一、流程

Spring MVC 主框架將 ServletRequest • 物件及目標方法的入參例項傳遞給 WebDataBinderFactory 例項，以建立 DataBinder 例項物件2. DataBinder 呼叫裝配在 Spring MVC 上下文中的 ConversionService 元件進行資料型別轉換、資料格式化工作。將 Servlet 中的請求資訊填充到入參物件中3. 呼叫 Validator 元件對已經綁定了請求訊息的入參物件進行資料合法性校驗，並最終生成資料繫結結果BindingData 物件4. Spring MVC 抽取 BindingResult 中的入參物件和校驗錯誤物件，將它們賦給處理方法的響應入參

二、原始碼分析：

public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResolver, HandlerMethodReturnValueHandler {
    protected final Log logger = LogFactory.getLog(this.getClass());
    private final boolean annotationNotRequired;

    public ModelAttributeMethodProcessor(boolean annotationNotRequired) {
        this.annotationNotRequired = annotationNotRequired;
    }

    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.hasParameterAnnotation(ModelAttribute.class) || this.annotationNotRequired && !BeanUtils.isSimpleProperty(parameter.getParameterType());
    }

    public final Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
        String name = ModelFactory.getNameForParameter(parameter);
        ModelAttribute ann = (ModelAttribute)parameter.getParameterAnnotation(ModelAttribute.class);
        if (ann != null) {
            mavContainer.setBinding(name, ann.binding());
        }

        Object attribute = mavContainer.containsAttribute(name) ? mavContainer.getModel().get(name) : this.createAttribute(name, parameter, binderFactory, webRequest);
        WebDataBinder binder = binderFactory.createBinder(webRequest, attribute, name);
        if (binder.getTarget() != null) {
            if (!mavContainer.isBindingDisabled(name)) {
                this.bindRequestParameters(binder, webRequest);
            }

            this.validateIfApplicable(binder, parameter);
            if (binder.getBindingResult().hasErrors() && this.isBindExceptionRequired(binder, parameter)) {
                throw new BindException(binder.getBindingResult());
            }
        }
 


        Map<String, Object> bindingResultModel = binder.getBindingResult().getModel();
        mavContainer.removeAttributes(bindingResultModel);
        mavContainer.addAllAttributes(bindingResultModel);
        return binder.convertIfNecessary(binder.getTarget(), parameter.getParameterType(), parameter);
    }