spring如何知道post请求正文中是否提供了参数值？

Question

I am building web service with spring and come across with following problem. There is a post service as follow.

    @RequestMapping(value = "/postSomething", method = RequestMethod.POST)    
    public ResponseDTO postSomething(@RequestBody ADto aDto){
         //post data
         //return response
    } 

    public class ADto{
       private String firstParam;

       private String secondParam;

       // getter setter
    }

So, my question is how can I know whether value of firstParam and secondParam is provided in request body or not.

   RequestBody: { paramFirst: null, paramSecond: null}

Edit1: Sorry for incomplete question: For RequestBody: {paramFirst: first Value} and for above request value of paramSecond will be null. So, how would I know whether paramSecond is included in request or not.

Edit2: I don't want to validate. What I want to know is whether request contains a particular parameter or not. Because there are two different cases, one is value of a parameter is given null and other is paramter is not included in request.

Answer 1

You could use the @Valid annotation like so (pseudo code, didn't test it):

@RequestMapping(value = "/postSomething", method = RequestMethod.POST)    
public ResponseDTO postSomething(@Valid @RequestBody ADto aDto){
     // MethodArgumentNotValidException will be thrown if validation fails.
} 

You'll need an exception handler to handle the validation error.

@ExceptionHandler
@ResponseBody
@ResponseStatus(value = HttpStatus.BAD_REQUEST)
public Error handleException(MethodArgumentNotValidException exception) {
    //do something with the validation message: exception.getBindingResult()
}

And your class.

public class ADto{

   @NotNull(message = "First parameter can not be null")
   private String firstParam;

   @NotNull(message = "Second parameter can not be null")
   private String secondParam;

   // getter setter
}

Answer 2

Try using Hibernate Validator ( http://hibernate.org/validator/ ), it's really easy to integrate it with Spring. That way, you'll need to annotate your Dto to enforce validation of required params and then call validate.

public class ADto{
   @NotNull
   private String firstParam;
   @NotNull 
   private String secondParam;

   // getter setter
}

@RequestMapping(value = "/postSomething", method = RequestMethod.POST)    
public ResponseDTO postSomething(@RequestBody ADto aDto){
     validator.validate(aDto)
     //post data
     //return response
} 

Answer 3

You could make firstParam and secondParam type Optional :

ADto class

public class ADto {
  private Optional<String> firstParam;
  private Optional<String> secondParam;

  // getter setter
}

postSomething method

@RequestMapping(value = "/postSomething", method = RequestMethod.POST)    
public ResponseDTO postSomething(@RequestBody ADto aDto) {

    if (Optional.ofNullable(aDto.getFirstParam()).isPresent()) {
        // firstParam is provided in the request
    } else {
        // firstParam is not provided in the request
    }

    if (Optional.ofNullable(aDto.getSecondParam()).isPresent()) {
        // secondtParam is provided in the request
    } else {
        // secondtParam is not provided in the request
    }

}

Note that isPresent() will return false if and only if firstParam (as well as for secondParam ) is not present in the request. Otherwise, even if the value is set to null, it will return true.