@RequestParam @RequestBody @PathVariable 等参数绑定注解详解

本文转自CSDN，作者：walkerJong，时间：2015-4-2

引言：

接上一篇文章，对@RequestMapping进行地址映射讲解之后，该篇主要讲解request 数据到handler method 参数数据的绑定所用到的注解和什么情形下使用；

简介：

handler method 参数绑定常用的注解,我们根据他们处理的Request的不同内容部分分为四类：（主要讲解常用类型）A、处理requet uri 部分（这里指uri template中variable，不含queryString部分）的注解： @PathVariable;B、处理request header部分的注解： @RequestHeader, @CookieValue;C、处理request body部分的注解：@RequestParam, @RequestBody;D、处理attribute类型是注解： @SessionAttributes, @ModelAttribute;

1、 @PathVariable

当使用@RequestMapping URI template 样式映射时， 即 someUrl/{paramId}, 这时的paramId可通过 @Pathvariable注解绑定它传过来的值到方法的参数上。示例代码：[java] viewplaincopy@Controller@RequestMapping("/owners/{ownerId}")public class RelativePathUriTemplateController {@RequestMapping("/pets/{petId}")public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {// implementation omitted}}上面代码把URI template 中变量 ownerId的值和petId的值，绑定到方法的参数上。若方法参数名称和需要绑定的uri template中变量名称不一致，需要在@PathVariable("name")指定uri template中的名称。

2、 @RequestHeader、@CookieValue

@RequestHeader 注解，可以把Request请求header部分的值绑定到方法的参数上。示例代码：这是一个Request 的header部分：[plain] viewplaincopyHost localhost:8080Accept text/html,application/xhtml+xml,application/xml;q=0.9Accept-Language fr,en-gb;q=0.7,en;q=0.3Accept-Encoding gzip,deflateAccept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7Keep-Alive 300[java] viewplaincopy@RequestMapping("/displayHeaderInfo.do")public void displayHeaderInfo(@RequestHeader("Accept-Encoding") String encoding,@RequestHeader("Keep-Alive") long keepAlive) {//...}上面的代码，把request header部分的 Accept-Encoding的值，绑定到参数encoding上了， Keep-Alive header的值绑定到参数keepAlive上。@CookieValue 可以把Request header中关于cookie的值绑定到方法的参数上。例如有如下Cookie值：[java] viewplaincopyJSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84参数绑定的代码：[java] viewplaincopy@RequestMapping("/displayHeaderInfo.do")public void displayHeaderInfo(@CookieValue("JSESSIONID") String cookie) {//...}即把JSESSIONID的值绑定到参数cookie上。

3、@RequestParam, @RequestBody

@RequestParamA） 常用来处理简单类型的绑定，通过Request.getParameter() 获取的String可直接转换为简单类型的情况（ String--> 简单类型的转换操作由ConversionService配置的转换器来完成）；因为使用request.getParameter()方式获取参数，所以可以处理get 方式中queryString的值，也可以处理post方式中 body data的值；B）用来处理Content-Type: 为

application/x-www-form-urlencoded

编码的内容，提交方式GET、POST；C) 该注解有两个属性： value、required； value用来指定要传入值的id名称，required用来指示参数是否必须绑定；示例代码：[java] viewplaincopy@Controller@RequestMapping("/pets")@SessionAttributes("pet")public class EditPetForm {// ...@RequestMapping(method = RequestMethod.GET)public String setupForm(@RequestParam("petId") int petId, ModelMap model) {Pet pet = this.clinic.loadPet(petId);model.addAttribute("pet", pet);return "petForm";}// ...@RequestBody该注解常用来处理Content-Type: 不是

application/x-www-form-urlencoded

编码的内容，例如application/json, application/xml等；它是通过使用HandlerAdapter 配置的

HttpMessageConverters

来解析post data body，然后绑定到相应的bean上的。因为配置有FormHttpMessageConverter，所以也可以用来处理

application/x-www-form-urlencoded

的内容，处理完的结果放在一个MultiValueMap<String, String>里，这种情况在某些特殊需求下使用，详情查看FormHttpMessageConverter api;示例代码：[java] viewplaincopy@RequestMapping(value = "/something", method = RequestMethod.PUT)public void handle(@RequestBody String body, Writer writer) throws IOException {writer.write(body);}

4、@SessionAttributes, @ModelAttribute

@SessionAttributes:该注解用来绑定HttpSession中的attribute对象的值，便于在方法中的参数里使用。该注解有value、types两个属性，可以通过名字和类型指定要使用的attribute 对象；示例代码：[java] viewplaincopy@Controller@RequestMapping("/editPet.do")@SessionAttributes("pet")public class EditPetForm {// ...}@ModelAttribute该注解有两个用法，一个是用于方法上，一个是用于参数上；用于方法上时： 通常用来在处理@RequestMapping之前，为请求绑定需要从后台查询的model；用于参数上时： 用来通过名称对应，把相应名称的值绑定到注解的参数bean上；要绑定的值来源于：A） @SessionAttributes 启用的attribute 对象上；B） @ModelAttribute 用于方法上时指定的model对象；C） 上述两种情况都没有时，new一个需要绑定的bean对象，然后把request中按名称对应的方式把值绑定到bean中。用到方法上@ModelAttribute的示例代码：[java] viewplaincopy// Add one attribute// The return value of the method is added to the model under the name "account"// You can customize the name via @ModelAttribute("myAccount")@ModelAttributepublic Account addAccount(@RequestParam String number) {return accountManager.findAccount(number);}这种方式实际的效果就是在调用@RequestMapping的方法之前，为request对象的model里put（“account”， Account）；用在参数上的@ModelAttribute示例代码：[java] viewplaincopy@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)public String processSubmit(@ModelAttribute Pet pet) {}首先查询 @SessionAttributes有无绑定的Pet对象，若没有则查询@ModelAttribute方法层面上是否绑定了Pet对象，若没有则将URI template中的值按对应的名称绑定到Pet对象的各属性上。

补充讲解：

问题： 在不给定注解的情况下，参数是怎样绑定的？

通过分析AnnotationMethodHandlerAdapter和RequestMappingHandlerAdapter的源代码发现，方法的参数在不给定参数的情况下：若要绑定的对象时简单类型： 调用@RequestParam来处理的。若要绑定的对象时复杂类型： 调用@ModelAttribute来处理的。这里的简单类型指java的原始类型(boolean, int 等)、原始类型对象（Boolean, Int等）、String、Date等ConversionService里可以直接String转换成目标对象的类型；下面贴出AnnotationMethodHandlerAdapter中绑定参数的部分源代码：[java] viewplaincopyprivate Object[] resolveHandlerArguments(Method handlerMethod, Object handler,NativeWebRequest webRequest, ExtendedModelMap implicitModel) throws Exception {Class[] paramTypes = handlerMethod.getParameterTypes();Object[] args = new Object[paramTypes.length];for (int i = 0; i < args.length; i++) {MethodParameter methodParam = new MethodParameter(handlerMethod, i);methodParam.initParameterNameDiscovery(this.parameterNameDiscoverer);GenericTypeResolver.resolveParameterType(methodParam, handler.getClass());String paramName = null;String headerName = null;boolean requestBodyFound = false;String cookieName = null;String pathVarName = null;String attrName = null;boolean required = false;String defaultValue = null;boolean validate = false;Object[] validationHints = null;int annotationsFound = 0;Annotation[] paramAnns = methodParam.getParameterAnnotations();for (Annotation paramAnn : paramAnns) {if (RequestParam.class.isInstance(paramAnn)) {RequestParam requestParam = (RequestParam) paramAnn;paramName = requestParam.value();required = requestParam.required();defaultValue = parseDefaultValueAttribute(requestParam.defaultValue());annotationsFound++;}else if (RequestHeader.class.isInstance(paramAnn)) {RequestHeader requestHeader = (RequestHeader) paramAnn;headerName = requestHeader.value();required = requestHeader.required();defaultValue = parseDefaultValueAttribute(requestHeader.defaultValue());annotationsFound++;}else if (RequestBody.class.isInstance(paramAnn)) {requestBodyFound = true;annotationsFound++;}else if (CookieValue.class.isInstance(paramAnn)) {CookieValue cookieValue = (CookieValue) paramAnn;cookieName = cookieValue.value();required = cookieValue.required();defaultValue = parseDefaultValueAttribute(cookieValue.defaultValue());annotationsFound++;}else if (PathVariable.class.isInstance(paramAnn)) {PathVariable pathVar = (PathVariable) paramAnn;pathVarName = pathVar.value();annotationsFound++;}else if (ModelAttribute.class.isInstance(paramAnn)) {ModelAttribute attr = (ModelAttribute) paramAnn;attrName = attr.value();annotationsFound++;}else if (Value.class.isInstance(paramAnn)) {defaultValue = ((Value) paramAnn).value();}else if (paramAnn.annotationType().getSimpleName().startsWith("Valid")) {validate = true;Object value = AnnotationUtils.getValue(paramAnn);validationHints = (value instanceof Object[] ? (Object[]) value : new Object[] {value});}}if (annotationsFound > 1) {throw new IllegalStateException("Handler parameter annotations are exclusive choices - " +"do not specify more than one such annotation on the same parameter: " + handlerMethod);}if (annotationsFound == 0) {// 若没有发现注解Object argValue = resolveCommonArgument(methodParam, webRequest); //判断WebRquest是否可赋值给参数if (argValue != WebArgumentResolver.UNRESOLVED) {args[i] = argValue;}else if (defaultValue != null) {args[i] = resolveDefaultValue(defaultValue);}else {Class<?> paramType = methodParam.getParameterType();if (Model.class.isAssignableFrom(paramType) || Map.class.isAssignableFrom(paramType)) {if (!paramType.isAssignableFrom(implicitModel.getClass())) {throw new IllegalStateException("Argument [" + paramType.getSimpleName() + "] is of type " +"Model or Map but is not assignable from the actual model. You may need to switch " +"newer MVC infrastructure classes to use this argument.");}args[i] = implicitModel;}else if (SessionStatus.class.isAssignableFrom(paramType)) {args[i] = this.sessionStatus;}else if (HttpEntity.class.isAssignableFrom(paramType)) {args[i] = resolveHttpEntityRequest(methodParam, webRequest);}else if (Errors.class.isAssignableFrom(paramType)) {throw new IllegalStateException("Errors/BindingResult argument declared " +"without preceding model attribute. Check your handler method signature!");}else if (BeanUtils.isSimpleProperty(paramType)) {// 判断是否参数类型是否是简单类型，若是在使用@RequestParam方式来处理,否则使用@ModelAttribute方式处理paramName = "";}else {attrName = "";}}}if (paramName != null) {args[i] = resolveRequestParam(paramName, required, defaultValue, methodParam, webRequest, handler);}else if (headerName != null) {args[i] = resolveRequestHeader(headerName, required, defaultValue, methodParam, webRequest, handler);}else if (requestBodyFound) {args[i] = resolveRequestBody(methodParam, webRequest, handler);}else if (cookieName != null) {args[i] = resolveCookieValue(cookieName, required, defaultValue, methodParam, webRequest, handler);}else if (pathVarName != null) {args[i] = resolvePathVariable(pathVarName, methodParam, webRequest, handler);}else if (attrName != null) {WebDataBinder binder =resolveModelAttribute(attrName, methodParam, implicitModel, webRequest, handler);boolean assignBindingResult = (args.length > i + 1 && Errors.class.isAssignableFrom(paramTypes[i + 1]));if (binder.getTarget() != null) {doBind(binder, webRequest, validate, validationHints, !assignBindingResult);}args[i] = binder.getTarget();if (assignBindingResult) {args[i + 1] = binder.getBindingResult();i++;}implicitModel.putAll(binder.getBindingResult().getModel());}}return args;}RequestMappingHandlerAdapter中使用的参数绑定，代码稍微有些不同，有兴趣的同仁可以分析下，最后处理的结果都是一样的。示例：[java] viewplaincopy@RequestMapping ({"/", "/home"})public String showHomePage(String key){logger.debug("key="+key);return "home";}这种情况下，就调用默认的@RequestParam来处理。[java] viewplaincopy@RequestMapping (method = RequestMethod.POST)public String doRegister(User user){if(logger.isDebugEnabled()){logger.debug("process url[/user], method[post] in "+getClass());logger.debug(user);}return "user";}这种情况下，就调用@ModelAttribute来处理。

参考文档：

1、 Spring Web Doc：

spring-3.1.0/docs/spring-framework-reference/html/mvc.html

参考文献：[1]walkerJong. @RequestParam@RequestBody @PathVariable 等参数绑定注解详解.[EB/OL]. /article/8221349.htmlt3