public interface JsonSerializer<T>

Interface representing a custom serializer for Json. You should write a custom serializer, if you are not happy with the default serialization done by Gson. You will also need to register this serializer through GsonBuilder.registerTypeAdapter(Type, Object).

Let us look at example where defining a serializer will be useful. The Id class defined below has two fields: clazz and value.

 public class Id<T> {
   private final Class<T> clazz;
   private final long value;

   public Id(Class<T> clazz, long value) {
     this.clazz = clazz;
     this.value = value;
   }

   public long getValue() {
     return value;
   }
 }
 

The default serialization of Id(com.foo.MyObject.class, 20L) will be {"clazz":com.foo.MyObject,"value":20}. Suppose, you just want the output to be the value instead, which is 20 in this case. You can achieve that by writing a custom serializer:

 class IdSerializer implements JsonSerializer<Id>() {
   public JsonElement serialize(Id id, Type typeOfId, JsonSerializationContext context) {
     return new JsonPrimitive(id.getValue());
   }
 }
 

You will also need to register IdSerializer with Gson as follows:

 Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdSerializer()).create();
 

New applications should prefer TypeAdapter, whose streaming API is more efficient than this interface's tree API.