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

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：

@Controller
@RequestMapping("/owners/{ownerId}")
public class RelativePathUriTemplateController {

  @RequestMapping("/pets/{petId}")
  public void findPet(@PathVariable String ownerId,
@PathVariable String petId, Model model) {
    // implementation omitted
  }
}

@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：

Host                    localhost:8080
Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language         fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding         gzip,deflate
Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive              300

Host                    localhost:8080
Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language         fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding         gzip,deflate
Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive              300

@RequestMapping("/displayHeaderInfo.do")
public void displayHeaderInfo(@RequestHeader("Accept-Encoding") String encoding,
                              @RequestHeader("Keep-Alive")
long keepAlive)  {

  //...

}

@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：

JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84

JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84

参数绑定的代码：

@RequestMapping("/displayHeaderInfo.do")
public void displayHeaderInfo(@CookieValue("JSESSIONID") String cookie)  {

  //...

}

@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

@RequestParam

A） 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：

@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";
    }

    // ...

@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：

@RequestMapping(value =
"/something", method = RequestMethod.PUT)
public void handle(@RequestBody String body, Writer writer)
throws IOException {
  writer.write(body);
}

@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：

@Controller
@RequestMapping("/editPet.do")
@SessionAttributes("pet")
public class EditPetForm {
    // ...
}

@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：

// 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")

@ModelAttribute
public Account addAccount(@RequestParam String number) {
    return accountManager.findAccount(number);
}

// 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")

@ModelAttribute
public 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：

@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
public String processSubmit(@ModelAttribute Pet pet) {

}

@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：

private 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;
    }

private 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：

@RequestMapping ({"/",
"/home"})
    public String showHomePage(String key){

        logger.debug("key="+key);

        return "home";
    }

@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。

@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";
}

@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。