I\'m saving an object with a java.util.Date field into a MongoDB 3.2 instance.
ObjectMapper mapper = new ObjectMapper();
String json = mapper.writeValueAsStr
This looks like Mongo Java driver bug, where Document.toJson profuces non-standard JSON even if JsonMode.STRICT is used. This problem is described in the following bug https://jira.mongodb.org/browse/JAVA-2173 for which I encourage you to vote.
A workaround is to use com.mongodb.util.JSON.serialize(document).