模块：应用依赖关系

implementation 'com.squareup.retrofit2:retrofit:2.6.0'
implementation 'com.squareup.retrofit2:converter-gson:2.6.0'

ApiClient

public class ApiClient {

    private static final String BASE_URL = "https://fcm.googleapis.com/";
    private static Retrofit retrofit = null;

    public static Retrofit getClient() {
        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .build();
        }
        return retrofit;
    }
}

ApiInterface

public interface ApiInterface {

    @Headers({"Authorization: key=" + ConstantKey.SERVER_KEY, "Content-Type:application/json"})
    @POST("fcm/send")
    Call<ResponseBody> sendNotification(@Body RootModel root);
}

RootModel

public class RootModel {

    @SerializedName("to") //  "to" changed to token
    private String token;

    @SerializedName("notification")
    private NotificationModel notification;

    @SerializedName("data")
    private DataModel data;

    public RootModel(String token, NotificationModel notification, DataModel data) {
        this.token = token;
        this.notification = notification;
        this.data = data;
    }

    public String getToken() {
        return token;
    }

    public void setToken(String token) {
        this.token = token;
    }

    public NotificationModel getNotification() {
        return notification;
    }

    public void setNotification(NotificationModel notification) {
        this.notification = notification;
    }

    public DataModel getData() {
        return data;
    }

    public void setData(DataModel data) {
        this.data = data;
    }
}

NotificationModel

public class NotificationModel {

    private String title;
    private String body;

    public NotificationModel(String title, String body) {
        this.title = title;
        this.body = body;
    }

    public String getBody() {
        return body;
    }

    public void setBody(String body) {
        this.body = body;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

}

数据模型

public class DataModel {

    private String name;
    private String age;

    public DataModel(String name, String age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAge() {
        return age;
    }

    public void setAge(String age) {
        this.age = age;
    }

}

使用此方法发送通知

private void sendNotificationToUser(String token) {
    RootModel rootModel = new RootModel(token, new NotificationModel("Title", "Body"), new DataModel("Name", "30"));

    ApiInterface apiService =  ApiClient.getClient().create(ApiInterface.class);
    retrofit2.Call<ResponseBody> responseBodyCall = apiService.sendNotification(rootModel);

    responseBodyCall.enqueue(new Callback<ResponseBody>() {
        @Override
        public void onResponse(retrofit2.Call<ResponseBody> call, retrofit2.Response<ResponseBody> response) {
            Log.d(TAG,"Successfully notification send by using retrofit.");
        }

        @Override
        public void onFailure(retrofit2.Call<ResponseBody> call, Throwable t) {

        }
    });
}