java Spring的启动原理详解

<!--上下文监听器，用于监听servlet的启动过程-->
    <listener>
        <description>ServletContextListener</description>
        <!--这里是自定义监听器，个性化定制项目启动提示-->
        <listener-class>com.trace.app.framework.listeners.ApplicationListener</listener-class>
    </listener>
    <!--dispatcherServlet的配置，这个servlet主要用于前端控制，这是springMVC的基础-->
    <servlet>
        <servlet-name>service_dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>/WEB-INF/spring/services/service_dispatcher-servlet.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <!--spring资源上下文定义，在指定地址找到spring的xml配置文件-->
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/spring/application_context.xml</param-value>
    </context-param>
    <!--spring的上下文监听器-->
    <listener>
        <listener-class>
            org.springframework.web.context.ContextLoaderListener
        </listener-class>
    </listener>
    <!--Session监听器，Session作为公共资源存在上下文资源当中，这里也是自定义监听器-->
    <listener>
        <listener-class>
            com.trace.app.framework.listeners.MySessionListener
        </listener-class>
    </listener>

<context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/spring/application_context.xml</param-value>
</context-param>

<listener>
        <listener-class>
            org.springframework.web.context.ContextLoaderListener
        </listener-class>
 </listener>

public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
}

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;
        }