Spring MVC源码解析

三产

发布于 2021-01-12 10:04:29

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XXXAware

XXXAware在Spring中该类接口用于通过Spring自动向XXXAware实现类中实现的setXXX(XXX xxx)方法中注入XXX对象，方便在实现类中调用XXX对象。例如： A类需要使用当前的ApplicationContext，那么只要是A实现ApplicationContextAware接口，然后实现接口中的setApplicationContext(ApplicationContext applicationContext)方法，Spring就会自动调用setApplicationContext方法将applicationContext传给A类。

XXXCapable

实现XXXCapable接口后说明该实现类具有提供XXX的能力，当Spring需要XXX时会通过该类的getXXX方法来获取XXX对象。

Environment

Environment具体功能与Servlet中的ServletContext类似。 HttpServletBean中Environment使用的是StandardServletEnvironment。

    @Override
    public ConfigurableEnvironment getEnvironment() {
        if (this.environment == null) {
            this.environment = this.createEnvironment();
        }
        return this.environment;
    }

    protected ConfigurableEnvironment createEnvironment() {
        return new StandardServletEnvironment();
    }

StandardServletEnvironment中封装了ServletContext、ServletConfig、JndiProperty、系统环境变量以及系统属性。

StandardServletEnvironment{
    activeProfiles=[

    ],
    defaultProfiles=[
        default
    ],
    propertySources=[
        servletConfigInitParams,
        servletContextInitParams,
        jndiProperties,
        systemProperties,
        systemEnvironment
    ]
}

具体属性大家可以调试一下查看

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Servlet在创建时会直接调用无参init方法 HttpServletBean的init方法如下：

    @Override
    public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
            logger.debug("Initializing servlet '" + getServletName() + "'");
        }
        try {
            // 将web.xml中init-param参数封装到pvs变量中，requiredProperties是指必须的一些参数，没有的话会报错
            PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
            //bw是对DispatcherServlet的封装
            //【BeanWrpper是Spring提供的用于操作JavaBean属性的一个工具类，通过它封装一个对象后可以直接修改对象的属性】
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            //对DispatcherServlet做一些初始化的工作
            initBeanWrapper(bw);
            //将配置值设置到DispatcherServlet
            bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
            throw ex;
        }

        // 子类通过该方法初始化【该类中只是声明了该方法】
        initServletBean();

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

    protected void initServletBean() throws ServletException {
    }

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FrameworkServlet，通过之前HttpServletBean详解，我们知道，FrameworkServlet是通过initServletBean来实现自身的初始化的。我们看一下initServletBean：

    @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();
            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");
        }
    }

    protected void initFrameworkServlet() throws ServletException {
    }

可以看到这个方法中的核心语句：

this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();

其中initFrameworkServlet是供子类Override后实现自身的一些初始化工作【DispatcherServlet中并没有对initFrameworkServlet进行Override】。可见，FrameworkServlet在构建工程中主要作用就是初始化webApplicationContext：

protected WebApplicationContext initWebApplicationContext() {
//获取rootContext
        WebApplicationContext rootContext =WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;
        //判断是否在构造方法中以及初始化了webApplicationContext
        if (this.webApplicationContext != null) {
            // 一个WebApplicationContext实例在构建实例的时候被注入
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    // 上下文尚未Refresh -> 配置并Refresh
                    // 设置父上下文,设置WebApplicationContext id等等
                    if (cwac.getParent() == null) {
                        // 如果上下文被注入时没有明确的设置Parent,则在这里设置
                        // 设置rootContext为Parent【可能为null】 
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        if (wac == null) {
            //如果在构建实例时没有注入WebApplicationContext
            //查找WebApplicationContext是否已经存在于ServletContext中【通过配置在Servlet中的contextAttribute获取】
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            // 如果还是没有WebApplicationContext,那么创建一个
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            // 如果还是没有WebApplicationContext 既不是一个ConfigurableApplicationContext支持Refresh，并且WebApplicationContext注入时没有Refresh- >手动触发onRefresh【供子类Override】。
            onRefresh(wac);
        }

        if (this.publishContext) {
            // 将WebApplicationContex保存到ServletContext
            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) {
        // For subclasses: do nothing by default.
    }

initWebApplicationContext做了三件事： 1. 获取Spring都得rootContext Spring默认将rootContext设置于ServletContext的属性中，属性名为ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";

设置WebApplicationContext并根据情况调用onRefresh方法设置WebApplicationContext一共三种方法：一、构造方法传入这种方法主要用于Servlet3.0以后的环境中，在程序中使用ServletContext.addServlet方式注册Servlet，这时可以在新建FrameworkServlet和其子类时通过构造方法传递WebApplicationContext 二、WebApplicationContext已经存在ServletContext中，这是只要将DispatcherServlet配置是将WebApplicationContext配置到contextAttribute就可以。三、创建一个WebApplicationContext【最终调用下面的方法】

    protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
        //获取要创建的类型
        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 + "]");
        }
        //检查获取到的类似是否是ConfigurableWebApplicationContext的子类
        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");
        }
        //具体创建
        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

        wac.setEnvironment(getEnvironment());
        wac.setParent(parent);
        //将servlet配置地址【web.xml contextConfigLocation设置】
        wac.setConfigLocation(getContextConfigLocation());

        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());
        //添加ContextRefreshListener
        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);
        wac.refresh();
    }

将WebApplicationContext设置到ServletContext中

总结一下可配置项： 1. contextAttribute：在ServletContext中，用作WebApplicationContext的属性名称 2. contextClass：创建的WebApplicationContext的类型 3. contextConfigLocation：SpringMVC配置文件的位置 4. publishContext：是否将WebApplicationContext设置到ServletContext中。

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通过之前FrameworkServlet的分析，我们知道DispatcherServlet的入口方法是onRefresh。

    @Override
    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调用了initStrategies，initStrategies中初始化了九大组件，其中的initXXXX方法基本实现都是通过WebApplicationContext.getBean(名称, class);获取注册的对应组件，如果找不到会通过getDefaultStrategy方法获取默认的组件。

protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
        List<T> strategies = getDefaultStrategies(context, strategyInterface);
        if (strategies.size() != 1) {
            throw new BeanInitializationException(
                    "DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
        }
        return strategies.get(0);
    }
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>();
        }
    }

    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, DispatcherServlet.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
        }
    }

getDefaultStrategy调用了getDefaultStrategies，而getDefaultStrategies是通过defaultStrategies来查找value值的。defaultStrategies对应的是DispatcherServlet.properties:

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

这里的默认设置并不是最优配置，也不是Spring推荐配置，只是为了防止没有配置。 这些默认配置是在没有配置时才会使用，使用<mvc:annotation-driven/>之后并不会全部使用默认配置。<mvc:annotation-driven/>会配置HandlerMapping、HandlerAdapter、HandlerExceptionResolver等。

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原始发表：2016/05/03 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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