Android中使用Retrifit2下载文件

```java
Retrofit retrofit = new Retrofit.Builder().baseUrl("http://tv.speechb.com/").build();
                try {
                    Response<ResponseBody> response = retrofit.create(HoldGirlBookApi.class).downloadFile("/book/minanfangfa.pdf").execute();
                    InputStream inputStream = response.body().byteStream();
                    StringBuffer sb = new StringBuffer();
                    sb.append(SDCardUtil.getSDCardBaseDir()).append("/accost_learn/book/download");
                    File file = new File(sb.toString());
                    if (!file.exists()) {
                        file.mkdirs();
                    }
                    OutputStream fos = new FileOutputStream(sb + "/1.pdf");
                    int len;
                    byte[] bytes = new byte[4096];

while ((len = inputStream.read(bytes)) != -1) {
                        fos.write(bytes,0,len);
                    }

fos.close();
                    inputStream.close();
                } catch (IOException e1) {
                    e1.printStackTrace();
                }
```