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I am setting up a controller for a Spring Boot application. I want the controller to produce an instance of an implementation of an interface based on the key-value pairs of the incoming JSON string.

The interface:

public interface ShoppableItem {}

Implementation 1:

public class GroceryItem implements ShoppableItem {



@JsonProperty("transactionId")

private Long transactionId;



@JsonProperty

@NotNull

private String itemName;



@JsonProperty

@NotNull

private Float weight;

}

Implementation 2:

public class ClothingItem implements ShoppableItem {



@JsonProperty("transactionId")

private Long transactionId;



@JsonProperty

@NotNull

private String itemName;



}

(Note: If a weight is provided, it is a grocery item and it cannot be null or "". If a weight is not provided, then it is a clothing item).

Controller

@Controller

public class Total {



  @PostMapping(value = "total")

  @ResponseStatus(HttpStatus.ACCEPTED)

  public String tester(@Valid @RequestBody ShoppableItem shoppableItem) {

    return shoppableItem.toString();

  }

}

JSON body

{

"transactionId" : "123",

"itemName" : "whatever"

}

The desired result was to create an instance of type ClothingItem. When I tried doing this, I get the following exception:

org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Can not construct instance of com.test.controllers.Models.ShoppableItem: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information; nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.test.controllers.Models.ShoppableItem: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information

Is this a feature I am implementing incorrectly? I understand this may not be the best approach, as it would be easier to just create 2 endpoints to handle each case.

You can achieve this by adding a static factory method
(annotated with @JsonCreator) to your ShoppableItem Interface.
Note that you need to annotate all of its arguments with @JsonProperty
so that Jackson will know how to map the JSON properties to method arguments.

public interface ShoppableItem {



    @JsonCreator

    public static ShoppableItem create(@JsonProperty("transactionId") Long transactionId,

                                       @JsonProperty("itemName") String itemName,

                                       @JsonProperty("weight") Float weight) {

        if (weight == null)

            return new ClothingItem(transactionId, itemName);

        else

            return new GroceryItem(transactionId, itemName, weight);

    }

}

Then Jackson will use that method when deserializing a ShoppableItem.