Using #JsonIgnore selectively on parent class fields - json

Question

I have a Class A which is extended by multiple classes, say Class B, Class C and Class D.
However I want only Class D to ignore super class fields during serialization.
How do I implement this? If I use #JsonIgnore annotation on parent Class A, all child classes get impacted.

Answer 1

I can see 2 ways:
1 - You can use a JacksonAnnotationIntrospector to dynamically ignore fields, here we test if the field is from class A (see example below when serialising class C)
class CustomIntrospector extends JacksonAnnotationIntrospector {
#Override
public boolean hasIgnoreMarker(final AnnotatedMember m) {
return m.getDeclaringClass() == A.class;
}
}
2 - You can use the #JsonIgnoreProperties annotation to ignore the fields you don't want (see example below on the definition of class D)
Then with the following class
class A {
public String fieldA = "a";
}
class B extends A {
public String fieldB = "b";
}
class C extends A {
public String fieldC = "c";
}
#JsonIgnoreProperties(value = { "fieldA" })
class D extends A {
public String fieldD = "d";
}
Then use the ObjectMapper
public static void main(String[] args) throws Exception {
A a = new A();
String jsonA = new ObjectMapper().writeValueAsString(a);
System.out.println(jsonA);
// No filtering, will output all fields
B b = new B();
String jsonB = new ObjectMapper().writeValueAsString(b);
System.out.println(jsonB);
// Using the CustomIntrospector to filter out fields from class A
C c = new C();
ObjectMapper mapper = new ObjectMapper().setAnnotationIntrospector(new CustomIntrospector());
String jsonC = mapper.writeValueAsString(c);
System.out.println(jsonC);
// Using #JsonIgnoreProperties to filter out fields from class A
D d = new D();
String jsonD = new ObjectMapper().writeValueAsString(d);
System.out.println(jsonD);
}
outputs
{"fieldA":"a"}
{"fieldA":"a","fieldB":"b"}
{"fieldC":"c"}
{"fieldD":"d"}