Polymorphic deserialization causes IllegalArgumentException in Jackson 2.8.1

There seems to be an issue trying to deserialize an object after serialization. The error message is odd because it says that the parent is not a subtype of the child, which is correct because that is of course backwards to what the relationship actually is.

Here is a very simple test case that works fine with Jackson 2.7.5 but is broken with Jackson 2.8.1.

package jackson.test;

import org.junit.Test;
import com.fasterxml.jackson.databind.ObjectMapper;

public class BrokenJackson {

    @Test
    public void test() throws Exception {
        ObjectMapper objectMapper = new ObjectMapper();
        final JacksonChild childObject = new JacksonChild();

        String value = objectMapper.writeValueAsString(childObject);
        objectMapper.readValue(value, JacksonChild.class);
    }
}

package jackson.test;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_EMPTY)
public class JacksonChild extends JacksonParent {

}

package jackson.test;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY, property = "type",
    defaultImpl = JacksonParent.class)
@JsonSubTypes({ @JsonSubTypes.Type(value = JacksonChild.class, name = "child") })
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_EMPTY)
public class JacksonParent {

}

If you run the test case with these 3 classes, the resulting stacktrace is:

java.lang.IllegalArgumentException: Class jackson.test.JacksonParent not subtype of [simple type, class jackson.test.JacksonChild]
    at com.fasterxml.jackson.databind.type.TypeFactory.constructSpecializedType(TypeFactory.java:359)
    at com.fasterxml.jackson.databind.jsontype.impl.StdTypeResolverBuilder.buildTypeDeserializer(StdTypeResolverBuilder.java:118)
    at com.fasterxml.jackson.databind.deser.BasicDeserializerFactory.findTypeDeserializer(BasicDeserializerFactory.java:1363)
    at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:481)
    at com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer(ObjectMapper.java:3890)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3785)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2833)
    at jackson.test.BrokenJackson.test(BrokenJackson.java:16)
.....

[cowtowncoder]

This is by design: as exception says, JacksonParent is not a subtype of JacksonChild. Problem being that although you ask for JacksonChild, it is possible that due to defaultImpl, JacksonParent would be returned; and that is not assignment-compatible. Earlier versions did not verify this, although they would have resulted in ClassCastException if this happened. In this case, fix would be to use:

  (JacksonChild) objectMapper.readValue(value, JacksonParent.class);

if you know for sure that defaultImpl is never used.

Now: it may be that ideally exception should be postponed and only thrown if problem occurs, not just because it might occur. I think there may already be an issue for this; but I’ll leave this open if not (if there is, I’ll close one as dup).

[cowtowncoder]

Closing, same as #1488 and there is I think an issue for improving handling for this particular case, to defer throwing exception.