Dynamic addition of fasterxml Annotation?

Question

Is there a way to set @JsonProperty annotation dynamically like:

class A {

    @JsonProperty("newB") //adding this dynamically
    private String b;

}

or can I simply rename field of an instance? If so, suggest me an idea. Also, in what way an ObjectMapper can be used with serialization?

Nonoccurrence

Read about MixIn - wiki.fasterxml.com/JacksonMixInAnnotations. Also see this tutorial: wiki.fasterxml.com/JacksonInFiveMinutes – Malapropism

I see the whole class mapping with another JSON or like files, rather I need to change only a particular field of a class to be modified as in @JsonProperty, sorry if I missed anything from your link – Nonoccurrence

@Nonoccurrence you are missing the fact that you can define mix-ins any way you want, and include only those methods, fields, annotations that you want to use as overrides. So, for example, your class A could be mix-in class to use for one or more target classes, to make field "b" be serialized as "newB". – Westley

@Westley Can a serialized result be an Object or it necessarily be a String from writeValueAsString() – Nonoccurrence

@Nonoccurrence I'm sorry, I don't quite understand what you are asking here. If you mean whether field or method signature of mix-in has to be the same, yes. But it sounds like you might be asking something else. – Westley

@Westley Here is my changed question, link – Nonoccurrence

Answer 1

Assume that your POJO class looks like this:

class PojoA {

    private String b;

    // getters, setters
}

Now, you have to create MixIn interface:

interface PojoAMixIn {

    @JsonProperty("newB")
    String getB();
}

Simple usage:

PojoA pojoA = new PojoA();
pojoA.setB("B value");

System.out.println("Without MixIn:");
ObjectMapper mapper = new ObjectMapper();
System.out.println(mapper.writerWithDefaultPrettyPrinter().writeValueAsString(pojoA));

System.out.println("With MixIn:");
ObjectMapper mapperWithMixIn = new ObjectMapper();
mapperWithMixIn.addMixInAnnotations(PojoA.class, PojoAMixIn.class);
System.out.println(mapperWithMixIn.writerWithDefaultPrettyPrinter().writeValueAsString(pojoA));

Above program prints:

Without MixIn:
{
  "b" : "B value"
}
With MixIn:
{
  "newB" : "B value"
}

Helwig

what if I have multiple fields in the class, do I need to add that in Interface too ? – Nonoccurrence

You have to provide only these methods for which you want to change configuration. If you want to update only one property, you have to provide only one method. – Malapropism

Michael, Thanks for your response but my intent is to get back the same object with modification in field name not a String, could you guide me a solution? – Nonoccurrence

Sorry, but I do not understand your problem. Could you update your question and provide the example which shows what do you want to do? – Malapropism

Yes using MixIn's I can actually do the process as you said, but Can I be able to work on object pojoA and return the mixIn pojoA and not a String from writeValueAsString() – Nonoccurrence

Hmm. Yes, you can deserialize returned JSON to PojoA object but why do you want to do this? – Malapropism

Will deserializing maps newB to b? again in need of MixIn, if so how? – Nonoccurrence

Answer 2

this is a very late answer but, if it helps you or others, you should be able to change annotations at runtime. Check this link:

https://www.baeldung.com/java-reflection-change-annotation-params

Modifying annotations might be a bit messy and I prefer other options.

Mixin's are a good static option but if you need to change properties at runtime you can use a custom serializer (or deserializer). Then register your serializer with the ObjectMapper of your choosing (writing formats like json / xml are now provided for free via Jackson). Here are some additional examples:

custom serializer: https://www.baeldung.com/jackson-custom-serialization

custom deserializer: https://www.baeldung.com/jackson-deserialization

i.e.:

    class A {
        //        @JsonProperty("newB") //adding this dynamically
        String b;
    }

    class ASerializer extends StdSerializer<A> {

        public ASerializer() {
            this(null);
        }

        public ASerializer(Class<A> a) {
            super(a);
        }

        @Override
        public void serialize(A a, JsonGenerator gen, SerializerProvider provider) throws IOException {

            if (a == null) {
                gen.writeNull();

            } else {
                gen.writeStartObject();
                gen.writeStringField("newB", a.b);
                gen.writeEndObject();
            }
        }
    }

    @Test
    public void test() throws JsonProcessingException {
        A a = new A();
        a.b = "bbb";
        String exp = "{\"newB\":\"bbb\"}";

        ObjectMapper mapper = new ObjectMapper();

        SimpleModule module = new SimpleModule();
        module.addSerializer(A.class, new ASerializer());
        mapper.registerModule(module);

        assertEquals(exp, mapper.writeValueAsString(a));
    }

Lymphoblast