要在每个方法上注释@Headers("Content-Type: application/json")，太麻烦。用拦截器+反射，上代码：

public class RetrofitUtil {
    public static OkHttpClient genericClient() {
        OkHttpClient httpClient = new OkHttpClient();
        httpClient.interceptors().add(chain -> {
            Request request = chain.request();
            MediaType mediaType = request.body().contentType();
            try {
                Field field = mediaType.getClass().getDeclaredField("mediaType");
                field.setAccessible(true);
                field.set(mediaType, "application/json");
            } catch (NoSuchFieldException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
            return chain.proceed(request);
        });
        return httpClient;
    }
}

调用的时候这么写：

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(Http.URL)
                .addConverterFactory(GsonConverterFactory.create())
                .client(RetrofitUtil.genericClient())
                .build();