How deserialize json object array as array of json strings?

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Consider json input:

{
    companies: [
        {
            "id": 1,
            "name": "name1"
        },
        {
            "id": 1,
            "name": "name1"
        }
    ],
    nextPage: 2
}

How deserialize this into class:

public class MyClass {
    List<String> companies;
    Integer nextPage;
}

Where List<String> companies; consists of strings:

{"id": 1,"name": "name1"}
{"id": 1,"name": "name1"}

@JsonRawValue doesn't work for List<String> companies;

Is there a way to configure Jackson serialization to keep companies array with raw json string with annotations only? (E.g. without writing custom deserializator)

jackson json java
1 Answers

There is no annotation-only solution for your problem. Somehow you have to convert JSON Object to java.lang.String and you need to specify that conversion.

You can:

  1. Write custom deserializer which is probably most obvious solution but forbidden in question.
  2. Register custom com.fasterxml.jackson.databind.deser.DeserializationProblemHandler and handle com.fasterxml.jackson.databind.exc.MismatchedInputException situation in more sophisticated way.
  3. Implement com.fasterxml.jackson.databind.util.Converter interface and convert JsonNode to String. It is semi-annotational way to solve a problem but we do not implement the worst part - deserialisation.

Let's go to point 2. right away.

2. DeserializationProblemHandler

Solution is pretty simple:

ObjectMapper mapper = new ObjectMapper();
mapper.addHandler(new DeserializationProblemHandler() {
    @Override
    public Object handleUnexpectedToken(DeserializationContext ctxt, JavaType targetType, JsonToken t, JsonParser p, String failureMsg) throws IOException {
        if (targetType.getRawClass() == String.class) {
            // read as tree and convert to String
            return p.readValueAsTree().toString();
        }
        return super.handleUnexpectedToken(ctxt, targetType, t, p, failureMsg);
    }
});

Read a whole piece of JSON as TreeNode and convert it to String using toString method. Helpfully, toString generates valid JSON. Downside, this solution has a global scope for given ObjectMapper instance.

3. Custom Converter

This solution requires to implement com.fasterxml.jackson.databind.util.Converter interface which converts com.fasterxml.jackson.databind.JsonNode to String:

class JsonNode2StringConverter implements Converter<JsonNode, String> {
    @Override
    public String convert(JsonNode value) {
        return value.toString();
    }

    @Override
    public JavaType getInputType(TypeFactory typeFactory) {
        return typeFactory.constructType(new TypeReference<JsonNode>() {
        });
    }

    @Override
    public JavaType getOutputType(TypeFactory typeFactory) {
        return typeFactory.constructType(new TypeReference<String>() {
        });
    }
}

and now, you can use annotation like below:

@JsonDeserialize(contentConverter = JsonNode2StringConverter.class)
private List<String> companies;

Solutions 2. and 3. solve this problem almost in the same way - read node and convert it back to JSON, but uses different approaches.

If, you want to avoid deserialising and serialising process you can take a look on solution provided in this article: Deserializing JSON property as String with Jackson and take a look at:

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