Web容器启动时加载Spring

怼烎@ 2022-06-18 04:56 239阅读 0赞

在应用程序web.xml中做了以下配置信息时，当启动Web容器时就会自动加载Spring容器。

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     <pre><code>&lt;listener&gt;

&lt;listener-class&gt;org.springframework.web.context.ContextLoaderListener&lt;/listener-class&gt;

&lt;/listener&gt;</code></pre> 
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##### ContextLoaderListener类实现了javax.servlet.ServletContextListener接口并且继承了org.springframework.web.context.ContextLoader类。ServletContextListener事件类是Web容器的一部分，处理Web应用的Servlet上下文(context)的监听。实现ServletContextListener接口中的contextInitialized和contextDestroyed方法。当Web容器启动时会自动调用contextInitialized方法，进行初始化Spring Web应用程序上下文，主要加载web.xml中contextConfigLocation的配置文件；当Web容器关闭之前会调用contextDestroyed方法，进行销毁Spring Web应用程序上下文。ContextLoader类实现了Spring上下文初始化的工作，执行initWebApplicationContext方法返回WebApplicationContext。Spring实现的contextInitialized和contextDestroyed代码如下： #####

public void contextInitialized(ServletContextEvent event) {
    
           this.contextLoader = createContextLoader();
    
           if (this.contextLoader ==null) {
    
               this.contextLoader = this;
    
           }
    
        this.contextLoader.initWebApplicationContext(event.getServletContext());
    
        }
    
     
    
    public void contextDestroyed(ServletContextEvent event) {
    
           if (this.contextLoader !=null) {

>     this.contextLoader.closeWebApplicationContext(event.getServletContext());
> 
>     }
>     
>         ContextCleanupListener.cleanupAttributes(event.getServletContext());
> 
>     }

##### Spring执行实现Spring上下文的方法 #####

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) {
    
                    configureAndRefreshWebApplicationContext((ConfigurableWebApplicationContext)this.context, 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;
    
            }
    
        }