杰克逊反序列化缺少的默认值

Question

I try to deserialize a JSON object that I receive in my API using the following code:

ObjectMapper mapper = new ObjectMapper();
ExampleDto ed = mapper.readValue(req.body(), ExampleDto.class);

My class uses Lombok to generate constructors, getters and setters, and looks like this:

@Data
@AllArgsConstructor
@NoArgsConstructor
public class ExampleDto {
    private String name = "";
    private List<String> values = new LinkedList<>();
}

Both properties should be optional, and use the default value specified in the class definition if they are not provided. However, if I now try to deserialize the JSON

{name: "Foo"}

the values field is null . From my understanding, and all example code I found, values should be an empty list.

Edit: Not a duplicate, as I'm using Lombok without Optionals

Answer 1

@AllArgsConstructor creates the following constructor

@ConstructorProperties({"name", "values"})
ExampleDto(String name, List<String> values) {
    this.name = name;
    this.values = values;
}

The constructor is annotated with @ConstructorProperties which means a property-based creator (argument-taking constructor or factory method) is available to instantiate values from JSON object so jackson-databind uses this constructor to instantiate an object from ExampleDto class.

When the following line executes

mapper.readValue("{\"name\": \"Foo\"}", ExampleDto.class);

because there's no value for values in the provided JSON, null is passed for the second argument when the constructor is invoked.

If you remove @AllArgsConstructor annotation jackson-databind would use setter methods to initialize the object and in this case values would not be null