下面是一个使用Retrofit进行多部分请求的示例代码：

首先，首先确保你已添加以下依赖项到你的build.gradle文件中：

implementation 'com.squareup.retrofit2:retrofit:2.9.0'
implementation 'com.squareup.retrofit2:converter-gson:2.9.0'
implementation 'com.squareup.okhttp3:logging-interceptor:4.8.1'

然后，创建一个ApiService接口来定义你的API请求：

import okhttp3.MultipartBody;
import okhttp3.RequestBody;
import retrofit2.Call;
import retrofit2.http.Multipart;
import retrofit2.http.POST;
import retrofit2.http.Part;

public interface ApiService {

    @Multipart
    @POST("your_endpoint")
    Call uploadFile(
        @Part("description") RequestBody description,
        @Part MultipartBody.Part file);

}

接下来，创建一个RetrofitClient类来创建Retrofit实例：

import okhttp3.OkHttpClient;
import okhttp3.logging.HttpLoggingInterceptor;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class RetrofitClient {

    private static Retrofit retrofit;

    public static Retrofit getRetrofitInstance() {
        if (retrofit == null) {
            OkHttpClient.Builder builder = new OkHttpClient.Builder();

            // 添加日志拦截器
            HttpLoggingInterceptor loggingInterceptor = new HttpLoggingInterceptor();
            loggingInterceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
            builder.addInterceptor(loggingInterceptor);

            retrofit = new Retrofit.Builder()
                .baseUrl("your_base_url")
                .addConverterFactory(GsonConverterFactory.create())
                .client(builder.build())
                .build();
        }
        return retrofit;
    }

}

最后，在你的代码中使用这些类来发送多部分请求：

import okhttp3.MediaType;
import okhttp3.MultipartBody;
import okhttp3.RequestBody;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class MainActivity extends AppCompatActivity {

    private ApiService apiService;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        apiService = RetrofitClient.getRetrofitInstance().create(ApiService.class);

        // 创建文件描述
        RequestBody description = RequestBody.create(MediaType.parse("text/plain"), "Your file description");

        // 创建文件的 RequestBody
        File file = new File("your_file_path");
        RequestBody fileRequest = RequestBody.create(MediaType.parse("image/*"), file);
        MultipartBody.Part filePart = MultipartBody.Part.createFormData("file", file.getName(), fileRequest);

        // 发送多部分请求
        Call call = apiService.uploadFile(description, filePart);
        call.enqueue(new Callback() {
            @Override
            public void onResponse(Call call, Response response) {
                // 请求成功处理
            }

            @Override
            public void onFailure(Call call, Throwable t) {
                // 请求失败处理
            }
        });
    }
}

请注意，上述示例为了简化表达，只上传了一个文件部分。你可以根据你的需求添加更多的部分。