ContextLoaderListener初始化

ContextLoaderListener的作用就是啟動web容器時，自動裝配ApplicationContext的配置資訊。因為它實現了ServletContextListener介面，當web容器啟動時，會觸發ServletContextListener的contextInitialized方法。

public interface ServletContextListener extends EventListener {

    public void contextInitialized ( ServletContextEvent sce );

    public
 
 void contextDestroyed ( ServletContextEvent sce );
}

一般在web.xml中都會有如下配置，ContextLoaderListener是一個上下文載入器，同時它實現了ServletContextListener介面，也是一個最常用的Servlet監聽器。

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

ServletContext啟動之後會呼叫ServletContextListener的contextInitialized方法，我們進入到ContextLoaderListener的contextInitialized方法體。

public class ContextLoaderListener implements ServletContextListener {

    private ContextLoader contextLoader;

    public void contextInitialized(ServletContextEvent event) {
        this.contextLoader = createContextLoader();
        this.contextLoader.initWebApplicationContext(event.getServletContext());
    }

    protected
 
 ContextLoader createContextLoader() {
        return new ContextLoader();
    }
    ……
}

public WebApplicationContext initWebApplicationContext(ServletContext servletContext)
            throws IllegalStateException, BeansException {

        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!");
        }

        try {
            //載入根上下文的雙親上下文
            ApplicationContext parent = loadParentContext(servletContext);

            this.context = createWebApplicationContext(servletContext, parent);
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
            currentContextPerThread.put(Thread.currentThread().getContextClassLoader(), this.context);
            ……
            return this.context;
        }
        ……
    }

上述程式碼首先判斷在ServletContext中是否已經有根上下文存在了，如果存在，則丟擲異常提示建立失敗，如果不存在。

protected WebApplicationContext createWebApplicationContext(
            ServletContext servletContext, ApplicationContext parent) throws BeansException {

        Class contextClass = determineContextClass(servletContext);
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }

        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
        wac.setParent(parent);
        wac.setServletContext(servletContext);
        wac.setConfigLocation(servletContext.getInitParameter(CONFIG_LOCATION_PARAM));
        customizeContext(servletContext, wac);
        wac.refresh();

        return wac;
    }

    protected Class determineContextClass(ServletContext servletContext) throws ApplicationContextException {
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName);
            }
            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);
            }
        }
    }

    static {
        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());
        }
    }

    private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";

根據上述靜態程式碼塊的內容，我們檢視當前類ContextLoader同樣目錄下的ContextLoader.properties檔案，內容如下：

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

綜上分析，在初始化的過程中，首先讀取ContextLoader.properties內容，根據其中的配置提取將要實現WebApplicationContext介面的實現類，並根據這個實現類通過反射方式建立例項物件。

再回到ContextLoader的initWebApplicationContext方法中，createWebApplicationContext方法執行結束後，需要將例項物件記錄到servletContext中，並對映當前的類載入器與建立的例項物件到全域性變數currentContextPerThread中。

DispatcherServlet初始化

在完成對ContextLoaderListener的初始化之後，Web容器開始初始化DispatcherServlet。在Servlet的初始化階段會呼叫其init方法，DispatcherServlet繼承自FrameworkServlet，而FrameworkServlet繼承自HttpServletBean，在HttpServletBean中找到了init方法。

    public final void init() throws ServletException {
        ……
        PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                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) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }

        initServletBean();

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

DispatcherServlet的初始化過程主要是先將web.xml中的init節點內容解析成Servlet型別例項，再將Servlet型別例項轉換成BeanWrapper型別例項。最後進入initServletBean方法進行具體的初始化，FrameworkServlet擴充套件了此方法。

    protected final void initServletBean() throws ServletException {
        ……
        try {
            this.webApplicationContext = initWebApplicationContext();
            initFrameworkServlet();
        }
        ……
    }

    protected WebApplicationContext initWebApplicationContext() {
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        if (wac == null) {
            // No context instance was injected at construction time -> see if one
            // has been registered in the servlet context. If one exists, it is assumed
            // that the parent context (if any) has already been set and that the
            // user has performed any initialization such as setting the context id
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            // Either the context is not a ConfigurableApplicationContext with refresh
            // support or the context injected at construction time had already been
            // refreshed -> trigger initial onRefresh manually here.
            onRefresh(wac);
        }

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

        return wac;
    }

    protected void onRefresh(ApplicationContext context) {
        initStrategies(context);
    }


    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }

onRefresh是FrameworkServlet類中提供的模板方法，在其子類DispatcherServlet中進行了重寫：

• 初始化MultipartResolver
  在Spring中，MultipartResolver主要用於處理檔案上傳，預設情況下，Spring是沒有MultipartResolver的，如果想使用，則需要在Web應用的上下文中新增MultipartResolver解析器。

    private void initMultipartResolver(ApplicationContext context) {
        try {
            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");
            }
        }
    }

• 初始化LocaleResolver
  在Spring的國際化配置中一種有3中使用方式：基於URL引數的配置、基於session的配置、基於cookie的國際化配置。

    private void initLocaleResolver(ApplicationContext context) {
        try {
            this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using LocaleResolver [" + this.localeResolver + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate LocaleResolver with name '" + LOCALE_RESOLVER_BEAN_NAME +
                        "': using default [" + this.localeResolver + "]");
            }
        }
    }

• 初始化ThemeResolver

    private void initThemeResolver(ApplicationContext context) {
        try {
            this.themeResolver = context.getBean(THEME_RESOLVER_BEAN_NAME, ThemeResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using ThemeResolver [" + this.themeResolver + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.themeResolver = getDefaultStrategy(context, ThemeResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate ThemeResolver with name '" + THEME_RESOLVER_BEAN_NAME +
                        "': using default [" + this.themeResolver + "]");
            }
        }
    }

• 初始化HandlerMapping

private void initHandlerMappings(ApplicationContext context) {
        this.handlerMappings = null;

        if (this.detectAllHandlerMappings) {
            // 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<>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerMappings);
            }
        }
        else {
            try {
                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");
            }
        }
    }

• 初始化HandlerAdapter

    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<>(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");
            }
        }
    }

• 初始化HandlerExceptionResolver

    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<>(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");
            }
        }
    }

• 初始化RequestToViewNameTranslator

    private void initRequestToViewNameTranslator(ApplicationContext context) {
        try {
            this.viewNameTranslator =
                    context.getBean(REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using RequestToViewNameTranslator [" + this.viewNameTranslator + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate RequestToViewNameTranslator with name '" +
                        REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME + "': using default [" + this.viewNameTranslator +
                        "]");
            }
        }
    }

• 初始化ViewResolvers

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<>(matchingBeans.values());
                // We keep ViewResolvers in sorted order.
                AnnotationAwareOrderComparator.sort(this.viewResolvers);
            }
        }
        else {
            try {
                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");
            }
        }
    }

• 初始化FlashMapManager

    private void initFlashMapManager(ApplicationContext context) {
        try {
            this.flashMapManager = context.getBean(FLASH_MAP_MANAGER_BEAN_NAME, FlashMapManager.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using FlashMapManager [" + this.flashMapManager + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.flashMapManager = getDefaultStrategy(context, FlashMapManager.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate FlashMapManager with name '" +
                        FLASH_MAP_MANAGER_BEAN_NAME + "': using default [" + this.flashMapManager + "]");
            }
        }
    }