SpringMVC初始化阶段流程源码分析

1、都知道SpringMVC项目启动的时候都会初始化一个类：DispatcherServlet,看这个类的源码我们可以发现他其实就是一个servlet，

为什么这么说呢？请看：

DispatcherServlet extends FrameworkServlet
FrameworkServlet extends HttpServletBean
HttpServletBean extends HttpServlet
初始化这个DispatcherServlet的时候我们可以看到这个类里面有一个静态代码块：根据代码可以看出代码块里面会去读取DispatcherServlet.properties配置文件里面的配置，load到Properties集合里面

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

private static final Properties defaultStrategies;

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

2、我们都知道一个servlet初始化阶段 ，会调用init()方法，那么这边会先调用HttpServlet的子类HttpServletBean里面的init方法：org.springframework.web.servlet.HttpServletBean#init，这个方法里面调用了initServletBean这个方法：org.springframework.web.servlet.FrameworkServlet#initServletBean

@Override
    public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
            logger.debug("Initializing servlet '" + getServletName() + "'");
        }

        // Set bean properties from init parameters.
        PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
                ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
                bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
                initBeanWrapper(bw);
                bw.setPropertyValues(pvs, true);
            }
            catch (BeansException ex) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();

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

org.springframework.web.servlet.FrameworkServlet#initServletBean里面调用了org.springframework.web.servlet.FrameworkServlet#initWebApplicationContext

@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 | 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");
        }
    }

org.springframework.web.servlet.FrameworkServlet#initWebApplicationContext

    protected WebApplicationContext initWebApplicationContext() {
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                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 -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        if (wac == null) {
            // No context instance was injected at construction time -> see if one
            // has been registered in the servlet context. If one exists, it is assumed
            // that the parent context (if any) has already been set and that the
            // user has performed any initialization such as setting the context id
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            // Either the context is not a ConfigurableApplicationContext with refresh
            // support or the context injected at construction time had already been
            // refreshed -> trigger initial onRefresh manually here.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            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;
    }

里面有一个onRefresh方法：里面做了一些列的初始化操作，具体哪个方法对应什么功能呢？请大家自行看里面的源码，我这边主要说的是

initHandlerMappings(context);
initHandlerAdapters(context);
这两个方法就是初始化我们的映射器和我们的适配器的，到此我们的初始化就结束了；

@Override
    protected void onRefresh(ApplicationContext context) {
        initStrategies(context);
    }

    protected void initStrategies(ApplicationContext context) {
        //此方法主要做文件上传处理的
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        //此方法主要是获取配置文件DispatcherServlet.properties里面的handlerMapping类型对象
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }