GSON Null Object Support

The default behaviour that is implemented in Gson is that null object fields are ignored.  This allows for a more compact output format; however, the client must define a default value for these fields as the JSON format is converted back into its Java.

Here’s how you would configure a Gson instance to output null:
Gson gson = new GsonBuilder().serializeNulls().create();

NOTE: when serializing nulls with Gson, it will add a JsonNull element to the JsonElement structure.  Therefore, this object can be used in custom serialization/deserialization.

Here’s an example:

public class Foo {
  private final String s;
  private final int i;

  public Foo() {
    this(null, 5);
  }

  public Foo(String s, int i) {
    this.s = s;
    this.i = i;
  }
}

Gson gson = new GsonBuilder().serializeNulls().create();
Foo foo = new Foo();
String json = gson.toJson(foo);
System.out.println(json);

json = gson.toJson(null);
System.out.println(json);

======== OUTPUT ========
{"s":null,"i":5}
null

Ref: https://sites.google.com/site/gson/gson-user-guide#TOC-Null-Object-Support