我需要创建一个库,其中包含同步和异步方法。
executeSynchronous()
- 等待直到获得结果,然后返回结果。executeAsynchronous()
- 立即返回一个 Future,如果需要,可以在其他事情完成后进行处理。
我的库的核心逻辑
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 DataKey 对象构造一个 URL,并通过执行它来对该 URL 进行 HTTP 客户端调用,在我们以 JSON 字符串形式返回响应后,我们将该 JSON 字符串发送回我们的客户通过创建 DataResponse 对象来实现。有些客户会调用 executeSynchronous()
,有些客户可能会调用 executeAsynchronous()
,所以这就是为什么我需要在我的库中单独提供两个方法。
界面:
public interface Client {
// for synchronous
public DataResponse executeSynchronous(DataKey key);
// for asynchronous
public Future<DataResponse> executeAsynchronous(DataKey key);
}
然后我的 DataClient
实现了上述 Client
接口(interface):
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:-
当我开始研究此解决方案时,我并没有终止已超时的任务。我向客户端报告超时,但任务继续在线程池中运行(可能长时间占用我有限的 10 个线程之一)。所以我在网上做了一些研究,发现我可以通过在 future
上使用 cancel
来取消那些超时的任务,如下所示 -
future.cancel(true);
但我想确定,我在 executeSynchronous
方法中取消超时任务的方式看起来是否正确?
因为我在Future
上调用cancel()
,如果任务仍在队列中,这将阻止它运行,所以我不确定我在做什么对还是不对?执行此操作的正确方法是什么?
如果有更好的方法,那么有人可以提供一个例子吗?
最佳答案
如果任务仍在队列中,则只需调用 future.cancel()
即可取消它,但显然您不知道该任务是否在队列中。此外,即使您要求 future
中断任务,它也可能无法工作,因为您的任务仍然可以执行忽略线程中断状态的操作。
因此您可以使用 future.cancel(true)
但您需要确保您的任务(线程)确实考虑线程中断状态。例如,正如您提到的,您进行 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。
请引用下面的例子。
我尝试实现任务取消场景。通常,线程可以检查 isInterrupted() 并尝试终止自身。但是,当您使用可调用的线程池执行器并且任务实际上并不像 while(!Thread.isInterrupted()) {//执行任务}
时,这会变得更加复杂。
在此示例中,任务正在写入文件(为了保持简单,我没有使用 http 调用)。线程池执行器开始运行任务,但调用者希望在 100 毫秒后取消它。现在 future 向线程发送中断信号,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 会维护一个它将要使用的 IO 资源列表,一旦将来想要取消该任务,它只需对所有 IO 资源调用 cancel()
即可以 IOException 终止该任务然后线程完成。
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,您需要在可运行任务中创建 RestTemplate
时使用此工厂。
RestTemplate template = new RestTemplate(new CancellableSimpleClientHttpRequestFactory(this.cancellableResources));
确保您传递的可取消资源列表与您在 CancellableTask
中维护的列表相同。
现在您需要像这样修改 CancellableTask
中的 cancel()
方法 -
synchronized (lock) {
for (Object cancellableResource : cancellableResources) {
if (cancellableResource instanceof HttpURLConnection) {
((HttpURLConnection) cancellableResource).disconnect();
success = true;
}
}
}
关于java - 如何终止多线程中超时的任务?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/29130563/