public interface HttpDownloader {    public void setDownloadManager(HttpDownloadManager manager);    public HttpDownloadManager getDownloadManager();    public void executeAction();}

public class MyAsyncTask extends AsyncTask<String, Void, Bitmap> implements HttpDownloader{    private HttpDownloadManager httpDownloadManager;    protected Bitmap doInBackground(String... params) {        httpDownloadManager.downloadStarted();        ...    }    protected void onPostExecute(Bitmap bitmap) {        httpDownloadManager.downloadCompleted();        ...    }    public void setDownloadManager(HttpDownloadManager httpDownloadManager){        this.httpDownloadManager = httpDownloadManager;    }    public HttpDownloadManager getDownloadManager(){        return httpDownloadManager    }    public void executeAction(){        this.execute();    }}

public class HttpDownloadManager{    private LinkedList<HttpDownloader> tasks = new LinkedList<HttpDownloader>();    private int mMaxTaskNumber = 1;    private int mActiveTaskNumber = 0;    private boolean isRunning = false;    private ExectorService mService = Executors.newSingleThreadExecutor();        public HttpDownloadManager() {        }        public void addTask(HttpDownloader task) {        try{            task.setDownloadManager(this);            tasks.offer(task);        } catch (Exception e) {            e.printStackTrace();        }        if (!isRunning && !mService.isShutdown()) {            mService.submit(new DoTask());        }    }        public void deleteTasks() {        tasks.clear();    }        public void destroy() {        if (!mService.isShutdown) {            mService.shutdown();        }    }        public final void setMaxTaskNumber(int maxTaskNumber) {        mMaxTaskNumber = maxTaskNumber;    }        public void downloadStarted() {        mActiveTaskNumber++;    }        public void downloadCompleted() {        mActiveTaskNumber--;    }        public boolean isAvailable() {        if (mActiveTaskNumber <= mMaxTaskNumber) {            return true;        } else {            return false;        }    }        private class DoTask implements Runnable {        public void run() {            isRunning = true;            while(!tasks.isEmpty()) {                if (HttpDownloadManager.this.isAvailable()) {                    HttpDownloader task = tasks.poll();                    task.executeAction();                }            }            isRunning = false;        }    }}