Detailed explanation of parameter binding annotations such as @RequestParam @RequestBody @PathVariable-javaTutorial-php.cn

Introduction:

handler method Parameter binding commonly used annotations, we divide them into four categories according to the different content parts of the Request they handle: (mainly explaining common types)

A. Processing Annotations for the requet uri part (here refers to the variable in the uri template, excluding the queryString part): @PathVariable;

B. Annotations for processing the request header part: @RequestHeader, @CookieValue;

C , annotations for processing the request body part: @RequestParam, @RequestBody;

D. Annotations for processing attribute types: @SessionAttributes, @ModelAttribute;

1, @PathVariable

When using @RequestMapping URI template style mapping, that is, someUrl/{paramId}, the paramId at this time can be bound to the value passed by it to the parameter of the method through the @Pathvariable annotation.

Sample code:

@Controller 
@RequestMapping("/owners/{ownerId}") 
publicclass RelativePathUriTemplateController
 { 
  
  @RequestMapping("/pets/{petId}") 
  publicvoid findPet(@PathVariable String
 ownerId, @PathVariable String petId,
 Model model) {     
    // implementation
 omitted 
  } 
}

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

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The above code binds the value of the variable ownerId and petId in the URI template to the parameters of the method. If the method parameter name is inconsistent with the variable name in the uri template that needs to be bound, you need to specify the name in the uri template in @PathVariable("name").

2. @RequestHeader, @CookieValue

@RequestHeader annotation can bind the value of the Request header part to the parameters of the method.

Sample code:

This is the header part of a Request:

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

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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

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

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

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The above code binds the value of Accept-Encoding in the request header part to the parameter encoding, and the value of the Keep-Alive header is bound to the parameter keepAlive.

@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：

1
JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84
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1	`JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84`

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参数绑定的代码：

@RequestMapping("/displayHeaderInfo.do") 
publicvoid displayHeaderInfo(@CookieValue("JSESSIONID")
 String cookie)  { 
  
  //... 
  
}

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

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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") 
publicclass 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"; 
    } 
  
    // ...

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@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";
    }
 
    // ...

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@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) 
publicvoid handle(@RequestBody String
 body, Writer writer) throws IOException
 { 
  writer.write(body); 
}

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

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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") 
publicclass EditPetForm
 { 
    // ... 
}

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

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@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); 
}

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// 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);
}

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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)
 { 
     
}

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

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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：

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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++; 
                } 
                elseif (RequestHeader.class.isInstance(paramAnn))
 { 
                    RequestHeader requestHeader = (RequestHeader) paramAnn; 
                    headerName = requestHeader.value(); 
                    required = requestHeader.required(); 
                    defaultValue = parseDefaultValueAttribute(requestHeader.defaultValue()); 
                    annotationsFound++; 
                } 
                elseif (RequestBody.class.isInstance(paramAnn))
 { 
                    requestBodyFound = true; 
                    annotationsFound++; 
                } 
                elseif (CookieValue.class.isInstance(paramAnn))
 { 
                    CookieValue cookieValue = (CookieValue) paramAnn; 
                    cookieName = cookieValue.value(); 
                    required = cookieValue.required(); 
                    defaultValue = parseDefaultValueAttribute(cookieValue.defaultValue()); 
                    annotationsFound++; 
                } 
                elseif (PathVariable.class.isInstance(paramAnn))
 { 
                    PathVariable pathVar = (PathVariable) paramAnn; 
                    pathVarName = pathVar.value(); 
                    annotationsFound++; 
                } 
                elseif (ModelAttribute.class.isInstance(paramAnn))
 { 
                    ModelAttribute attr = (ModelAttribute) paramAnn; 
                    attrName = attr.value(); 
                    annotationsFound++; 
                } 
                elseif (Value.class.isInstance(paramAnn))
 { 
                    defaultValue = ((Value) paramAnn).value(); 
                } 
                elseif (paramAnn.annotationType().getSimpleName().startsWith("Valid"))
 { 
                    validate = true; 
                    Object value = AnnotationUtils.getValue(paramAnn); 
                    validationHints = (value instanceof Object[]
 ? (Object[]) value : new Object[]
 {value}); 
                } 
            } 
  
            if (annotationsFound
 > 1) { 
                thrownew 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; 
                } 
                elseif (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()))
 { 
                            thrownew 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; 
                    } 
                    elseif (SessionStatus.class.isAssignableFrom(paramType))
 { 
                        args[i] = this.sessionStatus; 
                    } 
                    elseif (HttpEntity.class.isAssignableFrom(paramType))
 { 
                        args[i] = resolveHttpEntityRequest(methodParam, webRequest); 
                    } 
                    elseif (Errors.class.isAssignableFrom(paramType))
 { 
                        thrownew IllegalStateException("Errors/BindingResult
 argument declared " + 
                                "without
 preceding model attribute. Check your handler method signature!"); 
                    } 
                    elseif (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); 
            } 
            elseif (headerName
 != null)
 { 
                args[i] = resolveRequestHeader(headerName, required, defaultValue, methodParam, webRequest, handler); 
            } 
            elseif (requestBodyFound)
 { 
                args[i] = resolveRequestBody(methodParam, webRequest, handler); 
            } 
            elseif (cookieName
 != null)
 { 
                args[i] = resolveCookieValue(cookieName, required, defaultValue, methodParam, webRequest, handler); 
            } 
            elseif (pathVarName
 != null)
 { 
                args[i] = resolvePathVariable(pathVarName, methodParam, webRequest, handler); 
            } 
            elseif (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; 
    }

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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;
    }

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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"; 
    }

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@RequestMapping ({"/", "/home"})
public String showHomePage(String key){
logger.debug("key="+key);
return "home";
}

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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"; 
}

Copy after login

@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";
}