Jackson null list to empty array serialization

I want to serialize null List to empty array.

So given:

class MyBean { List values; }

And given an instance with null for values, it should be serialized to:

{ "values": [] }

I want this to be global behaviour for all Lists in all classes. I do not want to add any annotations or specific handling for classes.

I have read all related questions I found, and could not come up with anything that works. Seems any custom serializer I try to register for List class is not kicking in.

If you have this working on your project, let me know how did you manage to do that.

In cases like this you need to customize JacksonAnnotationIntrospector class. Te serialize null-s, by default, Jackson uses com.fasterxml.jackson.databind.ser.std.NullSerializer class. You can extend default introspector class and override findNullSerializer.

See below example:

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.introspect.Annotated;
import com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;
import java.io.IOException;
import java.util.List;
public class Main {
public static void main(String[] args) throws IOException {
ObjectMapper mapper = new ObjectMapper();
mapper.setAnnotationIntrospector(new EmptyArrayJacksonAnnotationIntrospector());
mapper.writeValue(System.out, new ListWrapper());
}
}
class EmptyArrayJacksonAnnotationIntrospector extends JacksonAnnotationIntrospector {
@Override
public Object findNullSerializer(Annotated a) {
if (List.class.isAssignableFrom(a.getRawType())) {
return ArrayNullSerializer.INSTANCE;
}
return super.findNullSerializer(a);
}
}
final class ArrayNullSerializer extends StdSerializer<Object> {
public static final ArrayNullSerializer INSTANCE = new ArrayNullSerializer();
protected ArrayNullSerializer() {
super(Object.class);
}
@Override
public void serialize(Object value, JsonGenerator gen, SerializerProvider provider) throws IOException {
gen.writeStartArray();
gen.writeEndArray();
}
}
class ListWrapper {
private List values;
public List getValues() {
return values;
}
public void setValues(List values) {
this.values = values;
}
}

Above code prints:

{"values":[]}