How can I include raw JSON in an object using Jackson?

I am trying to include raw JSON inside a Java object when the object is (de)serialized using Jackson. In order to test this functionality, I wrote the following test:

public static class Pojo {
	public String foo;
	
	@JsonRawValue
	public String bar;
}

@Test
public void test() throws JsonGenerationException, JsonMappingException, IOException {
	
	String foo = "one";
	String bar = "{\"A\":false}";

	Pojo pojo = new Pojo();
	pojo.foo = foo;
	pojo.bar = bar;

	String json = "{\"foo\":\"" + foo + "\",\"bar\":" + bar + "}";
	
	ObjectMapper objectMapper = new ObjectMapper();
	String output = objectMapper.writeValueAsString(pojo);
	System.out.println(output);
	assertEquals(json, output);
	
	Pojo deserialized = objectMapper.readValue(output, Pojo.class);
	assertEquals(foo, deserialized.foo);
	assertEquals(bar, deserialized.bar);
}

The code outputs the following line:

{"foo":"one","bar":{"A":false}}

The JSON is exactly how I want things to look. Unfortunately, the code fails with an exception when attempting to read the JSON back in to the object. Here is the exception:

> org.codehaus.jackson.map.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token at [Source: java.io.StringReader@d70d7a; line: 1, column: 13] (through reference chain: com.tnal.prism.cobalt.gather.testing.Pojo["bar"])

Why does Jackson function just fine in one direction but fail when going the other direction? It seems like it should be able to take its own output as input again. I know what I'm trying to do is unorthodox (the general advice is to create an inner object for bar that has a property named A), but I don't want to interact with this JSON at all. My code is acting as a pass-through for this code -- I want to take in this JSON and send it back out again without touching a thing, because when the JSON changes I don't want my code to need modifications.

Thanks for the advice.

EDIT: Made Pojo a static class, which was causing a different error.

@JsonRawValue is intended for serialization-side only, since the reverse direction is a bit trickier to handle. In effect it was added to allow injecting pre-encoded content.

I guess it would be possible to add support for reverse, although that would be quite awkward: content will have to be parsed, and then re-written back to "raw" form, which may or may not be the same (since character quoting may differ). This for general case. But perhaps it would make sense for some subset of problems.

But I think a work-around for your specific case would be to specify type as 'java.lang.Object', since this should work ok: for serialization, String will be output as is, and for deserialization, it will be deserialized as a Map. Actually you might want to have separate getter/setter if so; getter would return String for serialization (and needs @JsonRawValue); and setter would take either Map or Object. You could re-encode it to a String if that makes sense.

Following @StaxMan answer, I've made the following works like a charm:

public class Pojo {
  Object json;

  @JsonRawValue
  public String getJson() {
    // default raw value: null or "[]"
    return json == null ? null : json.toString();
  }

  public void setJson(JsonNode node) {
    this.json = node;
  }
}

And, to be faithful to the initial question, here is the working test:

public class PojoTest {
  ObjectMapper mapper = new ObjectMapper();

  @Test
  public void test() throws IOException {
    Pojo pojo = new Pojo("{\"foo\":18}");

    String output = mapper.writeValueAsString(pojo);
    assertThat(output).isEqualTo("{\"json\":{\"foo\":18}}");

    Pojo deserialized = mapper.readValue(output, Pojo.class);
    assertThat(deserialized.json.toString()).isEqualTo("{\"foo\":18}");
    // deserialized.json == {"foo":18}
  }
}

I was able to do this with a custom deserializer (cut and pasted from here)

package etc;

import java.io.IOException;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.TreeNode;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;

/**
 * Keeps json value as json, does not try to deserialize it
 * @author roytruelove
 *
 */
public class KeepAsJsonDeserializer extends JsonDeserializer<String> {
	
	@Override
	public String deserialize(JsonParser jp, DeserializationContext ctxt)
			throws IOException {
		
		TreeNode tree = jp.getCodec().readTree(jp);
		return tree.toString();
	}
}

Use it by annotating the desired member like this:

@JsonDeserialize(using = KeepAsJsonDeserializer.class)
private String value;

@JsonSetter may help. See my sample ('data' is supposed to contain unparsed JSON):

class Purchase
{
    String data;

    @JsonProperty("signature")
    String signature;

    @JsonSetter("data")
    void setData(JsonNode data)
    {
        this.data = data.toString();
    }
}

This is a problem with your inner classes. The Pojo class is a non-static inner class of your test class, and Jackson cannot instantiate that class. So it can serialize, but not deserialize.

Redefine your class like this:

public static class Pojo {
    public String foo;

    @JsonRawValue
    public String bar;
}

Note the addition of static

Adding to Roy Truelove's great answer, this is how to inject the custom deserialiser in response to appearance of @JsonRawValue:

import com.fasterxml.jackson.databind.Module;

@Component
public class ModuleImpl extends Module {

    @Override
    public void setupModule(SetupContext context) {
        context.addBeanDeserializerModifier(new BeanDeserializerModifierImpl());
    }
}

---

import java.util.Iterator;

import com.fasterxml.jackson.annotation.JsonRawValue;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.DeserializationConfig;
import com.fasterxml.jackson.databind.deser.BeanDeserializerBuilder;
import com.fasterxml.jackson.databind.deser.BeanDeserializerModifier;
import com.fasterxml.jackson.databind.deser.SettableBeanProperty;

public class BeanDeserializerModifierImpl extends BeanDeserializerModifier {
    @Override
    public BeanDeserializerBuilder updateBuilder(DeserializationConfig config, BeanDescription beanDesc, BeanDeserializerBuilder builder) {
        Iterator<SettableBeanProperty> it = builder.getProperties();
        while (it.hasNext()) {
            SettableBeanProperty p = it.next();
            if (p.getAnnotation(JsonRawValue.class) != null) {
                builder.addOrReplaceProperty(p.withValueDeserializer(KeepAsJsonDeserialzier.INSTANCE), true);
            }
        }
        return builder;
    }
}

This easy solution worked for me:

public class MyObject {
    private Object rawJsonValue;

    public Object getRawJsonValue() {
        return rawJsonValue;
    }

    public void setRawJsonValue(Object rawJsonValue) {
        this.rawJsonValue = rawJsonValue;
    }
}

So I was able to store raw value of JSON in rawJsonValue variable and then it was no problem to deserialize it (as object) with other fields back to JSON and send via my REST. Using @JsonRawValue didnt helped me because stored JSON was deserialized as String, not as object, and that was not what I wanted.

This even works in a JPA entity:

private String json;

@JsonRawValue
public String getJson() {
    return json;
}

public void setJson(final String json) {
    this.json = json;
}

@JsonProperty(value = "json")
public void setJsonRaw(JsonNode jsonNode) {
    // this leads to non-standard json, see discussion: 
    // setJson(jsonNode.toString());
    
    StringWriter stringWriter = new StringWriter();
    ObjectMapper objectMapper = new ObjectMapper();
    JsonGenerator generator = 
      new JsonFactory(objectMapper).createGenerator(stringWriter);
    generator.writeTree(n);
    setJson(stringWriter.toString());
}

Ideally the ObjectMapper and even JsonFactory are from the context and are configured so as to handle your JSON correctly (standard or with non-standard values like 'Infinity' floats for example).

Here is a full working example of how to use Jackson modules to make @JsonRawValue work both ways (serialization and deserialization):

public class JsonRawValueDeserializerModule extends SimpleModule {

    public JsonRawValueDeserializerModule() {
        setDeserializerModifier(new JsonRawValueDeserializerModifier());
    }

    private static class JsonRawValueDeserializerModifier extends BeanDeserializerModifier {
        @Override
        public BeanDeserializerBuilder updateBuilder(DeserializationConfig config, BeanDescription beanDesc, BeanDeserializerBuilder builder) {
            builder.getProperties().forEachRemaining(property -> {
                if (property.getAnnotation(JsonRawValue.class) != null) {
                    builder.addOrReplaceProperty(property.withValueDeserializer(JsonRawValueDeserializer.INSTANCE), true);
                }
            });
            return builder;
        }
    }

    private static class JsonRawValueDeserializer extends JsonDeserializer<String> {
        private static final JsonDeserializer<String> INSTANCE = new JsonRawValueDeserializer();

        @Override
        public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
            return p.readValueAsTree().toString();
        }
    }
}

Then you can register the module after creating the ObjectMapper:

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new JsonRawValueDeserializerModule());

String json = "{\"foo\":\"one\",\"bar\":{\"A\":false}}";
Pojo deserialized = objectMapper.readValue(json, Pojo.class);

I had the exact same issue. I found the solution in this post : https://stackoverflow.com/questions/15659950/parse-json-tree-to-plain-class-using-jackson-or-its-alternatives

Check out the last answer. By defining a custom setter for the property that takes a JsonNode as parameter and calls the toString method on the jsonNode to set the String property, it all works out.

Using an object works fine both ways... This method has a bit of overhead deserializing the raw value in two times.

ObjectMapper mapper = new ObjectMapper();
RawJsonValue value = new RawJsonValue();
value.setRawValue(new RawHello(){{this.data = "universe...";}});
String json = mapper.writeValueAsString(value);
System.out.println(json);
RawJsonValue result = mapper.readValue(json, RawJsonValue.class);
json = mapper.writeValueAsString(result.getRawValue());
System.out.println(json);
RawHello hello = mapper.readValue(json, RawHello.class);
System.out.println(hello.data);

RawHello.java

public class RawHello {

    public String data;
}

RawJsonValue.java

public class RawJsonValue {

    private Object rawValue;

    public Object getRawValue() {
        return rawValue;
    }

    public void setRawValue(Object value) {
        this.rawValue = value;
    }
}

I had a similar problem, but using a list with a lot of JSON itens (List<String>).

public class Errors {
    private Integer status;
    private List<String> jsons;
}

I managed the serialization using the @JsonRawValue annotation. But for deserialization I had to create a custom deserializer based on Roy's suggestion.

public class Errors {

    private Integer status;

    @JsonRawValue
    @JsonDeserialize(using = JsonListPassThroughDeserialzier.class)
    private List<String> jsons;

}

Below you can see my "List" deserializer.

public class JsonListPassThroughDeserializer extends JsonDeserializer<List<String>> {

    @Override
    public List<String> deserialize(JsonParser jp, DeserializationContext cxt) throws IOException, JsonProcessingException {
        if (jp.getCurrentToken() == JsonToken.START_ARRAY) {
            final List<String> list = new ArrayList<>();
            while (jp.nextToken() != JsonToken.END_ARRAY) {
                list.add(jp.getCodec().readTree(jp).toString());
            }
            return list;
        }
        throw cxt.instantiationException(List.class, "Expected Json list");
    }
}