简单介绍Spring的ContextLoaderListener

在开发Spring的Web项目中，通常我们都会在web.xml中配置一个Spring的核心监听器，就是把Spring的IOC容器纳入Servlet容器中，配置如下：

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

查看ContextLoaderListener源码如下：

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());
    }

}

ContextLoaderListener官方源码定义是Spring的root(也就是WebApplicationContext)启动和关闭的引导监听器，当调用ContextLoaderListener的无参构造器时，就会创建一个新的ContextLoaderListener,也将会创建一个web应用上下文基于web.xml中的contextLoader和contextConfigLocation的servlet context-param参数，创建的应用上下文会被注册到ServletContext中通过WebApplicationContext里的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE这个属性。

ContextLoaderListener继承自ContextLoader并且实现了ServletContextListener接口，实现ServletContextListener作用不言而喻，就是要让Spring和Servlet容器关联起来，让Spring容器伴随着Servlet容器的启动而启动，其实这个类中最主要的一个方法就是contextInitialized()，作用就是Spring上下文初始化，里面的initWebApplicationContext()是从父类ContextLoader中继承过来的，查看源码：

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) {
                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;
        }
    }

 关于多次出现的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE这个参数，我个人理解的就是它是Servlet容器识别Spring的一个标识，如果为null，说明Spring还有没注册进Servlet容器中，

 紧接着就是创建WebApplicationContext，configureAndRefreshWebApplicationContext()这个方法最主要就是配置applicationContext.xml这个文件，

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);这句代码就是把Spring容器注册进ServletContext中。

关于WebApplicationContext这个接口：

public interface WebApplicationContext extends ApplicationContext {

    String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";

    String SCOPE_REQUEST = "request";

    String SCOPE_SESSION = "session";

    String SCOPE_GLOBAL_SESSION = "globalSession";

    String SCOPE_APPLICATION = "application";

    String SERVLET_CONTEXT_BEAN_NAME = "servletContext";

    String CONTEXT_PARAMETERS_BEAN_NAME = "contextParameters";

    String CONTEXT_ATTRIBUTES_BEAN_NAME = "contextAttributes";

    ServletContext getServletContext();

}

 WebApplicationContext继承自ApplicationContext，这个接口添加了一个getServletContext()方法，并且定义了一些root context在启动进程中必须绑定的application属性，和一般的应用上下文一样，web application contexts也是分层的，每一个应用都有一个root context,并且应用里的每个Servlet都有自己的子context,除了标准的应用上下文声明周期，WebApplicationContext实现需要探测ServletContextAware beans和调用setServletContext()方法。

WebApplicationContext里面定义了一些常用的bean作用域属性，比如request,session,globalSession,application,servletContext.

其实可以在相应的类中打断点调试，这样更能加深印象，有一点不明白的是

else if (ccl != null) {
    currentContextPerThread.put(ccl, this.context);
}

这段代码中ccl明明不为空，可是断点调试却不会执行，不知道哪位能解释一下。