CODEWITHSUNDEEP
javajacksonmixinsbuilderjson-deserialization
Deepika Misra
Deepika Misra

Reputation: 33

Jackson deserialization with builder pattern: "Build method has bad return type, not compatible with POJO type"

I have a simple class that uses a 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 object of the interface type and not the implemented type. I then mixin annotations (because the class Foo is actually a 3rd party class) for defining the sub types of the interface 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 mixin for defining the builder pattern of class Foo, like so:

@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 get the error Build method 'Foo$Builder#build(0 params) has bad return type (FooInterface), not compatible with POJO type (Foo)

It looks like this is the culprit, but I don't understand why returning a super type in a build method wont be acceptable. Any ideas?

Upvotes: 3

Views: 3193

Answers (1)

Alexey Gavrilov
Alexey Gavrilov

Reputation: 10853

It looks like a Jackson bug. I've reported an issue: https://github.com/FasterXML/jackson-databind/issues/761

Upvotes: 2

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