用 Java 下载时 APK 损坏，但浏览器下载工作正常

Question

I am trying to download an APK hosted in a server. When downloading via browser, it works fine. I get 14MB. But when downloading using Java, the APK seems corrupted since APK downloaded is just aroung 189kb. Below is my code when downloading using Apache Commons FileUtils:

URL format is similar to https://example.com/emm.apk . Entering this URL in a browser directly downloads the APK without problem.

    URL url = new URL("https://example.com/emm.apk");
    File file = new File(UUID.randomUUID().toString() +"-"+ FilenameUtils.getName(fileUrl.getPath()));
    FileUtils.copyURLToFile(url, file, 50_000, 50_000);

I also tried Java's built in File IO:

    URLConnection conn = url.openConnection();
    int contentLength = conn.getContentLength();

    DataInputStream stream = new DataInputStream(url.openStream());

    byte[] buffer = new byte[contentLength];
    stream.readFully(buffer);
    stream.close();

    DataOutputStream fos = new DataOutputStream(new FileOutputStream(file));
    fos.write(buffer);
    fos.flush();
    fos.close();

I only get this problem for that particular URL. I cannot pinpoint what's wrong since I don't have access with the host server. Clearly the problem is with how the APK is hosted. Is there requirements on how servers hosting APK should be configured? I came across application/vnd.android.package-archive , is this required for host servers? Why does the browser (Chrome) downloads it without problem while my Java code does not?

Answer 1

It seems that host server of the APK is blocking requests based on the User-Agent request header that's why browser download is okay but when downloading in different medium, it's not.

I tried manually setting the User-Agent of the URLConnection , and it's downloading properly.

URLConnection connection = fileUrl.openConnection();
connection.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36");
InputStream input = connection.getInputStream();
FileUtils.copyInputStreamToFile(input, file);

The 189kb is an HTML file saying the request was rejected.