請求聲明

// 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); 

如果要下載的文件是靜態資源（始終位于服務器上的同一位置），并且基本URL所引用的服務器上，您可以使用選項1,它看起來像一個常規的Retrofit 2請求聲明.
下載文件我們是使用ResponseBody作為返回類型，這里最好不要改成其他的類型
option2中可以輕松地將動態值作為完整網址傳遞到請求調用。

請求調用

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, "下載成功");
        } else {
            Log.d(TAG, "下載失敗");
        }
    }
    @Override
    public void onFailure(Call<ResponseBody> call, Throwable t) {
        Log.e(TAG, "error");
    }
});

FileDownloadService類是你實現的請求聲明，就是上面介紹的。
RetrofitClient代碼如下：

public class JuheRetrofit extends BaseRetrofit{

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

    /**
     * 獲取接口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;
    }

    /**
     * 設置HttpClient設置
     *
     * @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信息攔截器
     *
     * @return
     */
    protected static Interceptor getHttpLoggingInterceptor() {
        if (logging == null) {
            logging = new HttpLoggingInterceptor();
            logging.setLevel(HttpLoggingInterceptor.Level.BODY);
        }
        return logging;
    }

    /**
     * 獲取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;
    }
}

定義了聲明文件，也是現實請求調用，最后就要處理ResponseBody數據了，上面我們通過writeFileToSDCard()方法將文件保存到手機SDCard中

數據保存

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;
    }
}

有一個問題：在默認情況下，Retrofit在處理結果之前將整個服務器響應放入內存,這適用于一些JSON或XML響應，但大文件可以輕松導致內存不足錯誤
所以如果你下載的是大文件，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()方法放在異步線程中，否則可能會拋出android.os.NetworkOnMainThreadException異常。

以上就是Retrofit文件下載的所有內容了，如果不清楚了可以互相交流