Jackson deserialization with builder pattern: "Build method has wrong return type, incompatible with POJO type"

I have a simple class that uses the builder pattern.

public class Foo implements FooInterface {
  .....
  public static final class Builder {
     public Builder setValueA(String value) {...}
     public Builder setValueB(String value) {...}
     public FooInterface build() {...}
  }
}

      

        
        
        
      

    

Note that my Builder.build () method actually returns an interface type object, not an implemented type. Then I mix annotations (because the Foo class is actually a third-party class) to subtype the FooInterface like so:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = Foo.class, name = "foo")})
public abstract class FooInterfaceMixin {}

      

        
        
        
      

    

And a mixin to define a Foo class builder template, for example:

@JsonTypeName("foo")
@JsonDeserialize(builder=Foo.Builder.class)
public abstract class FooMixin {}

      

        
        
        
      

    

Finally, to deserialize, I do:

ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(FooInterface.class, FooInterfaceMixin.class);
mapper.addMixIn(Foo.class, FooMixin.class);

final String jsonString = "{\"type\":\"foo\", \"valueA\":\"a\", \"valueB\":\"b\"}";
FooInterface foo = mapper.readValue(jsonString, FooInterface.class);

      

        
        
        
      

    

I am getting the error Build method 'Foo $ Builder # build (0 params) has wrong return type (FooInterface), not POJO compatible type (Foo)

It looks like this one is the culprit, but I don't understand why returning a supertype in an assembly method would not be acceptable. Any ideas?