Spring Boot serialize parameteried type into JSON with type id

Spring Boot sample project:
https://github.com/yejianfengblue/spring-boot-jackson-serialize-generic-type

I have question in how Spring Boot serializes generic type into JSON with Jackson library. Please see my demo:

Fruit is a common interface of Apple and Banana.

Fruit

Apple

Banana

BasketController has 3 RESTful API:

BasketController

getFruitList()

List<Fruit>

getFruitBasket()

FruitBasket

List<Fruit>

getBasketOfFruit()

Basket<Fruit>

List<T>

In BasketController, I log the JSON string written from Jackson ObjectMapper and ObjectWriter. Please see the actual output in the comment below the log statement.

BasketController

BasketController.java

/** <li>Object<b>Mapper</b>.writeValueAsString, @type is lost due to java type erasure.
* <li>Object<b>Writer</b>.writeValueAsString, @type is kept */
@GetMapping(path = "getFruitList")
public List<Fruit> getFruitList() throws JsonProcessingException {

List<Fruit> fruitList = Arrays.asList(new Apple(1), new Banana(2));
log.info("List<Fruit> mapper.writeValueAsString: {}", mapper.writeValueAsString(fruitList));
// log: List<Fruit> mapper.writeValueAsString: [{"wgt":1},{"wgt":2}]
ObjectWriter writer = mapper.writerFor(new TypeReference<List<Fruit>>() {});
log.info("List<Fruit> writer.writeValueAsString: {}", writer.writeValueAsString(fruitList));
// log: List<Fruit> writer.writeValueAsString: [{"@type":"Apple","wgt":1},{"@type":"Banana","wgt":2}]
return fruitList;
}

/** Object<b>Mapper</b>.writeValueAsString, with a wrapper of {@code List<Fruit>}, @type is kept */
@GetMapping(path = "getFruitBasket")
public FruitBasket getFruitBasket() throws JsonProcessingException {

FruitBasket fruitBasket = new FruitBasket();
fruitBasket.getItems().add(new Apple(3));
fruitBasket.getItems().add(new Banana(4));
log.info("FruitBasket mapper.writeValueAsString: {}", mapper.writeValueAsString(fruitBasket));
// log: FruitBasket mapper.writeValueAsString: {"items":[{"@type":"Apple","wgt":3},{"@type":"Banana","wgt":4}]}
return fruitBasket;
}

/** <li>Object<b>Mapper</b>.writeValueAsString, @type is lost due to java type erasure.
* <li>Object<b>Writer</b>.writeValueAsString, @type is kept */
@GetMapping(path = "getBasketOfFruit")
public Basket<Fruit> getBasketOfFruit() throws JsonProcessingException {

Basket<Fruit> basketOfFruit = new Basket<Fruit>();
basketOfFruit.getItems().add(new Apple(5));
basketOfFruit.getItems().add(new Banana(6));
log.info("Basket<Fruit> mapper.writeValueAsString: {}", mapper.writeValueAsString(basketOfFruit));
// log: Basket<Fruit> mapper.writeValueAsString: {"items":[{"wgt":5},{"wgt":6}]}
ObjectWriter writer = mapper.writerFor(new TypeReference<Basket<Fruit>>() {});
log.info("Basket<Fruit> writer.writeValueAsString: {}", writer.writeValueAsString(basketOfFruit));
// log: Basket<Fruit> writer.writeValueAsString: {"items":[{"@type":"Apple","wgt":5},{"@type":"Banana","wgt":6}]}
return basketOfFruit;
}


I know Java type erasure that List<Fruit> is treated as List<?> in run-time.

List<Fruit>

List<?>

SpringBootBlueApplicationTests tests the 3 RESTful API in order, and prints the JSON string serialized by Spring Boot:

SpringBootBlueApplicationTests

testGetFruitList()

testGetFruitBasket()

testGetBasketOfFruit()

SpringBootBlueApplicationTests.java

@Autowired
private TestRestTemplate restTemplate;

/** Same json string as Object<b>Writer</b> writes with @type in BasketController.getFruitList() */
@Test
public void testGetFruitList() {

log.info("/getFruitList JSON: {}", restTemplate.getForObject("/getFruitList", String.class));
// log: /getFruitList JSON: [{"@type":"Apple","wgt":1},{"@type":"Banana","wgt":2}]
}

/** Same json string as Object<b>Mapper</b> writes with @type in BasketController.getFruitBasket() */
@Test
public void testGetFruitBasket() {

log.info("/getFruitBasket JSON: {}", restTemplate.getForObject("/getFruitBasket", String.class));
// log: /getFruitBasket JSON: {"items":[{"@type":"Apple","wgt":3},{"@type":"Banana","wgt":4}]}
}

/** Same json string as Object<b>Mapper</b> writes without @type in BasketController.getBasketOfFruit().
* What I want is the string as Object<b>Writer</b> writes with @type */
@Test
public void testGetBasketOfFruit() {

log.info("/getBasketOfFruit JSON: {}", restTemplate.getForObject("/getBasketOfFruit", String.class));
// log: /getBasketOfFruit JSON: {"items":[{"wgt":5},{"wgt":6}]}
}


During JSON serialization in RESTful API 1 getFruitList(), Spring Boot knows the actual runtime type Fruit of List<Fruit>.

getFruitList()

Fruit

List<Fruit>

Question:

How can I make Spring Boot serializes Basket<Fruit> in RESTful API 3 getBasketOfFruit, so that it contains @type?

Basket<Fruit>

getBasketOfFruit

I find a reference about Custom JSON Serializers and Deserializers in https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#boot-features-json-components.
But I have no clue in how to write the override method serialize().

serialize()

Preliminary Solution

Branch prelim_solution_with_jsonserializer https://github.com/yejianfengblue/spring-boot-jackson-serialize-generic-type/tree/prelim_solution_with_jsonserializer

I register 2 JsonSerializers and now Basket<Fruit> can be serialized with @type, but I don't know whether it's the best way to do it.

Basket<Fruit>

BasketJsonSerializer.java

@JsonComponent
public class BasketJsonSerializer {

public static class Serializer extends JsonSerializer<Basket<Fruit>> {
@Override
public void serialize(Basket<Fruit> value, JsonGenerator gen, SerializerProvider serializers)
throws IOException, JsonProcessingException {
gen.writeStartObject();
gen.writeObjectField("items", value.getItems());
gen.writeEndObject();
}
}
}


FruitListJsonSerializer.java

@JsonComponent
public class FruitListJsonSerializer {

public static class Serializer extends JsonSerializer<List<Fruit>> {
@Override
public void serialize(List<Fruit> list, JsonGenerator gen, SerializerProvider serializers)
throws IOException, JsonProcessingException {
gen.writeStartArray();
for (Fruit fruit : list)
gen.writeObject(fruit);
gen.writeEndArray();
}
}
}


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