at its default of "true" as well.
     * <p>Of specific interest is the ability for some AbstractUrlBasedView
     * implementations (FreeMarker, Velocity, Tiles) to check if an underlying
     * resource exists via {@link AbstractUrlBasedView#checkResource(Locale)}.
     * With this flag set to "false", an underlying resource that re-appears
     * is noticed and used. With the flag set to "true", one check is made only.
     */
    public void setCacheUnresolved(boolean cacheUnresolved) {
        this.cacheUnresolved = cacheUnresolved;
    }

    /**
     * Return if caching of unresolved views is enabled.
     */
    public boolean isCacheUnresolved() {
        return this.cacheUnresolved;
    }


    @Override
    public View resolveViewName(String viewName, Locale locale) throws Exception {
        if (!isCache()) {
            return createView(viewName, locale);
        }
        else {
            Object cacheKey = getCacheKey(viewName, locale);
            View view = this.viewAccessCache.get(cacheKey);
            if (view == null) {
                synchronized (this.viewCreationCache) {
                    view = this.viewCreationCache.get(cacheKey);
                    if (view == null) {
                        // Ask the subclass to create the View object.
                        view = createView(viewName, locale);
                        if (view == null && this.cacheUnresolved) {
                            view = UNRESOLVED_VIEW;
                        }
                        if (view != null) {
                            this.viewAccessCache.put(cacheKey, view);
                            this.viewCreationCache.put(cacheKey, view);
                            if (logger.isTraceEnabled()) {
                                logger.trace("Cached view [" + cacheKey + "]");
                            }
                        }
                    }
                }
            }
            return (view != UNRESOLVED_VIEW ? view : null);
        }
    }

    /**
     * Return the cache key for the given view name and the given locale.
     * <p>Default is a String consisting of view name and locale suffix.
     * Can be overridden in subclasses.
     * <p>Needs to respect the locale in general, as a different locale can
     * lead to a different view resource.
     */
    protected Object getCacheKey(String viewName, Locale locale) {
        return viewName + ‘_‘ + locale;
    }

    /**
     * Provides functionality to clear the cache for a certain view.
     * <p>This can be handy in case developer are able to modify views
     * (e.g. Velocity templates) at runtime after which you‘d need to
     * clear the cache for the specified view.
     * @param viewName the view name for which the cached view object
     * (if any) needs to be removed
     * @param locale the locale for which the view object should be removed
     */
    public void removeFromCache(String viewName, Locale locale) {
        if (!isCache()) {
            logger.warn("View caching is SWITCHED OFF -- removal not necessary");
        }
        else {
            Object cacheKey = getCacheKey(viewName, locale);
            Object cachedView;
            synchronized (this.viewCreationCache) {
                this.viewAccessCache.remove(cacheKey);
                cachedView = this.viewCreationCache.remove(cacheKey);
            }
            if (logger.isDebugEnabled()) {
                // Some debug output might be useful...
                if (cachedView == null) {
                    logger.debug("No cached instance for view ‘" + cacheKey + "‘ was found");
                }
                else {
                    logger.debug("Cache for view " + cacheKey + " has been cleared");
                }
            }
        }
    }

    /**
     * Clear the entire view cache, removing all cached view objects.
     * Subsequent resolve calls will lead to recreation of demanded view objects.
     */
    public void clearCache() {
        logger.debug("Clearing entire view cache");
        synchronized (this.viewCreationCache) {
            this.viewAccessCache.clear();
            this.viewCreationCache.clear();
        }
    }


    /**
     * Create the actual View object.
     * <p>The default implementation delegates to {@link #loadView}.
     * This can be overridden to resolve certain view names in a special fashion,
     * before delegating to the actual {@code loadView} implementation
     * provided by the subclass.
     * @param viewName the name of the view to retrieve
     * @param locale the Locale to retrieve the view for
     * @return the View instance, or {@code null} if not found
     * (optional, to allow for ViewResolver chaining)
     * @throws Exception if the view couldn‘t be resolved
     * @see #loadView
     */
    protected View createView(String viewName, Locale locale) throws Exception {
        return loadView(viewName, locale);
    }

    /**
     * Subclasses must implement this method, building a View object
     * for the specified view. The returned View objects will be
     * cached by this ViewResolver base class.
     * <p>Subclasses are not forced to support internationalization:
     * A subclass that does not may simply ignore the locale parameter.
     * @param viewName the name of the view to retrieve
     * @param locale the Locale to retrieve the view for
     * @return the View instance, or {@code null} if not found
     * (optional, to allow for ViewResolver chaining)
     * @throws Exception if the view couldn‘t be resolved
     * @see #resolveViewName
     */
    protected abstract View loadView(String viewName, Locale locale) throws Exception;

}