SpringMVC的启动

Spring MVC中的Servlet

Spring MVC中Servlet一共有三个层次，分别是HttpServletBean、FrameworkServlet和DispatcherServlet。HttpServletBean直接继承自Java的HttpServlet，其作用是将Servlet中配置的参数设置到相应的属性；FrameworkServlet初始化了WebApplicationContext；DispatcherServlet初始化了自身的9个组件。

DispatcherServlet

DispatcherServlet的入口方法是onRefresh方法，其中简单地调用了initStrategies方法，其中又调用了9个初始化方法，初始化策略组件。

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

protected void initStrategies(ApplicationContext context) {
    // 上传组件
    this.initMultipartResolver(context);
    // 国际化组件
    this.initLocaleResolver(context);
    this.initThemeResolver(context);
    this.initHandlerMappings(context);
    this.initHandlerAdapters(context);
    this.initHandlerExceptionResolvers(context);
    this.initRequestToViewNameTranslator(context);
    this.initViewResolvers(context);
    this.initFlashMapManager(context);
}

// 以initLocaleResolver为例分析初始化组件过程
private void initLocaleResolver(ApplicationContext context) {
    try {
        // 按照名称或类型在容器中获取
        // 我们只要在SpringMVC的配置文件中配置相应类型的组件，容器就可以自动找到
        this.localeResolver = (LocaleResolver)context.getBean("localeResolver", LocaleResolver.class);
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Using LocaleResolver [" + this.localeResolver + "]");
        }
    } catch (NoSuchBeanDefinitionException var3) {
        // 如果没有配置，使用默认策略
        this.localeResolver = (LocaleResolver)this.getDefaultStrategy(context, LocaleResolver.class);
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Unable to locate LocaleResolver with name 'localeResolver': using default [" + this.localeResolver + "]");
        }
    }
}

protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
    // 调用复数的方法
    List<T> strategies = this.getDefaultStrategies(context, strategyInterface);
    if (strategies.size() != 1) {
        throw new BeanInitializationException("DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
    } else {
        // 返回第一个
        return strategies.get(0);
    }
}
protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
    String key = strategyInterface.getName();
    // Properties类型的defaultStrategies在静态代码块中初始化
    String value = defaultStrategies.getProperty(key);
    if (value == null) {
        return new LinkedList();
    } else {
        // 如果有多个默认值，以逗号分隔数组
        String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
        List<T> strategies = new ArrayList(classNames.length);
        String[] var7 = classNames;
        int var8 = classNames.length;
        // 初始化组件
        for(int var9 = 0; var9 < var8; ++var9) {
            String className = var7[var9];
            try {
                Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
                Object strategy = this.createDefaultStrategy(context, clazz);
                strategies.add(strategy);
            } catch (ClassNotFoundException var13) {
                throw new BeanInitializationException("Could not find DispatcherServlet's default strategy class [" + className + "] for interface [" + key + "]", var13);
            } catch (LinkageError var14) {
                throw new BeanInitializationException("Error loading DispatcherServlet's default strategy class [" + className + "] for interface [" + key + "]: problem with class file or dependent class", var14);
            }
        }

        return strategies;
    }
}

static {
    try {
        // DispatcherServlet.properties在DispatcherServlet.java文件的旁边
        ClassPathResource resource = new ClassPathResource("DispatcherServlet.properties", DispatcherServlet.class);
        defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
    } catch (IOException var1) {
        throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + var1.getMessage());
    }
}

DispatcherServlet.properties文件内容：

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

一共定义了8个组件，上传组件MultipartResolver没有默认配置，因为不是每个应用都需要上传功能，即使需要上传也不一定就要使用MultipartResolver；HandlerMapping、HandlerAdapter和HandlerExceptionResolver都配置了多个；ViewResolver也可以配置多个，默认是一个。
默认配置并不是最优配置，也不是spring的推荐配置。
当使用<mvc:annotation-driven/>后，并不会全部使用默认配置，因为它配置了HandlerMapping、HandlerAdapter和HandlerExceptionResolver，而且还做了很多别的工作。

Spring MVC的启动

启动DispatcherServlet之前先创建HttpServletBean、FrameworkServlet，创建DispatcherServlet后调用其onRefresh方法，再调用initStrategies方法初始化其它组件。