struts1源码中的单例模式

protected Action processActionCreate(HttpServletRequest request,
                                         HttpServletResponse response,
                                         ActionMapping mapping)
        throws IOException {

        // Acquire the Action instance we will be using (if there is one)
        String className = mapping.getType();
        if (log.isDebugEnabled()) {
            log.debug(" Looking for Action instance for class " + className);
        }

        // :TODO: If there were a mapping property indicating whether
        // an Action were a singleton or not ([true]),
        // could we just instantiate and return a new instance here?

        Action instance = null;
        synchronized (actions) {
          //sss 每次ancton的创建都是从系统的action map里面根据类名来读取,所以是另外的一种单例模式,所有的同类的action共享一个实例 struts127
            // Return any existing Action instance of this class
            instance = (Action) actions.get(className);
            if (instance != null) {
                if (log.isTraceEnabled()) {
                    log.trace("  Returning existing Action instance");
                }
                return (instance);
            }

            // Create and return a new Action instance
            if (log.isTraceEnabled()) {
                log.trace("  Creating new Action instance");
            }
           
            try {
                instance = (Action) RequestUtils.applicationInstance(className);
                // :TODO: Maybe we should propagate this exception
                // instead of returning null.
            } catch (Exception e) {
                log.error(
                    getInternal().getMessage("actionCreate", mapping.getPath()),
                    e);
                   
                response.sendError(
                    HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
                    getInternal().getMessage("actionCreate", mapping.getPath()));
                   
                return (null);
            }