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我需要创建一个库，其中包含同步和异步方法。

  • executeSynchronous()- 等到我有结果，返回结果。
  • executeAsynchronous()- 立即返回一个 Future，如果需要，可以在其他事情完成后处理它。

我的图书馆的核心逻辑

客户将使用我们的库，他们将通过传递DataKey构建器对象来调用它。然后，我们将使用该对象构造一个 URL，并通过执行该 URL 对该 URL 进行 HTTP 客户端调用，在我们将响应作为 JSON 字符串返回后，我们将通过创建对象DataKey将该 JSON 字符串原样发送回我们的客户DataResponse. 有些客户会打电话executeSynchronous()，有些客户可能会打电话executeAsynchronous()，这就是为什么我需要在我的库中分别提供两种方法。

界面：

public interface Client {

    // for synchronous
    public DataResponse executeSynchronous(DataKey key);

    // for asynchronous
    public Future<DataResponse> executeAsynchronous(DataKey key);
}

然后我有我DataClient的实现上述Client接口：

public class DataClient implements Client {

    private RestTemplate restTemplate = new RestTemplate();
    private ExecutorService executor = Executors.newFixedThreadPool(10);

    // for synchronous call
    @Override
    public DataResponse executeSynchronous(DataKey key) {
        DataResponse dataResponse = null;
        Future<DataResponse> future = null;

        try {
            future = executeAsynchronous(key);
            dataResponse = future.get(key.getTimeout(), TimeUnit.MILLISECONDS);
        } catch (TimeoutException ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.TIMEOUT_ON_CLIENT, key);
            dataResponse = new DataResponse(null, DataErrorEnum.TIMEOUT_ON_CLIENT, DataStatusEnum.ERROR);
            // does this look right the way I am doing it?
            future.cancel(true); // terminating tasks that have timed out.
        } catch (Exception ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.CLIENT_ERROR, key);
            dataResponse = new DataResponse(null, DataErrorEnum.CLIENT_ERROR, DataStatusEnum.ERROR);
        }

        return dataResponse;
    }

    //for asynchronous call
    @Override
    public Future<DataResponse> executeAsynchronous(DataKey key) {
        Future<DataResponse> future = null;

        try {
            Task task = new Task(key, restTemplate);
            future = executor.submit(task); 
        } catch (Exception ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.CLIENT_ERROR, key);
        }

        return future;
    }
}

将执行实际任务的简单类：

public class Task implements Callable<DataResponse> {

    private DataKey key;
    private RestTemplate restTemplate;

    public Task(DataKey key, RestTemplate restTemplate) {
        this.key = key;
        this.restTemplate = restTemplate;
    }

    @Override
    public DataResponse call() {
        DataResponse dataResponse = null;
        String response = null;

        try {
            String url = createURL();
            response = restTemplate.getForObject(url, String.class);

            // it is a successful response
            dataResponse = new DataResponse(response, DataErrorEnum.NONE, DataStatusEnum.SUCCESS);
        } catch (RestClientException ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.SERVER_DOWN, key);
            dataResponse = new DataResponse(null, DataErrorEnum.SERVER_DOWN, DataStatusEnum.ERROR);
        } catch (Exception ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.CLIENT_ERROR, key);
            dataResponse = new DataResponse(null, DataErrorEnum.CLIENT_ERROR, DataStatusEnum.ERROR);
        }

        return dataResponse;
    }

    // create a URL by using key object
    private String createURL() {
        String url = somecode;
        return url;
    }
}

问题陈述：-

当我开始研究这个解决方案时，我并没有终止已经超时的任务。我正在向客户端报告超时，但任务继续在线程池中运行（可能会长时间占用我有限的 10 个线程之一）。cancel所以我在网上做了一些研究，我发现我可以通过使用on取消那些超时的任务future，如下所示 -

future.cancel(true);

但我想确定一下，我在executeSynchronous取消已超时任务的方法中所做的方式是否正确？

由于我正在调用which 如果任务仍在队列cancel()中Future，它将停止运行，所以我不确定我在做什么是正确的还是不正确的？这样做的正确方法是什么？

如果有更好的方法，那么任何人都可以提供一个例子吗？

4

1 回答 1

2

如果任务仍在队列中，则只需调用即可取消它，future.cancel()但显然您不知道它是否在队列中。此外，即使您要求future中断任务，它也可能无法正常工作，因为您的任务仍然可以做一些忽略线程中断状态的事情。

所以你可以使用，future.cancel(true)但你需要确保你的任务（线程）确实考虑到线程中断状态。例如，正如您提到的，您进行了 http 调用，因此您可能需要在线程中断后立即关闭 http 客户端资源。

请参考以下示例。

我试图实现任务取消场景。通常一个线程可以检查isInterrupted()并尝试终止自己。但是当您使用线程池执行器时，这变得更加复杂，可调用并且如果任务不是真的像while(!Thread.isInterrupted()) {// execute task}.

在这个例子中，一个任务是写一个文件（我没有使用 http 调用来保持它的简单）。线程池执行器开始运行任务，但调用者想在 100 毫秒后取消它。现在未来将中断信号发送到线程，但可调用任务在写入文件时无法立即检查它。因此，为了实现这一点，callable 维护了一个将要使用的 IO 资源列表，并且一旦将来想要取消任务，它就会调用cancel()所有 IO 资源，从而以 IOException 终止任务，然后线程完成。

public class CancellableTaskTest {

    public static void main(String[] args) throws Exception {
        CancellableThreadPoolExecutor threadPoolExecutor = new CancellableThreadPoolExecutor(0, 10, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());
        long startTime = System.currentTimeMillis();
        Future<String> future = threadPoolExecutor.submit(new CancellableTask());
        while (System.currentTimeMillis() - startTime < 100) {
            Thread.sleep(10);
        }
        System.out.println("Trying to cancel task");
        future.cancel(true);
    }
}

class CancellableThreadPoolExecutor extends ThreadPoolExecutor {

    public CancellableThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
    }

    @Override
    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new CancellableFutureTask<T>(callable);
    }
}

class CancellableFutureTask<V> extends FutureTask<V> {

    private WeakReference<CancellableTask> weakReference;

    public CancellableFutureTask(Callable<V> callable) {
        super(callable);
        if (callable instanceof CancellableTask) {
            this.weakReference = new WeakReference<CancellableTask>((CancellableTask) callable);
        }
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        boolean result = super.cancel(mayInterruptIfRunning);
        if (weakReference != null) {
            CancellableTask task = weakReference.get();
            if (task != null) {
                try {
                    task.cancel();
                } catch (Exception e) {
                    e.printStackTrace();
                    result = false;
                }
            }
        }
        return result;
    }
}

class CancellableTask implements Callable<String> {

    private volatile boolean cancelled;
    private final Object lock = new Object();
    private LinkedList<Object> cancellableResources = new LinkedList<Object>();

    @Override
    public String call() throws Exception {
        if (!cancelled) {
            System.out.println("Task started");
            // write file
            File file = File.createTempFile("testfile", ".txt");
            BufferedWriter writer = new BufferedWriter(new FileWriter(file));
            synchronized (lock) {
                cancellableResources.add(writer);
            }
            try {
                long lineCount = 0;
                while (lineCount++ < 100000000) {
                    writer.write("This is a test text at line: " + lineCount);
                    writer.newLine();
                }
                System.out.println("Task completed");
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                writer.close();
                file.delete();
                synchronized (lock) {
                    cancellableResources.clear();
                }
            }
        }
        return "done";
    }

    public void cancel() throws Exception {
        cancelled = true;
        Thread.sleep(1000);
        boolean success = false;
        synchronized (lock) {
            for (Object cancellableResource : cancellableResources) {
                if (cancellableResource instanceof Closeable) {
                    ((Closeable) cancellableResource).close();
                    success = true;
                }
            }
        }
        System.out.println("Task " + (success ? "cancelled" : "could not be cancelled. It might have completed or not started at all"));
    }
}

对于您的 REST Http 客户端相关要求，您可以修改工厂类，如下所示 -

public class CancellableSimpleClientHttpRequestFactory extends SimpleClientHttpRequestFactory {

    private List<Object> cancellableResources;

    public CancellableSimpleClientHttpRequestFactory() {
    }

    public CancellableSimpleClientHttpRequestFactory(List<Object> cancellableResources) {
        this.cancellableResources = cancellableResources;
    }

    protected HttpURLConnection openConnection(URL url, Proxy proxy) throws IOException {
        HttpURLConnection connection = super.openConnection(url, proxy);
        if (cancellableResources != null) {
            cancellableResources.add(connection);
        }
        return connection;
    }
}

在这里，您需要在创建RestTemplate可运行任务时使用此工厂。

    RestTemplate template = new RestTemplate(new CancellableSimpleClientHttpRequestFactory(this.cancellableResources));

确保您传递了与您在CancellableTask.

现在您需要像这样修改cancel()方法CancellableTask-

synchronized (lock) {
    for (Object cancellableResource : cancellableResources) {
        if (cancellableResource instanceof HttpURLConnection) {
            ((HttpURLConnection) cancellableResource).disconnect();
            success = true;
        }
    }
}
于 2015-03-18T19:22:24.513 回答