DispatcherServlet的初始化

Posted 2020-10-21 hxwang

DispatcherServlet的初始化在springmvc的启动中有讲过，这一篇在上一篇的基础上接着讲。DispatcherServlet作为springmvc的前端控制器，还需要初始化其他的模块。

回到initWebApplicationContext()函数，看到在wac = createWebApplicationContext(rootContext)后面，调用了onRefresh(was)函数，onRefresh函数在DispatcherServlet类中重写，代码如下：

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

可以看到MVC初始化是在DispatcherServlet的initStrategies()方法中完成，在该方法中初始化各种MVC框架实现元素，如至此request映射的HandlerMappings，以及handler适配处理器HandlerAdapters，以及视图生成器ViewResolver等。

/**
     * 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);
    }

比如初始化HandlerMappings，在SpringMVC中，HandlerMappings的作用是为Http请求找到对应的Controller控制器，来进一步处理请求。

    /**
     * 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) {
            // 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 {
                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");
            }
        }
    }

detectAllHandlerMappings变量默认为true，所以在初始化HandlerMapping接口默认实现类的时候，会把上下文中所有HandlerMapping类型的Bean都注册在handlerMappings这个List变量中。如果你手工将其设置为false，那么将尝试获取名为handlerMapping的Bean，新建一个只有一个元素的List，将其赋给handlerMappings。如果经过上面的过程，handlerMappings变量仍为空，那么说明你没有在上下文中提供自己HandlerMapping类型的Bean定义。此时，SpringMVC将采用默认初始化策略来初始化handlerMappings。
点进去getDefaultStrategies看一下。

/**
     * 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>();
        }
    }

这个DispatcherServlet.properties里面，以键值对的方式，记录了SpringMVC默认实现类，它在spring-webmvc-4.2.5.RELEASE.jar这个jar包内，在org.springframework.web.servlet包里面。

 

# 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

至此，我们分析完了initHandlerMappings(context)方法的执行过程，其他的初始化过程与这个方法非常类似。所有初始化方法执行完后，SpringMVC正式完成初始化，静静等待Web请求的到来。