如何强制执行Singleton（一个ScheduledExecutorService的全局实例），一次执行一项任务？

Question

我有一个需要从服务器请求数据的应用程序，有时是一次性请求，而其他时候则需要以固定速率轮询。

对于一个关闭的请求，我只使用如下线程：

    public void request() {
        Thread requestThread = new Thread(this);
        requestThread.start();
    }

这些没问题，先执行，然后完成。

但是，对于长时间的轮询任务，我似乎永远无法一次只执行其中之一。

我想要的是以下方法：

    public void longPoll() {
        try {
            pollingScheduledExecutor.scheduleAtFixedRate(this, 0, 3, TimeUnit.SECONDS);
        } catch (Exception ignored) {
        }
    }

曾经只能存在一次，但是现在发生的是，当我从第一个类调用它时，它通常开始，但是从第二个任务调用后，它永远不会停止。

如何强制该ScheduledExecutorService只有一个全局实例？

对于上下文，这是这些方法所在的类：

public abstract class AbstractCommunicationChannel implements Runnable {
    static String SERVER_ADDRESS = "http://0.0.0.0";

    private URL url;
    private JSONObject requestObject;
    private JSONObject responseObject;

    private volatile ScheduledExecutorService pollingScheduledExecutor = Executors.newSingleThreadScheduledExecutor();

    public void longPoll() {
        try {
            pollingScheduledExecutor.scheduleAtFixedRate(this, 0, 3, TimeUnit.SECONDS);
        } catch (Exception ignored) {
        }
    }

    public void request() {
        Thread requestThread = new Thread(this);
        requestThread.start();
    }

    AbstractCommunicationChannel(URL url, JSONObject requestObject) {
        this.url = url;
        this.requestObject = requestObject;
    }

    /**
     * This is the general purpose tool for hitting the server and getting a response back.
     */
    public void run() {
        Log.i("requestObject", requestObject.toString());
        try {
            HttpURLConnection httpUrlConnection = (HttpURLConnection) url.openConnection();
            httpUrlConnection.setDoOutput(true);
            httpUrlConnection.setRequestMethod("POST");
            httpUrlConnection.setRequestProperty("Content-Type", "application/json; charset=utf-8");

            OutputStreamWriter outputStreamWriter = new OutputStreamWriter(httpUrlConnection.getOutputStream());
            outputStreamWriter.write(requestObject.toString());
            outputStreamWriter.flush();
            outputStreamWriter.close();
            /* * */
            InputStream inputStream = httpUrlConnection.getInputStream();
            BufferedInputStream bufferedInputStream = new BufferedInputStream(inputStream);
            ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
            int result = bufferedInputStream.read();
            while (result != -1) {
                byteArrayOutputStream.write((byte) result);
                result = bufferedInputStream.read();
            }
            responseObject = new JSONObject(byteArrayOutputStream.toString("UTF-8"));
            httpUrlConnection.disconnect();
        } catch (Exception ignored) {
        }
        processResponse(responseObject);
    }

    protected abstract void processResponse(JSONObject responseObject);
}

Answer 1

又是我。

如果只需要一个正在运行的轮询任务，则必须取消前一个任务（如果已存在）。

例：

private static final Object lock = new Object();
private static ScheduledFuture<?> currentRunningTask;

    public void longPoll() {

        synchronized (lock) {

            if (currentRunningTask != null) {
                currentRunningTask.cancel(true);
            }

            try {
                currentRunningTask = pollingScheduledExecutor.scheduleAtFixedRate(this, 0, 3, TimeUnit.SECONDS);
            } catch (Exception ignored) {
                ignored.printStackTrace();
            }
        }
    }