请求声明

// option 1: a resource relative to your base URL
@GET("/resource/example.zip")
Call<ResponseBody> downloadFileWithFixedUrl();
// option 2: using a dynamic URL
@GET
Call<ResponseBody> downloadFileWithDynamicUrlSync(@Url String fileUrl); 

如果要下载的文件是静态资源（始终位于服务器上的同一位置），并且基本URL所引用的服务器上，您可以使用选项1,它看起来像一个常规的Retrofit 2请求声明.
下载文件我们是使用ResponseBody作为返回类型，这里最好不要改成其他的类型
option2中可以轻松地将动态值作为完整网址传递到请求调用。

请求调用

FileDownloadService downloadService = RetrofitClient.create(FileDownloadService.class);
Call<ResponseBody> call = downloadService.downloadFileWithDynamicUrlSync(fileUrl);
call.enqueue(new Callback<ResponseBody>() {  
    @Override
    public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
        if (response.isSuccess()) {
            boolean writtenToDisk = writeResponseBodyToDisk(response.body());
            Log.d(TAG, "下载成功");
        } else {
            Log.d(TAG, "下载失败");
        }
    }
    @Override
    public void onFailure(Call<ResponseBody> call, Throwable t) {
        Log.e(TAG, "error");
    }
});

FileDownloadService类是你实现的请求声明，就是上面介绍的。
RetrofitClient代码如下：

public class JuheRetrofit extends BaseRetrofit{

    private static OkHttpClient httpClient;
    private static Gson gson;
    private static HttpLoggingInterceptor logging;
    private static Retrofit retrofit;

    /**
     * 获取接口ApiService
     *
     * @return
     */
    public static JuheApiService getJuheApiService() {
        return getRetrofit().create(JuheApiService.class);
    }

    /**
     * 指定Gons格式
     *
     * @return
     */
    protected static Gson getGson() {
        if (gson == null) {
            gson = new GsonBuilder()
                    .setLenient()
                    .setDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
                    .create();
        }
        return gson;
    }

    /**
     * 设置HttpClient设置
     *
     * @return
     */
    protected static OkHttpClient getOkHttpClient() {
        if (httpClient == null) {
            OkHttpClient.Builder builder = new OkHttpClient.Builder();
            if (BuildConfig.DEBUG) {
                builder.addInterceptor(getHttpLoggingInterceptor());
            }
            builder.connectTimeout(20, TimeUnit.SECONDS);
            httpClient = builder.build();
        }
        return httpClient;
    }


    /**
     * Log信息拦截器
     *
     * @return
     */
    protected static Interceptor getHttpLoggingInterceptor() {
        if (logging == null) {
            logging = new HttpLoggingInterceptor();
            logging.setLevel(HttpLoggingInterceptor.Level.BODY);
        }
        return logging;
    }

    /**
     * 获取Retrofit
     *
     * @return
     */
    private static Retrofit getRetrofit() {
        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(UrlConstant.BASE_JUHE)
                    .client(getOkHttpClient())
                    .addConverterFactory(GsonConverterFactory.create(getGson()))
                    .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                    .build();
        }
        return retrofit;
    }
}

定义了声明文件，也是现实请求调用，最后就要处理ResponseBody数据了，上面我们通过writeFileToSDCard()方法将文件保存到手机SDCard中

数据保存

private boolean writeFileToSDCard(ResponseBody body) {  
    try {
        // todo change the file location/name according to your needs
        File futureStudioIconFile = new File(getExternalFilesDir(null) + File.separator + "Future Studio Icon.png");
        InputStream inputStream = null;
        OutputStream outputStream = null;
        try {
            byte[] fileReader = new byte[4096];
            long fileSize = body.contentLength();
            long fileSizeDownloaded = 0;
            inputStream = body.byteStream();
            outputStream = new FileOutputStream(futureStudioIconFile);
            while (true) {
                int read = inputStream.read(fileReader);
                if (read == -1) {
                    break;
                }
                outputStream.write(fileReader, 0, read);
                fileSizeDownloaded += read;
                Log.d(TAG, "file download: " + fileSizeDownloaded + " of " + fileSize);
            }
            outputStream.flush();
            return true;
        } catch (IOException e) {
            return false;
        } finally {
            if (inputStream != null) {
                inputStream.close();
            }
            if (outputStream != null) {
                outputStream.close();
            }
        }
    } catch (IOException e) {
        return false;
    }
}

有一个问题：在默认情况下，Retrofit在处理结果之前将整个服务器响应放入内存,这适用于一些JSON或XML响应，但大文件可以轻松导致内存不足错误
所以如果你下载的是大文件，你需要使用@Streaming注解，如下：

@Streaming
@GET
Call<ResponseBody> downloadFileWithDynamicUrlAsync(@Url String fileUrl); 

注意，如果你使用了@Streaming，不要忘了把上面的writeFileToSDCard()方法放在异步线程中，否则可能会抛出android.os.NetworkOnMainThreadException异常。

以上就是Retrofig文件下载的所有内容了，如果不清楚了可以互相交流