Gson自定义类型适配器仅在一个对象类型上序列化null？

Question

I'd like to serialize null for only one type in my JSON body that is going on a PUT. I don't want to serialize nulls for any other types in the object. I have something like this:

public class Patient {
    public Address address;
    public String first_name;
    public String last_name;
}

and I only want to serialize address if address is null. So for example

Patient patient = new Patient();
patient.address = null;
patient.last_name = "Doe";

will look something like this:

"address":null,
"last_name":"Doe"

Where address is assigned null, patient is left off of the object because by default Gson does not serialize nulls which is what I want, and last name retains the assigned string value.

Is there a Gson custom type adapter that I could use?

public class GsonCustomAdapter extends TypeAdapter<Address>

I'm not familiar at all with this concept and have been trying to understand it for awhile. Any help is much appreciated.

Answer 1

If, by default, you don't want to serialize nulls, you can tell the JsonWriter to serialize it only if you are actually reading an Address instance.

Let's assume the following class:

class Address {
    public String country = "UK";
    public String city = "London";
}

Now we create a specific type adapter for the Address class. This is here where you explicitly say that even if the JsonWriter is not supposed to write null values in the response, you allow it to do so just for the Address field (see the comments in the code).

class AddressAdapter extends TypeAdapter<Address> {
    @Override
    public void write(JsonWriter out, Address address) throws IOException {
        if (address == null) {
            //if the writer was not allowed to write null values
            //do it only for this field
            if (!out.getSerializeNulls()) {
                out.setSerializeNulls(true);
                out.nullValue();
                out.setSerializeNulls(false);
            } else {
                out.nullValue();
            }
        } else {
            out.beginObject();
            out.name("country");
            out.value(address.country);
            out.name("city");
            out.value(address.city);
            out.endObject();
        }
    }

    @Override
    public Address read(JsonReader in) throws IOException {
        if(in.peek() == JsonToken.NULL) {
            in.nextNull();
            return null;
        }
        in.beginObject();
        Address address = new Address();
        in.nextName();
        address.country = in.nextString();
        in.nextName();
        address.city = in.nextString();
        in.endObject();
        return address;
    }
}

Now you have to register this adapter so that the parser knows he has to use it when serializing/deserializing an Address field. To do that, use the annotation @JsonAdapter .

class Patient {
    @JsonAdapter(AddressAdapter.class)
    public Address address;
    public String first_name;
    public String last_name;
}

And it's done!

For instance let's take the patient in your example:

Patient patient = new Patient();
patient.last_name = "Doe";

With the parser set to serialize null values, you get:

{"address":null,"first_name":null,"last_name":"Doe"}

When you don't allow it (set by default):

{"address":null,"last_name":"Doe"}

by setting an address for the patient:

patient.address = new Address();
...
{"address":{"country":"UK","city":"London"},"last_name":"Doe"}

---

As a note if you want to stick with naming conventions on the Java side, you can use the annotation @SerializedName , for instance:

@SerializedName("first_name") public String firstName;
@SerializedName("last_name")  public String lastName;

Hope it helps ! :-)