杰克逊：使用类方法序列化POJO

Question

I know I can use (new ObjectMapper()).convertValue(this, ObjectNode.class) to serialise given object to JSON . Is there a way to tell the lib to use a method inside that class to serialise it ?

My classes:

interface A {
  ObjectNode toJSON();
}

abstract class Tokens {
  Map<String, String> tokends;
}

class Foo extends Tokens implements A {
  private long id;
  private Bar bar;

  // Constructors, getters & setters...

  @Override
  public ObjectNode toJSON() {
    ObjectNode node = (new ObjectMapper()).convertValue(this, ObjectNode.class);
    tokens.forEach(node::put);

    return node;
  }
}

class Bar extends Tokens implements A {
  private long id;

  // Constructors, getters & setters...

  @Override
  public ObjectNode toJSON() {
    ObjectNode node = (new ObjectMapper()).convertValue(this, ObjectNode.class);
    tokens.forEach(node::put);

    return node;
  }
}

Now if I call (new A()).toJSON() I will get:

{
  "id": 1,
  "bar": {
    "id": 2
  },
  "token1": "...",
  "token2": "...:,
}

Instead of:

{
  "id": 1,
  "bar": {
    "id": 2,
    "token3": "...",
    "token4": "..."
  },
  "token1": "...",
  "token2": "...:,
}

I understand that this happens because the first method modifies the serialised class but is not called by jackson when it goes into child objects.

Is there a way to tell jaskson to use toJSON method to create the child with tokens as well ?

Answer 1

To take a slightly different approach, @JsonAnyGetter provides a way to include the keys of the tokens map directly in the parent JSON:

abstract class Tokens {
    Map<String, String> tokens = new LinkedHashMap<>();

    @JsonAnyGetter
    public Map<String, String> getTokens() {
        return tokens;
    }

    @JsonAnySetter
    public void addToken(String key, String value) {
        tokens.put(key, value);
    }

}

Then such as:

new ObjectMapper().writeValueAsString(foo);

returns:

{
  "id" : 1,
  "bar" : {
    "id" : 2,
    "token3" : "three",
    "token4" : "four"
  },
  "token1" : "one",
  "token2" : "two"
}

Answer 2

Try the Jackson annotation @com.fasterxml.jackson.annotation.JsonValue on your toJSON() method. From the javadoc :

Marker annotation similar to javax.xml.bind.annotation.XmlValue that indicates that results of the annotated "getter" method (which means signature must be that of getters; non-void return type, no args) is to be used as the single value to serialize for the instance. Usually value will be of a simple scalar type (String or Number), but it can be any serializable type (Collection, Map or Bean).