如何获得Jackson JsonTypeInfo以使用相对路径

Question

I want to use JsonTypeInfo to for Polymorphic Type Handling, but JsonTypeInfo.Id.MINIMAL_CLASS doesn't behave the way that I think it should. Here is the setup.

    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.9.3</version>
    </dependency>

Base Class:

package foo.jackson;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
@JsonTypeInfo(
              use=JsonTypeInfo.Id.MINIMAL_CLASS,
              include=JsonTypeInfo.As.WRAPPER_OBJECT )
public class Example {
    protected String    example = "_example";
    public String getExample()                  { return example; }
    public void   setExample(String example)    { this.example = example; }
}

Child Class:

package foo.jackson.stuff;
public class Car extends foo.jackson.Example {
    protected   String  make = "Duesenberg";
    public String getMake()             { return make; }
    public void   setMake(String make)  { this.make = make; }
}

Invocation:

    foo.jackson.Example ex  = new foo.jackson.Example();
    foo.jackson.Example car = new foo.jackson.stuff.Car();

    try {
        ObjectMapper json   = new ObjectMapper();

        log.info("Base   {}",json.writeValueAsString(ex));
        String  stCar   = json.writeValueAsString(car);
        log.info("Child  {}",stCar);

        foo.jackson.Example parse   = json.readValue(stCar, foo.jackson.Example.class);
        log.info( "Got {}", parse.getClass().getCanonicalName() );
    }
    catch (JsonProcessingException jpe) {
        log.error("Whoops {}", jpe.toString());
    }

Which generates:

Base   {".Example":{"example":"_example"}}
Child  {".Car":{"example":"_example","make":"Duesenberg"}}
Whoops com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Could not resolve type id 'foo.jackson.Car' as a subtype of [simple type, class foo.jackson.Example]: no such class found

If I change to JsonTypeInfo.Id.CLASS, then I get:

Base   {"foo.jackson.Example":{"example":"_example"}}
Child  {"foo.jackson.stuff.Car":{"example":"_example","make":"Duesenberg"}}
Got foo.jackson.stuff.Car

My interpretation of the Jackson Documentation is that MINIMAL_CLASS is supposed to use the relative path and I should get:

Base   {".Example":{"example":"_example"}}
Child  {".stuff.Car":{"example":"_example","make":"Duesenberg"}}
Got foo.jackson.stuff.Car

1. Am I missing a step?
2. Did I misinterpret the Per-class annotations documentation?
3. Is this a Jackson issue and I should just use JsonTypeInfo.Id.CLASS?

Answer 1

Looks like I was missing a step. If I use writerWithType(foo.jackson.Example.class) when serializing, then I get my expected behavior:

    foo.jackson.Example ex  = new foo.jackson.Example();
    foo.jackson.Example car = new foo.jackson.stuff.Car();

    try {
        ObjectMapper json   = new ObjectMapper();

        log.info("Base   {}",json.writeValueAsString(ex));
        String  stCar   = json
            .writerWithType(foo.jackson.Example.class)
            .writeValueAsString(car);
        log.info("Child  {}",stCar);

        foo.jackson.Example parse   = json.readValue(stCar, foo.jackson.Example.class);
        log.info( "Got {}", parse.getClass().getCanonicalName() );
    }
    catch (JsonProcessingException jpe) {
        log.error("Whoops {}", jpe.toString());

generates:

Base   {".Example":{"example":"_example"}}
Child  {".stuff.Car":{"example":"_example","make":"Duesenberg"}}
Got foo.jackson.stuff.Car