Re: 在SimpleFormController中用“SessionForm”概念构建修改信息页面

是的，覆写formBackingObject方法就是利用Session Form。使用Session Form还必须将Form设置为Session Form。如下代码末端用红色序号标注的地方表示了通过formBackingObject在“GET"修改页面时将Command Object加入到Session的过程。用绿色序号标注的代码表示了如何在”POST“请求后从Session中取出Command对象。

此段代码拷贝自Spring的AbstractFormController类。

java 代码

protected ModelAndView handleRequestInternal(HttpServletRequest request, HttpServletResponse response)
        throws Exception {
    // Form submission or new form to show?
    if (isFormSubmission(request)) {
        // Fetch form object from HTTP session, bind, validate, process submission.
        try {
            Object command = getCommand(request);  1
            ...
        }
        catch (HttpSessionRequiredException ex) {
            ...
        }
    }

    else {
        // New form to show: render form view.
        return showNewForm(request, response); 1
    }
}

protected final ModelAndView showNewForm(HttpServletRequest request, HttpServletResponse response)
        throws Exception {

    logger.debug("Displaying new form");
    return showForm(request, response, getErrorsForNewForm(request)); 2
}

protected final BindException getErrorsForNewForm(HttpServletRequest request) throws Exception {
        // Create form-backing object for new form.
    Object command = formBackingObject(request); 3
    if (command == null) {
        throw new ServletException("Form object returned by formBackingObject() must not be null");
    }
    if (!checkCommand(command)) {
        throw new ServletException("Form object returned by formBackingObject() must match commandClass");
    }

    // Bind without validation, to allow for prepopulating a form, and for
    // convenient error evaluation in views (on both first attempt and resubmit).
    ServletRequestDataBinder binder = createBinder(request, command);
    BindException errors = new BindException(binder.getBindingResult());
    if (isBindOnNewForm()) {
        logger.debug("Binding to new form");
        binder.bind(request);
        onBindOnNewForm(request, command, errors);
    }

    // Return BindException object that resulted from binding.
    return errors;
}

protected final Object getCommand(HttpServletRequest request) throws Exception {
    // If not in session-form mode, create a new form-backing object.
    if (!isSessionForm()) {  2
        return formBackingObject(request);
    }

    // Session-form mode: retrieve form object from HTTP session attribute.
    HttpSession session = request.getSession(false);
    if (session == null) {
        throw new HttpSessionRequiredException("Must have session when trying to bind (in session-form mode)");
    }

    String formAttrName = getFormSessionAttributeName(request);
    Object sessionFormObject = session.getAttribute(formAttrName);  3
    if (sessionFormObject == null) {
        throw new HttpSessionRequiredException("Form object not found in session (in session-form mode)");
    }

    // Remove form object from HTTP session: we might finish the form workflow
    // in this request. If it turns out that we need to show the form view again,
    // we'll re-bind the form object to the HTTP session.
    if (logger.isDebugEnabled()) {
        logger.debug("Removing form session attribute [" + formAttrName + "]");
    }
    session.removeAttribute(formAttrName);

    return currentFormObject(request, sessionFormObject);
}

protected final ModelAndView showForm(
    HttpServletRequest request, BindException errors, String viewName, Map controlModel)
    throws Exception {

    // In session form mode, re-expose form object as HTTP session attribute.
    // Re-binding is necessary for proper state handling in a cluster,
    // to notify other nodes of changes in the form object.
    if (isSessionForm()) { 5
        String formAttrName = getFormSessionAttributeName(request);
        if (logger.isDebugEnabled()) {
            logger.debug("Setting form session attribute [" + formAttrName + "] to: " + errors.getTarget());
        }
        request.getSession().setAttribute(formAttrName, errors.getTarget());  6
    }

    ...
}

protected Object formBackingObject(HttpServletRequest request) throws Exception {
    return createCommand(); 4
}

如此看来覆写formBackingObject方法才是使用Session Form的正道。不过就是好像必须在form初始化时手动设置为Session Form。
现在还没有看出来Session中的Command对象是什么时候被修改的，这还是个问题。