spring mvc配置

我们知道要想使用springmvc，一般需要配置如下

web.xml中配置ContextLoaderListener来加载spring根配置文件。

<web-app>  
  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:application-context.xml</param-value>
  </context-param>
  <listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>
</web-app>

再加一个spring的根配置文件application-context.xml，可以什么内容都不写。大家有没有想过ContextLoaderListener的作用是什么，具体做了什么？如果先思考一下，带着问题再去看源码，更有益处！

当然为了能正常接收请求，还需要配置DispatcherServlet及对应的mvc配置文件，这里暂不配置，以专注springmvc初始化。

分析

很明显ContextLoaderListener是入口，首先看类继承层次图（intellij idea快捷键 ctrl+alt+shift+u）

ContextLoaderListener实现了ServletContextListener，这个接口的contextInitialized方法会在容器初始化时被调用。

ContextLoaderListener.java
/** * Initialize the root web application context. */
    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }

contextInitialized 方法很简单，调用了ContextLoader 的initWebApplicationContext

ContextLoader.java
/** * Initialize Spring's web application context for the given servlet context, * using the application context provided at construction time, or creating a new one * according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and * "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params. * @param servletContext current servlet context * @return the new WebApplicationContext * @see #ContextLoader(WebApplicationContext) * @see #CONTEXT_CLASS_PARAM * @see #CONFIG_LOCATION_PARAM */
    public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
        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!");
        }

        Log logger = LogFactory.getLog(ContextLoader.class);
        servletContext.log("Initializing Spring root WebApplicationContext");
        if (logger.isInfoEnabled()) {
            logger.info("Root WebApplicationContext: initialization started");
        }
        long startTime = System.currentTimeMillis();

        try {
            // Store context in local instance variable, to guarantee that
            // it is available on ServletContext shutdown.
            if (this.context == null) {
                this.context = createWebApplicationContext(servletContext);
            }
            if (this.context instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
                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 ->
                        // determine parent for root web application context, if any.
                        ApplicationContext parent = loadParentContext(servletContext);
                        cwac.setParent(parent);
                    }
                    configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

            ClassLoader ccl = Thread.currentThread().getContextClassLoader();
            if (ccl == ContextLoader.class.getClassLoader()) {
                currentContext = this.context;
            }
            else if (ccl != null) {
                currentContextPerThread.put(ccl, this.context);
            }

            if (logger.isDebugEnabled()) {
                logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
                        WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
            }
            if (logger.isInfoEnabled()) {
                long elapsedTime = System.currentTimeMillis() - startTime;
                logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
            }

            return this.context;
        }
        catch (RuntimeException ex) {
            logger.error("Context initialization failed", ex);
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
            throw ex;
        }
        catch (Error err) {
            logger.error("Context initialization failed", err);
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
            throw err;
        }
    }

首先看注释，这个方法就是为给定的servlet context初始化spring application context，spring application context有两个来源，要么是使用构造方法传入的，要么通过web.xml中context-param指定contextClass、contextConfigLocation新创建的 ，一句话概括，这个方法是用来初始化spring application context的。方法返回值是WebApplicationContext，类继承层次图

initWebApplicationContext（）方法内部，首先判断ServletContext是否已经存在spring application context，如果存在则抛出错误。否则创建上下文this.context = createWebApplicationContext(servletContext);， 然后**配置并刷新**WebApplicationContextconfigureAndRefreshWebApplicationContext，然后将spring application context设置到servletcontext。简单来说

这里最重要的两个方法是createWebApplicationContext、configureAndRefreshWebApplicationContext，坦白的说，createWebApplicationContext是创建了WebApplicationContext这个对象，在此基础上，configureAndRefreshWebApplicationContext修改了WebApplicationContext的属性。

/** * Instantiate the root WebApplicationContext for this loader, either the * default context class or a custom context class if specified. * <p>This implementation expects custom contexts to implement the * {@link ConfigurableWebApplicationContext} interface. * Can be overridden in subclasses. * <p>In addition, {@link #customizeContext} gets called prior to refreshing the * context, allowing subclasses to perform custom modifications to the context. * @param sc current servlet context * @return the root WebApplicationContext * @see ConfigurableWebApplicationContext */
    protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
        Class<?> contextClass = determineContextClass(sc);
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

看方法注释，使用默认的context class或者 自定义的context class实例化root webapplicationcontext。如果是自定义context class，那么它要实现ConfigurableWebApplicationContext接口。
看代码，通过determineContextClass()返回context class，如果不是ConfigurableWebApplicationContext，则抛出异常，也就是说，这个方法只能返回ConfigurableWebApplicationContext类型的ApplicaContext，为了一窥究竟，我们看determineContextClass()

protected Class<?> determineContextClass(ServletContext servletContext) {
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
            }
            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);
            }
        }
    }

web.xml中没有配置context_class_param，所以只能是contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
返回context class，继续跟踪defaultStrategies

ContextLoader.java
private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
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, ContextLoader.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
        }
    }

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

defaultStrategies返回的是XmlWebApplicationContext这是通过ContextLoader.properties指定的，通过查看类层次图，XmlWebApplicationContext是ConfigurableWebApplicationContext的子类

这里就清楚了，我们再继续看ContextLoader.configureAndRefreshWebApplicationContext()，对ApplicationContext做了哪些修改。

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {

        wac.setServletContext(sc);
        String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
        if (configLocationParam != null) {
            wac.setConfigLocation(configLocationParam);
        }

        // 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(sc, null);
        }

        customizeContext(sc, wac);
        wac.refresh();
    }

代码里将contextConfigLocation设置到ConfigurableWebApplicationContext，((ConfigurableWebEnvironment) env).initPropertySources(sc, null);初始化servlet context的property source，执行customizeContext()来执行自定义逻辑，最后执行refresh() 因为此时的wac是XmlWebApplicationContext，refresh()方法在父类AbstractApplicationContext中定义，所以执行AbstractApplicationContext.refresh()

AbstractApplicationContext.java
@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.
                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();
            }
        }
    }

大体上就是准备资源，然后创建BeanFactory、然后后处理BeanFactory（postProcessBeanFactory（））、然后执行BeanFactory的后置处理器invokeBeanFactoryPostProcessors、然后注册BeanPostProcessor……，这里不要犯迷糊，是先创建BeanFactory、然后执行BeanFactory的后置处理器，再注册BeanPostProcessor，然后在创建bean时，后置处理Bean。如果这个过程中失败，则确保已经创建的bean都毁灭。

总结

• spring mvc初始化，创建的ApplicationContext是XmlWebApplicationContext，BeanFactory是DefaultListableBeanFactory，最重要的两个方法是createWebApplicationContext()、AbstractApplicationContext.refresh() ,尤其是refresh()方法，下一步将此方法中的每个步骤搞明白。
• spring源码很优秀，整个springmvc初始化过程，逻辑很清晰；除此之外，在类层次结构设计上很庞大；
• 再说一点看源码的心得，看源码先大概流程，后细节，别一头扎进某个方法，看大概干了什么，梳理流程，然后再挖下去，这个过程中辅助类继承层次图（ctrl+alt+shift+u）至关重要，还有方法调用链使用ctrl+alt+h（intellij idea）也很实用。