再看SpringMVC是如何通过一个DispatcherServlet拦截http请求的

Posted 2020-10-14 小欢欢

初始入口:

org.springframework.web.context.ContextLoaderListener

org.springframework.web.context.ContextLoader

 

类的继承关系图如下:

可以看到这个Listener实现了Servlet标准提供的接口，这个listener只有两个方法是 用来监听web容器启动和关闭动作的：

    /**
     * Receives notification that the web application initialization
     * process is starting.
     *
     * <p>All ServletContextListeners are notified of context   // 这里已经说了，使用的观察者模式，容器启动会调用
     * initialization before any filters or servlets in the web
     * application are initialized.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being initialized
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     * Receives notification that the ServletContext is about to be
     * shut down.
     *
     * <p>All servlets and filters will have been destroyed before any
     * ServletContextListeners are notified of context
     * destruction.
     *
     * @param sce the ServletContextEvent containing the ServletContext  //这里也说了，容器关闭会调用 
     * that is being destroyed
     */
    public void contextDestroyed(ServletContextEvent sce);

当然ContextLoaderListener实现了这两个方法：

    /**
     * Initialize the root web application context.
     */
    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());  //这个方法在父类ContextLoader中实现 ，这里只是把ServletContext当参数传过去了，因为在执行逻辑中需要用到很多与web容器相关的参数
    }


    /**
     * Close the root web application context.
     */
    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());  //同上
    }

切换到父类ContextLoader 中initWebApplicationContext方法中:

/**
     * 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);                   //这里先创建Web环境的 ApplicationContext ，这个Context的地位就类似于ServletContext,是获取Bean的入口
            }
            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);          //这才是最核心的方法，看名字就知道 配置并刷新 ApplicationContext,这个动作完成了类示例的创建，初始化和 注册事件的发布，各种监听的执行, 为
                }                                                                            //DispatcherServlet这个核心类执行Handler的注册做好了万全准备
            }
            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);                //这里可以看到把当前Context绑定到当前线程，是为了做隔离使用，保证线程安全，稍后再说这个问题
            }

            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;
        }
    }

先来看看ApplicationContext的创建，进入createWebApplicationContext,毕竟它是获取Bean的入口。

/**
     * 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);
    }

    /**
     * Return the WebApplicationContext implementation class to use, either the
     * default XmlWebApplicationContext or a custom context class if specified.
     * @param servletContext current servlet context
     * @return the WebApplicationContext implementation class to use
     * @see #CONTEXT_CLASS_PARAM
     * @see org.springframework.web.context.support.XmlWebApplicationContext
     */
    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);
            }
        }
    }

这里先从ServletContext中获取参数是contextClass的值，也就是从web.xml读取contextParam或者硬编码的方式(servlet3.0以上的版本)中获取配置指定的ApplicationContext。如果没有，使用默认的策略 XmlWebApplicationContext，这个默认的策略 defaultStrategies是在ContextLoader开头的静态代码块中初始的：

/**
     * Name of the class path resource (relative to the ContextLoader class)
     * that defines ContextLoader\'s default strategy names.
     */
    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);  //这里从ContextLoader所在的包下加载一个叫ContextLoader.properties的文件，
                                                                                                              //这个文件在spring-web包的context子包中，内容只有一行，配置了默认的WebApplicationContext
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load \'ContextLoader.properties\': " + ex.getMessage());
        }
    }

有了ApplicationContext了，再回到上面initWebApplicationContext方法,下面就开始判断这个context是不是ConfigurableWebApplicationContext实例，如果是，并且没有执行过刷新，就执行配置刷新的方法，也就是上面提到的最核心的方法，用于bean的初始化，

这里在判断后，还获取了parent context并将它设置成当前context的parent.

 

当然最核心的入口，一切源头:

org.springframework.web.servlet.DispatcherServlet

该类的继承关系(使用Idea分析工具)如下:

 

DefaultListableBeanFactory类的继承关系:

 

与Bean创建时相关的Aware接口的三个直接子接口: