本文主要從falshMap初始化，存，取，消毀來進行原始碼分析，springmvc版本4.3.18。關於使用及驗證請參考另一篇https://www.cnblogs.com/pu20065226/p/10032048.html

1.初始化和呼叫，首先是入springMVC 入口webmvc包中org.springframework.web.servlet.DispatcherServlet中的doService方法進行呼叫

public class DispatcherServlet extends FrameworkServlet
{
  private FlashMapManager flashMapManager;
 
 
public static final String INPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".INPUT_FLASH_MAP";
  protected void initStrategies(ApplicationContext context)
  {
    initMultipartResolver(context);
    initLocaleResolver(context);
    initThemeResolver(context);
    initHandlerMappings(context);
    initHandlerAdapters(context);
    initHandlerExceptionResolvers(context);
    initRequestToViewNameTranslator(context);
    initViewResolvers(context);
　　//初始化
 
    initFlashMapManager(context);
  }
  private void initFlashMapManager(ApplicationContext context)
  {
    try
    {
　　　　//同級目錄下DispatcherServlet.properties中配置
　　　　//org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
      this.flashMapManager = ((FlashMapManager)context.getBean("flashMapManager", FlashMapManager.class
 
));
      if (this.logger.isDebugEnabled())
        this.logger.debug("Using FlashMapManager [" + this.flashMapManager + "]");

    }
    catch (NoSuchBeanDefinitionException ex)
    {
      this.flashMapManager = ((FlashMapManager)getDefaultStrategy(context, FlashMapManager.class));
      if (this.logger.isDebugEnabled())
        this.logger.debug("Unable to locate FlashMapManager with name 'flashMapManager': using default [" + this.flashMapManager + "]");
    }

  }
    
  protected void doService(HttpServletRequest request, HttpServletResponse response)
    throws Exception
  {
　　　//從session中獲取及session中刪除
　　　//此處執行的是模板類org.springframework.web.servlet.support.AbstractFlashMapManager中的retrieveAndUpdate方法
　　 FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
    if (inputFlashMap != null)//轉存request
      request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
}

AbstractFlashMapManager原始碼如下，

retrieveAndUpdate呼叫實現子類的retrieveFlashMaps取session中的flashmap,呼叫子類updateFlashMaps刪除session的方法

public abstract class AbstractFlashMapManager
  implements FlashMapManager
{
 public final FlashMap retrieveAndUpdate(HttpServletRequest request, HttpServletResponse response)
  {//從session中取falshmap
    List allFlashMaps = retrieveFlashMaps(request);
    if (CollectionUtils.isEmpty(allFlashMaps)) {
      return null;
    }

    if (this.logger.isDebugEnabled())
      this.logger.debug("Retrieved FlashMap(s): " + allFlashMaps);

    List mapsToRemove = getExpiredFlashMaps(allFlashMaps);
    FlashMap match = getMatchingFlashMap(allFlashMaps, request);
    if (match != null) {
      mapsToRemove.add(match);
    }

    if (!(mapsToRemove.isEmpty())) {
      if (this.logger.isDebugEnabled())
        this.logger.debug("Removing FlashMap(s): " + mapsToRemove);

      Object mutex = getFlashMapsMutex(request);
      if (mutex != null) {
        synchronized (mutex) {
          allFlashMaps = retrieveFlashMaps(request);
          if (allFlashMaps != null) {
            allFlashMaps.removeAll(mapsToRemove);
            updateFlashMaps(allFlashMaps, request, response);
          }
        }
      }
      else {
        allFlashMaps.removeAll(mapsToRemove);
　　　　//從session中刪除falshmap
        updateFlashMaps(allFlashMaps, request, response);
      }
    }

    return match;
  }
//具體實現在SessionFlashMapManager中

 　 protected abstract List<FlashMap> retrieveFlashMaps(HttpServletRequest paramHttpServletRequest);

  protected abstract void updateFlashMaps(List<FlashMap> paramList, HttpServletRequest paramHttpServletRequest, HttpServletResponse paramHttpServletResponse); 

}

 實現類SessionFlashMapManager中重寫retrieveFlashMaps和updateFlashMaps

public class SessionFlashMapManager extends AbstractFlashMapManager
{
  private static final String FLASH_MAPS_SESSION_ATTRIBUTE = SessionFlashMapManager.class.getName() + ".FLASH_MAPS";

  protected List<FlashMap> retrieveFlashMaps(HttpServletRequest request)
  {
    HttpSession session = request.getSession(false);
    return ((session != null) ? (List)session.getAttribute(FLASH_MAPS_SESSION_ATTRIBUTE) : null);
  }

  protected void updateFlashMaps(List<FlashMap> flashMaps, HttpServletRequest request, HttpServletResponse response)
  {//(!(flashMaps.isEmpty())) ? flashMaps : null)有值時,整個表示式返回null
    WebUtils.setSessionAttribute(request, FLASH_MAPS_SESSION_ATTRIBUTE, (!(flashMaps.isEmpty())) ? flashMaps : null);
  }

  protected Object getFlashMapsMutex(HttpServletRequest request)
  {
    return WebUtils.getSessionMutex(request.getSession());
  }
}

spring-web包org.springframework.web.util.WebUtils中WebUtils.setSessionAttribute

public static void setSessionAttribute(HttpServletRequest request, String name, Object value)
  {
    Assert.notNull(request, "Request must not be null");
    if (value != null) {
      request.getSession().setAttribute(name, value);
    }
    else {//進入此步
      HttpSession session = request.getSession(false);
      if (session != null)
        session.removeAttribute(name);//刪除falshmap
    }
  }

介面類org.springframework.web.servlet.FlashMapManager

public abstract interface FlashMapManager
{
  public abstract FlashMap retrieveAndUpdate(HttpServletRequest paramHttpServletRequest, HttpServletResponse paramHttpServletResponse);

  public abstract void saveOutputFlashMap(FlashMap paramFlashMap, HttpServletRequest paramHttpServletRequest, HttpServletResponse paramHttpServletResponse);
}