I am using Jackson version 2.16.1 and have encountered a peculiar behavior. According to the API documentation, it is recommended to use a common ObjectMapper instance. However, I observe different serialization results when using a shared instance versus a new instance. I have reproduced this issue with a simple example.

public class First {
  @JsonUnwrapped(prefix = "")
  final Thrid thrid = new Thrid();
}
public class Second {
  @JsonUnwrapped(prefix = "fromSecond")
  final Thrid thrid = new Thrid();
}
public class Thrid {
  @JsonUnwrapped(prefix = "fromThird")
  final Common common = new Common("1", "2");
}

@JsonInclude(JsonInclude.Include.NON_NULL)
public class Common {

  public String getA() {
    return a;
  }

  public String getB() {
    return b;
  }

  String a;
  String b;

  public Common() {

  }
  public Common(String a, String b) {
    this.a = a;
    this.b = b;
  }
}

public static void main(String[] args) throws JsonProcessingException {
    First first = new First();
    Second second = new Second();
    ObjectMapper objectMapper = new ObjectMapper();
    System.out.println(objectMapper.writeValueAsString(first));
    System.out.println(objectMapper.writeValueAsString(second));
    System.out.println(new ObjectMapper().writeValueAsString(second));
  }

Output-- 
{"fromThirda":"1","fromThirdb":"2"}
{"fromThirda":"1","fromThirdb":"2"}
{"fromSecondfromThirda":"1","fromSecondfromThirdb":"2"}

The output of the last two lines should be the same, but this is not the case. It appears that the @JsonUnwrapped annotation’s previous value for the Third class is being cached somewhere.

  public class First {
  @JsonUnwrapped(prefix = "")
  final Thrid thrid = new Thrid();
}

If I use a non-empty string instead of an empty one in the above class, the results are consistent.
Can someone please help us with this?