JsonIgnore аннотация, кажется, не работает для меня. Есть идеи почему?

public class JsonTest implements Serializable {

    @JsonIgnore
private static JsonTest instance = null;

@JsonIgnore
private transient Set<String> set = new CopyOnWriteArraySet<String>();

private JsonTest() {}

@JsonIgnore
public static JsonTest getInstance() {
    if (instance == null)
        instance = new JsonTest();
    return instance;
}

public void setSet(Set<String> set) {
    this.set = set;
}

@JsonIgnore
public Set<String> getSet() {
    return set;
}

public String toString() {
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(SerializationConfig.Feature.FAIL_ON_EMPTY_BEANS, false);
    try {
        return mapper.writeValueAsString(this);
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    }
}

public static void main(String[] args) {
    HashSet<String> set = new HashSet<String>();
    set.add("test");
    JsonTest.getInstance().setSet(set);
    System.out.println(JsonTest.getInstance().toString());
}

}

Вывод: {"set": ["test"]}