The Java unit test:
public void testAnimation() throws JsonParseException, JsonMappingException, IOException {
    ObjectMapper mapper = new ObjectMapper();
    Animation animation = mapper.readValue(new File("data/entityTest.json"), Animation.class);
}

The JSON:
{
    "frames" : [ {
        "ref" : "data/bullet.jpg"
    } ],
    "duration" : 1
}

The error:
org.codehaus.jackson.map.exc.UnrecognizedPropertyException: Unrecognized field "frames" (Class org.newdawn.slick.Animation), not marked as ignorable