Spring中很重要的ContextLoderListener类，你理解了吗？

作者：xlj3 原文：https://blog.csdn.net/luman1991/article/details/52229105

<listener>
    <description>spring监听器</description>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>
  <listener>
    <listener-class>org.springframework.web.util.IntrospectorCleanupListener</listener-class>
  </listener>
  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>
  classpath:spring-servlet.xml,classpath:spring-application.xml
    </param-value>
  </context-param>
  <context-param>
    <param-name>charset</param-name>
    <param-value>UTF-8</param-value>
  </context-param>

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
  private ContextLoader contextLoader;
 
  /**
   * Initialize the root web application context.
   */
  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()); 
}

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 {
      // Determine parent for root web application context, if any.
      ApplicationContext parent = loadParentContext(servletContext);
 
      // Store context in local instance variable, to guarantee that
      // it is available on ServletContext shutdown.
      this.context = createWebApplicationContext(servletContext, parent);
      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;
    }
  }

protected WebApplicationContext createWebApplicationContext(ServletContext sc, ApplicationContext parent) {
    Class<?> contextClass = determineContextClass(sc);
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
      throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
          "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    }
    ConfigurableWebApplicationContext wac =
        (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
 
    // Assign the best possible id value.
    if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
      // Servlet <= 2.4: resort to name specified in web.xml, if any.
      String servletContextName = sc.getServletContextName();
      wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
          ObjectUtils.getDisplayString(servletContextName));
    }
    else {
      // Servlet 2.5's getContextPath available!
      try {
        String contextPath = (String) ServletContext.class.getMethod("getContextPath").invoke(sc);
        wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
            ObjectUtils.getDisplayString(contextPath));
      }
      catch (Exception ex) {
        throw new IllegalStateException("Failed to invoke Servlet 2.5 getContextPath method", ex);
      }
    }
 
    wac.setParent(parent);
    wac.setServletContext(sc);
    wac.setConfigLocation(sc.getInitParameter(CONFIG_LOCATION_PARAM));
    customizeContext(sc, wac);
    wac.refresh();
    return wac;
  }

//关闭指定Servlet上下文中的Spring Web应用上下文  
public void closeWebApplicationContext(ServletContext servletContext) {  
        servletContext.log("Closing Spring root WebApplicationContext");  
        try {  
        //Spring Web应用上下文的类型都是ConfigurableWebApplicationContext  
            if (this.context instanceof ConfigurableWebApplicationContext) {  
                //关闭Spring Web应用上下文，释放资源，销毁所有缓存的单态模式Bean  
                ((ConfigurableWebApplicationContext) this.context).close();  
            }  
        }  
        finally {  
            //获取当前线程的上下文类加载器  
            ClassLoader ccl = Thread.currentThread().getContextClassLoader();  
            //将当前上下文对象设置为null  
            if (ccl == ContextLoader.class.getClassLoader()) {  
                currentContext = null;  
            }  
            //移除容器类加载器—>Web应用上下文Map集合中中key为指定类加载器的项  
            else if (ccl != null) {  
                currentContextPerThread.remove(ccl);  
            }  
        //移除Servlet上下文中Spring Web根上下文属性 servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);  
                //释放父容器引用  
            if (this.parentContextRef != null) {  
                this.parentContextRef.release();  
            }  
        }  
    }

public interface WebApplicationContext extends ApplicationContext {  
    //用于定义在Servlet上下文中存储Spring Web根上下文  
    String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";  
    //请求范围  
    String SCOPE_REQUEST = "request";  
    //会话范围  
    String SCOPE_SESSION = "session";  
    //全局会话范围  
    String SCOPE_GLOBAL_SESSION = "globalSession";  
    //应用程序范围  
    String SCOPE_APPLICATION = "application";  
    //容器中存储的Servlet上下文环境的Bean名称  
    String SERVLET_CONTEXT_BEAN_NAME = "servletContext";  
    //web.xml文件中的配置Spring初始化参数的属性  
    String CONTEXT_PARAMETERS_BEAN_NAME = "contextParameters";  
    //Servlet上下文属性  
    String CONTEXT_ATTRIBUTES_BEAN_NAME = "contextAttributes";  
//获取当前应用程序所以Web容器的标准Servlet上下文  
    ServletContext getServletContext();  
}