Use Jackson to get one Field from sub-object

Question

I don't expect this will be a tough question, but with all that Jackson has to offer, I hope there is a solution that will work for me. Am using Java 11 now.

Let's say I have an object that looks like this:

public class MyMainObject() {
    private Long id;
    private String myName;
    private SomeObject someObject;
    ... getters/setters
}
public class SomeObject() {
    private Long someId;
    private String someName;
}

Now, I know I can use Jackson to convert this code as follows to json:

MyMainObject myMainObject = new MyMainObject();
ObjectMapper mapper = new ObjectMapper();
String json = mapper.writeValueAsString(myMainObject);

I know that this will give me the entirety of 'SomeObject' in the JSON, which is fine. But now, I just want to get the 'someId' field from 'SomeObject'.

I am wondering if there is some Jackson Annotation I could use in the 'MyMainObject' with the field 'someObject' so I only get the 'someId' from 'SomeObject'.

I suppose I could forget this and just do a mapping from these two objects to some other DTO. Or, I could put @JsonIgnore for the SomeObject.someName and that would work also. But in this case, imagine SomeObject has many more fields than the ones I listed, and I don't want to use @JsonIgnore for all those fields.

So, if there is a quick Jackson Annotation I could use to just get the one field from the SomeObject, that would be great. I'll keep checking the Jackson Annotations to see if there is something.

Does this answer your question? [Jackson: Serialize only marked fields](https://stackoverflow.com/questions/22668374/jackson-serialize-only-marked-fields) — Youans, Jun 12 '23 at 21:14

R4N · Accepted Answer · 2023-06-12T22:09:36.203

If I understand you query correctly, you'd like to include just a single property of SomeObject when serializing it using Jackson. If that's correct, then I think @JsonIncludeProperties class annotation might be a good fit. Something along these lines:

@JsonIncludeProperties({"someId"})
public class SomeObject {
    private Long someId;
    private String someName;
// other properties to be ignored when serializing
...
// getters/setters

}

The output result would not include other properties of SomeObject in the serialization (only the property/properties specified by @JsonIncludeProperties):

{"id":null,"myName":null,"someObject":{"someId":123}}

It was added in Jackson Release 2.12: https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.12#jsonincludeproperties

Additional details linked from the GitHub release post: https://cowtowncoder.medium.com/jackson-2-12-most-wanted-2-5-bc45ef53ede7

Use Jackson to get one Field from sub-object

1 Answers1