线程安全启动和停止

 private volatile Thread udpSendThread;

class UdpSender extends Thread {
        @Override
        public void run() {
            if (udpSendThread == null) {
                return;
            }
            // other work
        }
    }

    private void startUdpSender() {
        if (udpSendThread == null || !udpSendThread.isAlive()) {
            udpSendThread = new UdpSender();
            udpSendThread.start();
        }
    }

    private void stopUdpSender() {
        Thread tmpThread = udpSendThread;
        udpSendThread = null;
        if (tmpThread != null) {
            tmpThread.interrupt();
        }
    }