public static void downloadAPK(String url, int num) {
		// Ϊ10̵̳߳߳أ̫Ļ»ס
		ExecutorService executor = Executors.newFixedThreadPool(10);
		log.info("init thread pool");
		for (int i = 0; i < num; i++) {
			Runnable worker = new DownloadTask(url,DEST_DIR);
			executor.execute(worker);
		}
		// This will make the executor accept no new threads and finish all
		// existing threads in the queue
		executor.shutdown();
		// Wait until all threads are finish,and also you can
		// use"executor.awaitTermination();" to wait
		while (!executor.isTerminated()) {
			try {
				Thread.sleep(5*ONE_MINUTE_IN_MISECONDS);
			} catch (InterruptedException e) {
				log.info(e.getMessage());
				e.printStackTrace();
			}
			log.info("downloading.......");
		}
		log.info("Finished all threads");
	}