Can I tell Jackson to conditionally serialize a private instance variable based on differently-typed method?

Question

To the best of my understanding, Jackson will

• serialize a public instance variable to the variable name

  public List<String> myStrings = new ArrayList<>();

  serializes to

  { 'myStrings' : [ ... ] }

• serialize a private instance variable to the variable name if it has a public getter named getVariable() :

   private List<String> myStrings = new ArrayList<>(); public List<String> getMyStrings() { return myStrings; } 

  serializes similar to

  { 'myStrings' : [ ... ] }

---

However, what I am trying to achieve is to serialize it to a String (instead of array of Strings) based on another method, but keep the JSON key (based on @JsonInclude(JsonInclude.Include.NON_NULL) suppressing the original accessor in some cases

    @JsonInclude(JsonInclude.Include.NON_NULL)

    private boolean firstStringOnly = true;
    private List<String> myStrings = new ArrayList<>();
    public List<String> getMyStrings() { return firstStringOnly ? null: myStrings; }
    public String getFirstString() { return firstStringOnly ? myStrings.get(0) : null; } 

Desired JSON serialization:

• For firstStringOnly==true : { 'myStrings' : 'first_String' } (using getFirstString() )

• For firstStringOnly==false : { 'myStrings' : [ ... ] } (using getMyStrings() )

Is this possible to do? I'm specifically looking to avoid using custom serializers, and do this via annotations only.

You can assume a reasonably recent version of Jackson and Java 8.

Just to re-iterate, the question constraints are:

* NO custom serializer

* Both use cases produce the same JSON key

Answer 1

EDIT: Sorry, i have misread your initial question. I assume you want to keep both of the typed getters. Would this work for you?

public class TestClass {

  private boolean firstStringOnly = true;
  private List<String> myStrings = new ArrayList<>();

  @JsonIgnore
  public boolean isFirstStringOnly() {
    return firstStringOnly;
  }

  public void setFirstStringOnly(boolean firstStringOnly) {
    this.firstStringOnly = firstStringOnly;
  }

  @JsonIgnore
  public List<String> getMyStrings() {
    return firstStringOnly ? null : myStrings;
  }

  @JsonIgnore
  public String getFirstString() { return firstStringOnly ? myStrings.get(0) : null; }

  @JsonProperty("myStrings")
  public Object getMyStringsForSerialization() {
    return firstStringOnly ? getFirstString() : getMyStrings();
  }

  public void setMyStrings(List<String> myStrings) {
    this.myStrings = myStrings;
  }

Answer 2

You can generalize getMyStrings() method and make it return Object. And inside check the flag and return first value or all values. Here is my sample

public class tst {
private static class YourObject {

    private boolean firstStringOnly;
    private List<String> myStrings = new ArrayList<>();

    public YourObject(boolean firstStringOnly) {
        this.firstStringOnly = firstStringOnly;
        this.myStrings.add("str1");
        this.myStrings.add("str2");
    }
    public Object getMyStrings(){
        return firstStringOnly ? myStrings.get(0) : myStrings;
    }

}

    public static void main(String[] args) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writeValueAsString(new YourObject(true)));
        System.out.println(mapper.writeValueAsString(new YourObject(false)));
    }
}

The output is

{"myStrings":"str1"}
{"myStrings":["str1","str2"]}