Support for @JsonCreator(mode = Mode.DELEGATING)

[nfekete]

There's a pattern for json serialization through Jackson that Immutables is using that works in Jackson but it's not currently supported by gwt-jackson.

The Immutables pattern looks like this. You define an abstract class/interface, annotate it with @Immutable, and Immutables will generate an immutable implementation of the abstract class/interface with Jackson serialization and deserialization support (in case Jackson is found on the classpath).

From an interface similar to this:

@JsonTypeInfo( use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY)
@JsonSerialize
public interface Person {
    String getName();
    ImmutablePerson withName(String name);
}

it generates a class similar to this (ommitting the things irrelevant to this issue):

public static class ImmutablePerson implements Person {

    private final String name;
    
    private ImmutablePerson(String name) {
        this.name = name;
    }

    @Override
    public String getName() {
        return name;
    }
    
    @Override
    public ImmutablePerson withName(String name) {
        return new ImmutablePerson(name);
    }
    
    @JsonCreator(mode = Mode.DELEGATING)
    static ImmutablePerson fromJson(MutablePerson delegate) {
        return new ImmutablePerson(delegate.name);
    }
    
    @JsonTypeInfo(use=Id.NONE)
    @JsonDeserialize
    @JsonAutoDetect(fieldVisibility = Visibility.NONE)
    static class MutablePerson implements Person {
        
        String name;
        
        @Override
        public String getName() {
            throw new UnsupportedOperationException();
        }
        public void setName(String name) {
            this.name = name;
        }
        @Override
        public ImmutablePerson withName(String name) {
            throw new UnsupportedOperationException();
        }
    } 
}

Note the @JsonCreator pattern that it's using on the fromJson method. It basically tells Jackson, that whenever the ImmutablePerson class is encountered when deserializing, it should use that factory method by deserializing the current JSON object under deserialization and calling the factory method with the delegate deserialized object as the argument.

Note that the json type that is used as a delegate here is marked with @JsonTypeInfo(use=Id.NONE), and this is actually important, as only types declared like this are usable for @JsonCreator(mode = Mode.DELEGATING) factory method. gwt-jackson doesn't support this use case, it will fail on

gwt-jackson/gwt-jackson/src/main/java/com/github/nmorel/gwtjackson/rebind/bean/BeanProcessor.java

Lines 459 to 461 in 4ba7f99

	default:
	logger.log( TreeLogger.Type.ERROR, "JsonTypeInfo.Id." + typeInfo.use() + " is not supported" );
	throw new UnableToCompleteException();

I will provide a PR with a JUnit test for this issue.