An empty JSON field which is a boolean/nullable field in Java model, is getting converted as null

Question

I have a specific requirement where in,

• if a (boolean type) field in the request JSON body is absent altogether, then the request is valid.
• If this field is set with a boolean value (true or false), then it is obviously valid.
• If the field has non-boolean values, it should throw an exception.
• If the field has empty value, it should throw an exception.

Even though the Java model throws an exception if the field is non-boolean by type checking, the empty value is being converted as null. Instead I need to be able to distinguish it and throw an exception. How can achieve this?

Here's how I defined the post body model and I am using the AutoValue generator so I don't have the setter function for the fields written by hand. Also, I am using the fasterxml library.

@JsonProperty("indicator")
@Nullable
public abstract Boolean getIndicator();

I have tried defining a custom annotation and writing the logic to check if the value is empty, but it didn't work.

Answer 1

You need to disable MapperFeature.ALLOW_COERCION_OF_SCALARS feature which allows coercion in cases like you have. See below example:

import com.fasterxml.jackson.databind.MapperFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        String json = "{\"indicator\":\"\"}";

        ObjectMapper mapper = new ObjectMapper();
        mapper.disable(MapperFeature.ALLOW_COERCION_OF_SCALARS);
        System.out.println(mapper.readValue(json, BaseClass.class));
    }
}

class BaseClass {
    private Boolean indicator;

    public Boolean getIndicator() {
        return indicator;
    }

    public void setIndicator(Boolean indicator) {
        this.indicator = indicator;
    }

    @Override
    public String toString() {
        return "BaseClass{" +
                "indicator=" + indicator +
                '}';
    }
}

Above code prints:

Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot coerce empty String ("") to Null value for type java.lang.Boolean (enable MapperFeature.ALLOW_COERCION_OF_SCALARS to allow) at [Source: (String)"{"indicator":""}"; line: 1, column: 14] (through reference chain: BaseClass["indicator"])