註冊controller的url有多種方式:

　　1. 你可以啥都不都幹，直接使用 @RequestMapping 註解上體路徑，然後訪問的時候就根據這個路徑來了；
　　2. 如果你想自定義一些路徑的訪問方式，那麼你可以自定義 RequestMappingHandlerMapping, 然後使用一個 bean 去掃描即可；

那麼，url 具體是如何映射出來的呢？

　　載入前面的細節就不多說了，總之一句話，tomcat轉到spring後，一切就開始了。

關於載入前情，有興趣可以展開閱讀下：

// org.apache.catalina.core.StandardContext.startInternal(), 接入 應用 

    
 
// 啟動
    /**
     * Start this component and implement the requirements
     * of {@link org.apache.catalina.util.LifecycleBase#startInternal()}.
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    @Override
    
 
protected synchronized void startInternal() throws LifecycleException {

        if(log.isDebugEnabled())
            log.debug("Starting " + getBaseName());

        // Send j2ee.state.starting notification
        if (this.getObjectName() != null) {
            Notification notification = new Notification("j2ee.state.starting",
                    
 
this.getObjectName(), sequenceNumber.getAndIncrement());
            broadcaster.sendNotification(notification);
        }

        setConfigured(false);
        boolean ok = true;

        // Currently this is effectively a NO-OP but needs to be called to
        // ensure the NamingResources follows the correct lifecycle
        if (namingResources != null) {
            namingResources.start();
        }

        // Post work directory
        postWorkDirectory();

        // Add missing components as necessary
        if (getResources() == null) {   // (1) Required by Loader
            if (log.isDebugEnabled())
                log.debug("Configuring default Resources");

            try {
                setResources(new StandardRoot(this));
            } catch (IllegalArgumentException e) {
                log.error(sm.getString("standardContext.resourcesInit"), e);
                ok = false;
            }
        }
        if (ok) {
            resourcesStart();
        }

        if (getLoader() == null) {
            WebappLoader webappLoader = new WebappLoader(getParentClassLoader());
            webappLoader.setDelegate(getDelegate());
            setLoader(webappLoader);
        }

        // An explicit cookie processor hasn't been specified; use the default
        if (cookieProcessor == null) {
            cookieProcessor = new Rfc6265CookieProcessor();
        }

        // Initialize character set mapper
        getCharsetMapper();

        // Validate required extensions
        boolean dependencyCheck = true;
        try {
            dependencyCheck = ExtensionValidator.validateApplication
                (getResources(), this);
        } catch (IOException ioe) {
            log.error(sm.getString("standardContext.extensionValidationError"), ioe);
            dependencyCheck = false;
        }

        if (!dependencyCheck) {
            // do not make application available if dependency check fails
            ok = false;
        }

        // Reading the "catalina.useNaming" environment variable
        String useNamingProperty = System.getProperty("catalina.useNaming");
        if ((useNamingProperty != null)
            && (useNamingProperty.equals("false"))) {
            useNaming = false;
        }

        if (ok && isUseNaming()) {
            if (getNamingContextListener() == null) {
                NamingContextListener ncl = new NamingContextListener();
                ncl.setName(getNamingContextName());
                ncl.setExceptionOnFailedWrite(getJndiExceptionOnFailedWrite());
                addLifecycleListener(ncl);
                setNamingContextListener(ncl);
            }
        }

        // Standard container startup
        if (log.isDebugEnabled())
            log.debug("Processing standard container startup");


        // Binding thread
        ClassLoader oldCCL = bindThread();

        try {
            if (ok) {
                // Start our subordinate components, if any
                Loader loader = getLoader();
                if (loader instanceof Lifecycle) {
                    ((Lifecycle) loader).start();
                }

                // since the loader just started, the webapp classloader is now
                // created.
                setClassLoaderProperty("clearReferencesRmiTargets",
                        getClearReferencesRmiTargets());
                setClassLoaderProperty("clearReferencesStopThreads",
                        getClearReferencesStopThreads());
                setClassLoaderProperty("clearReferencesStopTimerThreads",
                        getClearReferencesStopTimerThreads());
                setClassLoaderProperty("clearReferencesHttpClientKeepAliveThread",
                        getClearReferencesHttpClientKeepAliveThread());
                setClassLoaderProperty("clearReferencesObjectStreamClassCaches",
                        getClearReferencesObjectStreamClassCaches());

                // By calling unbindThread and bindThread in a row, we setup the
                // current Thread CCL to be the webapp classloader
                unbindThread(oldCCL);
                oldCCL = bindThread();

                // Initialize logger again. Other components might have used it
                // too early, so it should be reset.
                logger = null;
                getLogger();

                Realm realm = getRealmInternal();
                if(null != realm) {
                    if (realm instanceof Lifecycle) {
                        ((Lifecycle) realm).start();
                    }

                    // Place the CredentialHandler into the ServletContext so
                    // applications can have access to it. Wrap it in a "safe"
                    // handler so application's can't modify it.
                    CredentialHandler safeHandler = new CredentialHandler() {
                        @Override
                        public boolean matches(String inputCredentials, String storedCredentials) {
                            return getRealmInternal().getCredentialHandler().matches(inputCredentials, storedCredentials);
                        }

                        @Override
                        public String mutate(String inputCredentials) {
                            return getRealmInternal().getCredentialHandler().mutate(inputCredentials);
                        }
                    };
                    context.setAttribute(Globals.CREDENTIAL_HANDLER, safeHandler);
                }

                // Notify our interested LifecycleListeners
                fireLifecycleEvent(Lifecycle.CONFIGURE_START_EVENT, null);

                // Start our child containers, if not already started
                for (Container child : findChildren()) {
                    if (!child.getState().isAvailable()) {
                        child.start();
                    }
                }

                // Start the Valves in our pipeline (including the basic),
                // if any
                if (pipeline instanceof Lifecycle) {
                    ((Lifecycle) pipeline).start();
                }

                // Acquire clustered manager
                Manager contextManager = null;
                Manager manager = getManager();
                if (manager == null) {
                    if (log.isDebugEnabled()) {
                        log.debug(sm.getString("standardContext.cluster.noManager",
                                Boolean.valueOf((getCluster() != null)),
                                Boolean.valueOf(distributable)));
                    }
                    if ( (getCluster() != null) && distributable) {
                        try {
                            contextManager = getCluster().createManager(getName());
                        } catch (Exception ex) {
                            log.error("standardContext.clusterFail", ex);
                            ok = false;
                        }
                    } else {
                        contextManager = new StandardManager();
                    }
                }

                // Configure default manager if none was specified
                if (contextManager != null) {
                    if (log.isDebugEnabled()) {
                        log.debug(sm.getString("standardContext.manager",
                                contextManager.getClass().getName()));
                    }
                    setManager(contextManager);
                }

                if (manager!=null && (getCluster() != null) && distributable) {
                    //let the cluster know that there is a context that is distributable
                    //and that it has its own manager
                    getCluster().registerManager(manager);
                }
            }

            if (!getConfigured()) {
                log.error(sm.getString("standardContext.configurationFail"));
                ok = false;
            }

            // We put the resources into the servlet context
            if (ok)
                getServletContext().setAttribute
                    (Globals.RESOURCES_ATTR, getResources());

            if (ok ) {
                if (getInstanceManager() == null) {
                    javax.naming.Context context = null;
                    if (isUseNaming() && getNamingContextListener() != null) {
                        context = getNamingContextListener().getEnvContext();
                    }
                    Map<String, Map<String, String>> injectionMap = buildInjectionMap(
                            getIgnoreAnnotations() ? new NamingResourcesImpl(): getNamingResources());
                    setInstanceManager(new DefaultInstanceManager(context,
                            injectionMap, this, this.getClass().getClassLoader()));
                }
                getServletContext().setAttribute(
                        InstanceManager.class.getName(), getInstanceManager());
                InstanceManagerBindings.bind(getLoader().getClassLoader(), getInstanceManager());
            }

            // Create context attributes that will be required
            if (ok) {
                getServletContext().setAttribute(
                        JarScanner.class.getName(), getJarScanner());
            }

            // Set up the context init params
            mergeParameters();

            // Call ServletContainerInitializers
            for (Map.Entry<ServletContainerInitializer, Set<Class<?>>> entry :
                initializers.entrySet()) {
                try {
                    entry.getKey().onStartup(entry.getValue(),
                            getServletContext());
                } catch (ServletException e) {
                    log.error(sm.getString("standardContext.sciFail"), e);
                    ok = false;
                    break;
                }
            }

            // Configure and call application event listeners
            if (ok) {
                if (!listenerStart()) {
                    log.error(sm.getString("standardContext.listenerFail"));
                    ok = false;
                }
            }

            // Check constraints for uncovered HTTP methods
            // Needs to be after SCIs and listeners as they may programmatically
            // change constraints
            if (ok) {
                checkConstraintsForUncoveredMethods(findConstraints());
            }

            try {
                // Start manager
                Manager manager = getManager();
                if (manager instanceof Lifecycle) {
                    ((Lifecycle) manager).start();
                }
            } catch(Exception e) {
                log.error(sm.getString("standardContext.managerFail"), e);
                ok = false;
            }

            // Configure and call application filters
            if (ok) {
                if (!filterStart()) {
                    log.error(sm.getString("standardContext.filterFail"));
                    ok = false;
                }
            }

            // Load and initialize all "load on startup" servlets
            if (ok) {
                // 載入 servlet , startup 設定為1 的項 <load-on-startup>1</load-on-startup>
                if (!loadOnStartup(findChildren())){
                    log.error(sm.getString("standardContext.servletFail"));
                    ok = false;
                }
            }

            // Start ContainerBackgroundProcessor thread
            super.threadStart();
        } finally {
            // Unbinding thread
            unbindThread(oldCCL);
        }

        // Set available status depending upon startup success
        if (ok) {
            if (log.isDebugEnabled())
                log.debug("Starting completed");
        } else {
            log.error(sm.getString("standardContext.startFailed", getName()));
        }

        startTime=System.currentTimeMillis();

        // Send j2ee.state.running notification
        if (ok && (this.getObjectName() != null)) {
            Notification notification =
                new Notification("j2ee.state.running", this.getObjectName(),
                                 sequenceNumber.getAndIncrement());
            broadcaster.sendNotification(notification);
        }

        // The WebResources implementation caches references to JAR files. On
        // some platforms these references may lock the JAR files. Since web
        // application start is likely to have read from lots of JARs, trigger
        // a clean-up now.
        getResources().gc();

        // Reinitializing if something went wrong
        if (!ok) {
            setState(LifecycleState.FAILED);
        } else {
            setState(LifecycleState.STARTING);
        }
    }

    /**
     * Load and initialize all servlets marked "load on startup" in the
     * web application deployment descriptor.
     *
     * @param children Array of wrappers for all currently defined
     *  servlets (including those not declared load on startup)
     * @return <code>true</code> if load on startup was considered successful
     */
    public boolean loadOnStartup(Container children[]) {

        // Collect "load on startup" servlets that need to be initialized
        TreeMap<Integer, ArrayList<Wrapper>> map = new TreeMap<>();
        for (int i = 0; i < children.length; i++) {
            Wrapper wrapper = (Wrapper) children[i];
            int loadOnStartup = wrapper.getLoadOnStartup();
            if (loadOnStartup < 0)
                continue;
            Integer key = Integer.valueOf(loadOnStartup);
            ArrayList<Wrapper> list = map.get(key);
            if (list == null) {
                list = new ArrayList<>();
                map.put(key, list);
            }
            list.add(wrapper);
        }

        // Load the collected "load on startup" servlets
        for (ArrayList<Wrapper> list : map.values()) {
            for (Wrapper wrapper : list) {
                try {
                    // 呼叫 wrapper,
                    wrapper.load();
                } catch (ServletException e) {
                    getLogger().error(sm.getString("standardContext.loadOnStartup.loadException",
                          getName(), wrapper.getName()), StandardWrapper.getRootCause(e));
                    // NOTE: load errors (including a servlet that throws
                    // UnavailableException from the init() method) are NOT
                    // fatal to application startup
                    // unless failCtxIfServletStartFails="true" is specified
                    if(getComputedFailCtxIfServletStartFails()) {
                        return false;
                    }
                }
            }
        }
        return true;

    }

// StandardWrapper.load()
    /**
     * Load and initialize an instance of this servlet, if there is not already
     * at least one initialized instance.  This can be used, for example, to
     * load servlets that are marked in the deployment descriptor to be loaded
     * at server startup time.
     * <p>
     * <b>IMPLEMENTATION NOTE</b>:  Servlets whose classnames begin with
     * <code>org.apache.catalina.</code> (so-called "container" servlets)
     * are loaded by the same classloader that loaded this class, rather than
     * the classloader for the current web application.
     * This gives such classes access to Catalina internals, which are
     * prevented for classes loaded for web applications.
     *
     * @exception ServletException if the servlet init() method threw
     *  an exception
     * @exception ServletException if some other loading problem occurs
     */
    @Override
    public synchronized void load() throws ServletException {
        instance = loadServlet();

        if (!instanceInitialized) {
            initServlet(instance);
        }

        if (isJspServlet) {
            StringBuilder oname = new StringBuilder(getDomain());

            oname.append(":type=JspMonitor");

            oname.append(getWebModuleKeyProperties());

            oname.append(",name=");
            oname.append(getName());

            oname.append(getJ2EEKeyProperties());

            try {
                jspMonitorON = new ObjectName(oname.toString());
                Registry.getRegistry(null, null)
                    .registerComponent(instance, jspMonitorON, null);
            } catch( Exception ex ) {
                log.info("Error registering JSP monitoring with jmx " +
                         instance);
            }
        }
    }


    /**
     * Load and initialize an instance of this servlet, if there is not already
     * at least one initialized instance.  This can be used, for example, to
     * load servlets that are marked in the deployment descriptor to be loaded
     * at server startup time.
     * @return the loaded Servlet instance
     * @throws ServletException for a Servlet load error
     */
    public synchronized Servlet loadServlet() throws ServletException {

        // Nothing to do if we already have an instance or an instance pool
        if (!singleThreadModel && (instance != null))
            return instance;

        PrintStream out = System.out;
        if (swallowOutput) {
            SystemLogHandler.startCapture();
        }

        Servlet servlet;
        try {
            long t1=System.currentTimeMillis();
            // Complain if no servlet class has been specified
            if (servletClass == null) {
                unavailable(null);
                throw new ServletException
                    (sm.getString("standardWrapper.notClass", getName()));
            }

            InstanceManager instanceManager = ((StandardContext)getParent()).getInstanceManager();
            try {
                servlet = (Servlet) instanceManager.newInstance(servletClass);
            } catch (ClassCastException e) {
                unavailable(null);
                // Restore the context ClassLoader
                throw new ServletException
                    (sm.getString("standardWrapper.notServlet", servletClass), e);
            } catch (Throwable e) {
                e = ExceptionUtils.unwrapInvocationTargetException(e);
                ExceptionUtils.handleThrowable(e);
                unavailable(null);

                // Added extra log statement for Bugzilla 36630:
                // https://bz.apache.org/bugzilla/show_bug.cgi?id=36630
                if(log.isDebugEnabled()) {
                    log.debug(sm.getString("standardWrapper.instantiate", servletClass), e);
                }

                // Restore the context ClassLoader
                throw new ServletException
                    (sm.getString("standardWrapper.instantiate", servletClass), e);
            }

            if (multipartConfigElement == null) {
                MultipartConfig annotation =
                        servlet.getClass().getAnnotation(MultipartConfig.class);
                if (annotation != null) {
                    multipartConfigElement =
                            new MultipartConfigElement(annotation);
                }
            }

            // Special handling for ContainerServlet instances
            // Note: The InstanceManager checks if the application is permitted
            //       to load ContainerServlets
            if (servlet instanceof ContainerServlet) {
                ((ContainerServlet) servlet).setWrapper(this);
            }

            classLoadTime=(int) (System.currentTimeMillis() -t1);

            if (servlet instanceof SingleThreadModel) {
                if (instancePool == null) {
                    instancePool = new Stack<>();
                }
                singleThreadModel = true;
            }

            // 初始化 GenericServlet.servlet()
            initServlet(servlet);

            fireContainerEvent("load", this);

            loadTime=System.currentTimeMillis() -t1;
        } finally {
            if (swallowOutput) {
                String log = SystemLogHandler.stopCapture();
                if (log != null && log.length() > 0) {
                    if (getServletContext() != null) {
                        getServletContext().log(log);
                    } else {
                        out.println(log);
                    }
                }
            }
        }
        return servlet;

    }
    
    
// javax.servlet.GenericServlet.initServlet(),  注意，這是 tomcat 的包

    /**
     * Called by the servlet container to indicate to a servlet that the servlet
     * is being placed into service. See {@link Servlet#init}.
     * <p>
     * This implementation stores the {@link ServletConfig} object it receives
     * from the servlet container for later use. When overriding this form of
     * the method, call <code>super.init(config)</code>.
     *
     * @param config
     *            the <code>ServletConfig</code> object that contains
     *            configuration information for this servlet
     * @exception ServletException
     *                if an exception occurs that interrupts the servlet's
     *                normal operation
     * @see UnavailableException
     */
    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        // 轉到 HttpServletBean
        this.init();
    }

// 轉到 org.springframework.web.servlet.HttpServletBean.init()

    /**
     * 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;
        }

        // servlet 許可權移交，讓子類初始化一切,即web.xml中配置的 DispatcherServlet, FrameworkServlet.initServletBean()
        // Let subclasses do whatever initialization they like.
        initServletBean();

        if (logger.isDebugEnabled()) {
            logger.debug("Servlet '" + getServletName() + "' configured successfully");
        }
    }
    
// org.springframework.web.servlet.FrameworkServlet.initServletBean() 接收控制權，載入 spring
    /**
     * 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 {
            // 初始化上下文
            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");
        }
    }

    /**
     * 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
            // 建立 webApplicationContext, 一般可以是 org.springframework.web.context.support.XmlWebApplicationContext
            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) {
        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");
        }
        // 建立一個新的 org.springframework.web.context.support.XmlWebApplicationContext, 載入 bean...
        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

        wac.setEnvironment(getEnvironment());
        wac.setParent(parent);
        wac.setConfigLocation(getContextConfigLocation());

        configureAndRefreshWebApplicationContext(wac);

        return wac;
    }

    // 最後一步，為重新整理spring做好準備
    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);
        // 最後，呼叫 重新整理介面，終於到我們熟悉的 refresh() 了。
        wac.refresh();
    }
    

到了spring的 refresh()方法，即會進行所有的bean元件及其他元件的初始化操作了！忘文生義。

    
// org.springframework.context.support.AbstractApplicationContext, 熟悉的載入方式來了
    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                // 初始化引數時，註冊 url
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

如上，就是spring的整個載入程式框架，簡潔明瞭。不過，這不是我們要說的重點，我們要說的是 url 是如何暴露的？

　　我們只需要看這一句: finishBeanFactoryInitialization(beanFactory); url 是從這裡開始的！
　　這句的目的是，完成bean的初始化操作，聽起來還是有點抽象，我們來看一下細節:

    /**
     * Finish the initialization of this context's bean factory,
     * initializing all remaining singleton beans.
     */
    protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        // Initialize conversion service for this context.
        if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
                beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
            beanFactory.setConversionService(
                    beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
        }

        // Register a default embedded value resolver if no bean post-processor
        // (such as a PropertyPlaceholderConfigurer bean) registered any before:
        // at this point, primarily for resolution in annotation attribute values.
        if (!beanFactory.hasEmbeddedValueResolver()) {
            beanFactory.addEmbeddedValueResolver(new StringValueResolver() {
                @Override
                public String resolveStringValue(String strVal) {
                    return getEnvironment().resolvePlaceholders(strVal);
                }
            });
        }

        // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
        String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
        for (String weaverAwareName : weaverAwareNames) {
            getBean(weaverAwareName);
        }

        // Stop using the temporary ClassLoader for type matching.
        beanFactory.setTempClassLoader(null);

        // Allow for caching all bean definition metadata, not expecting further changes.
        beanFactory.freezeConfiguration();

        // Instantiate all remaining (non-lazy-init) singletons.
        // 繼續處理剩下的單例，即在此將 url 註冊
        beanFactory.preInstantiateSingletons();
    }

　　單從字面意思上看，更像獲取許多的bean, 而且還不會使用。其實這裡的意思是，通過 getBean 的方式，初始化一些特殊的bean, 比如: ConversionService, LoadTimeWeaverAware
　　但是最後一句不同: beanFactory.preInstantiateSingletons(); 意思是要初始化 單例啊！

到底要初始化啥單例呢？

// org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons()
    public void preInstantiateSingletons() throws BeansException {
        if (logger.isDebugEnabled()) {
            logger.debug("Pre-instantiating singletons in " + this);
        }

        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);

        // Trigger initialization of all non-lazy singleton beans...
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            // 只有 singleton 才會進入
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                if (isFactoryBean(beanName)) {
                    final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                    boolean isEagerInit;
                    if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                        isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                            @Override
                            public Boolean run() {
                                return ((SmartFactoryBean<?>) factory).isEagerInit();
                            }
                        }, getAccessControlContext());
                    }
                    else {
                        isEagerInit = (factory instanceof SmartFactoryBean &&
                                ((SmartFactoryBean<?>) factory).isEagerInit());
                    }
                    if (isEagerInit) {
                        getBean(beanName);
                    }
                }
                else {
                    // 明顯不是 factoryBean,只是採用繼承 RequestMappingHandlerMapping 的方式而已，所以僅僅是普通bean獲取而已
                    getBean(beanName);
                }
            }
        }

        // 如下只是給 SmartInitializingSingleton 的一條綠色通道而已，忽略
        // Trigger post-initialization callback for all applicable beans...
        for (String beanName : beanNames) {
            Object singletonInstance = getSingleton(beanName);
            if (singletonInstance instanceof SmartInitializingSingleton) {
                final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
                if (System.getSecurityManager() != null) {
                    AccessController.doPrivileged(new PrivilegedAction<Object>() {
                        @Override
                        public Object run() {
                            smartSingleton.afterSingletonsInstantiated();
                            return null;
                        }
                    }, getAccessControlContext());
                }
                else {
                    smartSingleton.afterSingletonsInstantiated();
                }
            }
        }
    }

下面我們還是簡單過一下 beans 元件的載入流程吧,下面是掃略時間：

// org.springframework.beans.factory.support.DefaultListableBeanFactory
// org.springframework.beans.factory.support.AbstractBeanFactory
    public Object getBean(String name) throws BeansException {
        return doGetBean(name, null, null, false);
    }

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * @param name the name of the bean to retrieve
     * @param requiredType the required type of the bean to retrieve
     * @param args arguments to use when creating a bean instance using explicit arguments
     * (only applied when creating a new instance as opposed to retrieving an existing one)
     * @param typeCheckOnly whether the instance is obtained for a type check,
     * not for actual use
     * @return an instance of the bean
     * @throws BeansException if the bean could not be created
     */
    @SuppressWarnings("unchecked")
    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {

        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            if (logger.isDebugEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // Check if bean definition exists in this factory.
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                String nameToLookup = originalBeanName(name);
                if (args != null) {
                    // Delegation to parent with explicit args.
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dep : dependsOn) {
                        if (isDependent(beanName, dep)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                        }
                        registerDependentBean(dep, beanName);
                        try {
                            getBean(dep);
                        }
                        catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }

                // 單例的建立肯定是走到這裡了
                // Create bean instance.
                if (mbd.isSingleton()) {
                    // 單例建立
                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            try {
                                // 稍後回撥
                                return createBean(beanName, mbd, args);
                            }
                            catch (BeansException ex) {
                                // Explicitly remove i