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I want to build an simple android download app without using download manager. I try to implement it with asynctask. But I don't know how to implement "pausing and resuming when Based on the internet condition". That is when I click the start button, it start to download if connected. If internet is unconnected, it pause, and then the internet is connected, it resume automatically. I know it can be finished by using download manager easily, but how can I implement it without that? I'm a totally beginner, hope someone can give me an simple example of that.

Edited by peter_budo because: Moved to Mobile Development section
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— AsyncTask isn't a good fit for a download that must pause automatically on network loss and resume later. AsyncTasks are tied to Activity lifecycles and can be killed; use a dedicated Service (foreground on modern Android) with a worker thread, detect connectivity separately, and implement HTTP range requests so you can resume where you left off.

Basic approach (high level)

  • Run the download in a Service so it survives UI changes and system limits.
  • Write bytes incrementally to disk (use RandomAccessFile) and keep a persisted record (or simply use the file length) as the resume offset.
  • When (re)connecting, send a Range header like Range: bytes=<offset>-. Servers that support resume reply 206 (Partial Content).
  • Pause by setting a paused flag; the worker thread should wait on a lock while paused. Resume by clearing the flag and notifying the lock.
  • Monitor network with ConnectivityManager (registerNetworkCallback on API21+, or a BroadcastReceiver for older devices) and call pause()/resume() automatically.

Minimal, illustrative worker loop (simplified)

// in a Service worker thread (simplified)
volatile boolean paused = false;
final Object pauseLock = new Object();

public void run() throws Exception {
    long downloaded = destFile.length();
    while (!stopped) {
        HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();
        conn.setRequestProperty("Range", "bytes=" + downloaded + "-");
        conn.setConnectTimeout(15000);
        conn.setReadTimeout(30000);
        conn.connect();
        int code = conn.getResponseCode();
        if (code == 206 || (code == 200 && downloaded == 0)) {
            try (InputStream in = conn.getInputStream();
                 RandomAccessFile raf = new RandomAccessFile(destFile, "rw")) {
                raf.seek(downloaded);
                byte[] buf = new byte[8192];
                int len;
                while ((len = in.read(buf)) != -1) {
                    synchronized (pauseLock) { while (paused) pauseLock.wait(); }
                    raf.write(buf, 0, len);
                    downloaded += len;
                    // send progress updates (LocalBroadcast / callback)
                }
                break;
            }
        } else {
            throw new IOException("Unexpected response: " + code);
        }
    }
}

public void pause()  { paused = true; }
public void resume() { synchronized (pauseLock) { paused = false; pauseLock.notifyAll(); } }

Troubleshooting and cautions

  • Check server support: look for Accept-Ranges and expect HTTP 206. If server returns 200 for a resumed request, you must restart or handle duplication.
  • Use foreground Service on Android O+ and handle battery-optimization issues.
  • If the connection drops mid-read you may get IOExceptions — catch them, set paused, and wait for connectivity to return, then reconnect using the saved offset.
  • Libraries like OkHttp simplify retries and cancellation; Android's DownloadManager handles pause/resume for you if you decide not to implement everything yourself.

's linked post points in the right direction; the code sketch above gives a concrete way to combine connection callbacks, a pause flag, and HTTP Range to achieve automatic pause/resume.

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