Spring的ContextLoaderListener 簡介

在開發Spring的Web項目中，通常我們都會在web.xml中配置一個Spring的核心監聽器，就是把Spring的IOC容器納入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源碼如下：

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官方源碼定義是Spring的root(也就是WebApplicationContext)啟動和關閉的引導監聽器，當調用ContextLoaderListener的無參構造器時，就會創建一個新的ContextLoaderListener,也將會創建一個web應用上下文基於web.xml中的contextLoader和contextConfigLocation的servlet context-param參數，創建的應用上下文會被注冊到ServletContext中通過WebApplicationContext裡的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE這個屬性。

ContextLoaderListener繼承自ContextLoader並且實現了ServletContextListener接口，實現ServletContextListener作用不言而喻，就是要讓Spring和Servlet容器關聯起來，讓Spring容器伴隨著Servlet容器的啟動而啟動，其實這個類中最主要的一個方法就是contextInitialized()，作用就是Spring上下文初始化，裡面的initWebApplicationContext()是從父類ContextLoader中繼承過來的，查看源碼：

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;
        }
    }

關於多次出現的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE這個參數，我個人理解的就是它是Servlet容器識別Spring的一個標識，如果為null，說明Spring還有沒注冊進Servlet容器中，

緊接著就是創建WebApplicationContext，configureAndRefreshWebApplicationContext()這個方法最主要就是配置applicationContext.xml這個文件，

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);這句代碼就是把Spring容器注冊進ServletContext中。

關於WebApplicationContext這個接口：

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繼承自ApplicationContext，這個接口添加了一個getServletContext()方法，並且定義了一些root context在啟動進程中必須綁定的application屬性，和一般的應用上下文一樣，web application contexts也是分層的，每一個應用都有一個root context,並且應用裡的每個Servlet都有自己的子context,除了標准的應用上下文聲明周期，WebApplicationContext實現需要探測ServletContextAware beans和調用setServletContext()方法。

WebApplicationContext裡面定義了一些常用的bean作用域屬性，比如request,session,globalSession,application,servletContext.

其實可以在相應的類中打斷點調試，這樣更能加深印象，有一點不明白的是

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

這段代碼中ccl明明不為空，可是斷點調試卻不會執行，不知道哪位能解釋一下。