如何强制执行单例（一个全局实例）ScheduledExecutorService，一次一个任务？

Question

我有一个应用程序需要从服务器请求数据，有时作为一次性请求，有时它需要以固定速率轮询。

对于一次性请求，我只使用这样的线程：

    public void request() {

        Thread requestThread = new Thread(this);

        requestThread.start();

    }

这些没关系，它们执行然后就完成了。

但是，对于长轮询任务，我似乎永远不能一次只执行其中一个。

我想要的是以下方法：

    public void longPoll() {

        try {

            pollingScheduledExecutor.scheduleAtFixedRate(this, 0, 3, TimeUnit.SECONDS);

        } catch (Exception ignored) {

        }

    }

只能存在一次，但现在发生的情况是，当我从第一堂课调用它时，它会正常开始，但在第二个任务调用后它永远不会停止。

如何强制执行只有一个全局实例 this ScheduledExecutorService？

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");

            }

}

Answer 1

又是我。如果您只想要一个正在运行的轮询任务，则必须取消前一个（如果已经存在）。例子：private static final Object lock = new Object();private static ScheduledFuture<?> currentRunningTask;&nbsp; &nbsp; public void longPoll() {&nbsp; &nbsp; &nbsp; &nbsp; synchronized (lock) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (currentRunningTask != null) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; currentRunningTask.cancel(true);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; currentRunningTask = pollingScheduledExecutor.scheduleAtFixedRate(this, 0, 3, TimeUnit.SECONDS);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } catch (Exception ignored) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ignored.printStackTrace();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }