Dynamic addition of fasterxml Annotation?

问题

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?

回答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"
}

回答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));
    }

来源：https://stackoverflow.com/questions/25290915/dynamic-addition-of-fasterxml-annotation

标签

java

serialization

jackson

fasterxml