1 /** 2      * Resolve the value of the 'action' attribute. 3      * 
If the user configured an 'action' value then 4      * the result of evaluating this value is used. Otherwise, the 5      * {
    @link org.springframework.web.servlet.support.RequestContext#getRequestUri() originating URI} 6      * is used. 7      * @return the value that is to be used for the 'action' attribute 8      */ 9     protected String resolveAction() throws JspException {10         String action = getAction();11         if (StringUtils.hasText(action)) {12             action = getDisplayString(evaluate(ACTION_ATTRIBUTE, action));13             return processAction(action);14         }15         else {16             String requestUri = getRequestContext().getRequestUri();17             ServletResponse response = this.pageContext.getResponse();18             if (response instanceof HttpServletResponse) {19                 requestUri = ((HttpServletResponse) response).encodeURL(requestUri);20                 String queryString = getRequestContext().getQueryString();21                 if (StringUtils.hasText(queryString)) {22                     requestUri += "?" + HtmlUtils.htmlEscape(queryString);23                 }24             }25             if (StringUtils.hasText(requestUri)) {26                 return processAction(requestUri);27             }28             else {29                 throw new IllegalArgumentException("Attribute 'action' is required. " +30                         "Attempted to resolve against current request URI but request URI was null.");31             }32         }33     }

源码注释中写到，如果页面<form:form action="" method="POST">，如果action为空，则根据 getRequestUri()来设置action。