Spring MVC 解析之 DispatcherServlet之ContextLoaderListener容器初始化

ゝ一世哀愁。 2022-05-09 12:33 212阅读 0赞

ContextLoaderListener容器初始化  
[http://blog.csdn.net/qq924862077/article/details/52769754][http_blog.csdn.net_qq924862077_article_details_52769754]

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

在tomcat启动时，spring通过上述配置会初始化spring容器，注入applicationContext.xml中配置的bean以及其他一些配置。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    
        public ContextLoaderListener() {
        }
        public ContextLoaderListener(WebApplicationContext context) {
            super(context);
        }
        @Override
        public void contextInitialized(ServletContextEvent event) {
            initWebApplicationContext(event.getServletContext());
        }
        @Override
        public void contextDestroyed(ServletContextEvent event) {
            closeWebApplicationContext(event.getServletContext());
            ContextCleanupListener.cleanupAttributes(event.getServletContext());
        }
    }

ContextLoderListener实现ServletContextListener接口，而该接口继承java的监听标记接口EventListener，表示当servlet容器启动时，就会接受通知，进而进行一系列的初始化操作。

public interface ServletContextListener extends EventListener {
    
        /**
         * Receives notification that the web application initialization
         * process is starting.
         *
         * <p>All ServletContextListeners are notified of context
         * 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
         * that is being destroyed
         */
        public void contextDestroyed(ServletContextEvent sce);
    }

spring的具体初始化工作均在父类ContextLoder中进行，具体初始化步骤如下：

判断是否已经存在WebApplicationContext(spring容器)，如果存在，则抛出异常。  
创建WebApplicationContext容器，从web.xml中获取context-param名称为contextClass的value，该类必须实现WebApplicationContext接口，如果没有指定，则默认使用xmlWebApplicationContext。  
判断创建的WebApplicationContext容器是否为ConfigurableApplicationContext的实例或子类，如果是，则判断该容器是否至少刷新一次并且还没有关闭（是否有效），如果无效，则重新设置容器的id，并加载对应的contextConfigLocation配置（spring配置)，  
加载父容器，spring提供一个模板方法，如果需要配置父容器，可以继承ContextLoder，并覆盖loadParentContext方法。  
线程绑定容器

/**
         * 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) {
                    //创建容器
                    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);
                        }
                        //如果容器无效，则重置contextId，且重新加载contextConfigLocation配置
                        configureAndRefreshWebApplicationContext(cwac, servletContext);
                    }
                }
                servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
                //线程绑定容器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;
            }
        }

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本文来自 fengziciwei 的CSDN 博客 ，全文地址请点击：[https://blog.csdn.net/fengziciwei/article/details/76240141?utm\_source=copy][https_blog.csdn.net_fengziciwei_article_details_76240141_utm_source_copy]

[http_blog.csdn.net_qq924862077_article_details_52769754]: http://blog.csdn.net/qq924862077/article/details/52769754
[https_blog.csdn.net_fengziciwei_article_details_76240141_utm_source_copy]: https://blog.csdn.net/fengziciwei/article/details/76240141?utm_source=copy