inconsistent org.joda.time.DateTime serialization result when Spring Boot from 1.3.X to 1.4.0

Question

After upgrading a Spring Boot upgrade from 1.3.3 to 1.4.0, I have noticed a change of behavior when serializing a org.joda.time.DateTime.

Let's build up a simple Spring boot + Maven Project.

The pom.xml dependencies:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>1.3.7.RELEASE</version>
</parent>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-rest</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-mongodb</artifactId>
    </dependency>
    <dependency>
        <groupId>joda-time</groupId>
        <artifactId>joda-time</artifactId>
    </dependency>
    <dependency>
        <groupId>com.fasterxml.jackson.datatype</groupId>
        <artifactId>jackson-datatype-joda</artifactId>
    </dependency>
<dependencies>

The domain object:

@Document(collection="items")
@TypeAlias("item")
public class Item {
    @Id
    private String id;
    private DateTime date;

    /* getters/setters omitted */
}

The repository:

public interface ItemRepository extends MongoRepository<Item, String>{}

Now, if I GET any item resource, I get the following result:

{
  "_embedded" : {
    "items" : [ {
      "id" : "57bf084a452105f14763cce7",
      "date" : "1970-01-01T00:00:00.000Z",
      "_links" : {
        "self" : {
          "href" : "http://localhost:8082/items/57bf084a452105f14763cce7"
        },
        "item" : {
          "href" : "http://localhost:8082/items/57bf084a452105f14763cce7"
        }
      }
    } ]
  },
  /* omitted links and meta data */
}

Which is fine.

Now, if i upgrade Spring Boot to 1.4.0.RELEASE, without touching anything else in my code base, I get the following:

{
  "_embedded" : {
    "items" : [ {
      "id" : "57bf084a452105f14763cce7",
      "date" : {
        "content" : "1970-01-01T00:00:00.000Z"
      },
      "_links" : {
        "self" : {
          "href" : "http://localhost:8082/items/57bf084a452105f14763cce7"
        },
        "item" : {
          "href" : "http://localhost:8082/items/57bf084a452105f14763cce7"
        }
      }
    } ]
  },
  /* links and metadata omitted */
}

The datetime is now serialized as { "content" : "1970-01-01T00:00:00.000Z" } instead of "1970-01-01T00:00:00.000Z"

This problem is easily solved by annotating the DateTime field with @JsonFormat:

@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd'T'HH:mm:ss.SSSZ")
private DateTime date;

Actually, I don't like using @JsonFormat. Instead, I use a custom Jackson2ObjectMapperBuilder (it lets me handle my other serializers and deserializers, manually or via @JsonComponent...)

@Configuration
public class JacksonConfiguration {

    @Bean
    public Jackson2ObjectMapperBuilder jacksonBuilder() {
        Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();

        builder.serializerByType(DateTime.class, new DateTimeSerializer());
        
        return builder;
    }

}

Bad news, I still get the same result. I've tried with a custom serializer, I still get the "content" key in the way. Only by using the com.fasterxml.jackson.databind.ser.std.ToStringSerializer instead of the com.fasterxml.jackson.datatype.joda.ser.DateTimeSerializer do I (almost) get what I want:

{
  "_embedded" : {
    "items" : [ {
      "id" : "57bf084a452105f14763cce7",
      "date" : "1970-01-01T01:00:00.000+01:00",
      "_links" : {
        "self" : {
          "href" : "http://localhost:8082/items/57bf084a452105f14763cce7"
        },
        "item" : {
          "href" : "http://localhost:8082/items/57bf084a452105f14763cce7"
        }
      }
    } ]
  },
  /* links and metadata omitted */
}

In a nutshell, what's happening? Where does that extra "content" key come from and should it be there? I'd like to report this bug, but I don't know who is concerned, the developers from Spring Boot, or Jackson?