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如何执行ScheduledExecutorService的单例(一个全局实例),一次执行一个任务?

threadpool scheduled-tasks multithreading java

smatthewenglish · 技术社区 · 6 年前

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,有时需要以固定速率轮询。

对于一次性请求,我只使用如下线程:

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

然而,对于长轮询任务,我似乎永远不会一次只执行其中一个任务。

我想要的是以下方法:

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

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

对于上下文,以下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);
}

1 回复 | 直到 6 年前

HPCS 6 年前

又是我。

如果只需要一个正在运行的轮询任务,则必须取消上一个任务(如果已存在)。

例子:

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();
            }
        }
    }