我需要创建一个库，在其中我将使用同步和异步方法。
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。我图书馆的核心逻辑
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，然后在我们将响应作为JSON字符串返回之后，我们将该JSON字符串发送回给我们的客户。它是通过创建DataResponse对象。有些客户会致电executeSynchronous()，有些可能会致电executeAsynchronous()，这就是为什么我需要在我的库中单独提供两种方法。
接口：
public interface Client {
// for synchronous
public DataResponse executeSynchronous(DataKey key);
// for asynchronous
public Future<DataResponse> executeAsynchronous(DataKey key);
}
然后我的DataClient实现了上面的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）。所以我在网上进行了一些研究，发现我可以使用cancel future取消已经超时的任务，如下所示 -
future.cancel(true);
但是我想确定，我在executeSynchronous方法中取消已经超时的任务的方式是否正确？
因为我在cancel()上调用Future，如果任务仍然在队列中，它将阻止它运行，所以我不确定我在做什么是对还是不对？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？
答案 0 :(得分：2)
如果任务仍然在队列中，那么只需调用future.cancel()就可以取消它，但显然你不知道它是否在队列中。即使你要求future中断任务，它也可能不起作用，因为你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)，但是你需要确保你的任务（线程）确实考虑了线程中断状态。例如，如您所述，您进行了http调用，因此您可能需要在线程中断时立即关闭http客户端资源。
请参阅下面的示例。
我尝试过实施任务取消方案。通常，线程可以检查isInterrupted()并尝试终止自身。但是，当您使用线程池执行程序时，这会变得更加复杂，可调用且任务不像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调用来保持简单）。线程池执行程序开始运行任务，但调用者想要在100毫秒后取消它。现在将来将中断信号发送给线程，但是可调用任务在写入文件时无法立即检查它。因此，为了使这种情况发生可调用，它会维护一个将要使用的IO资源列表，并且一旦未来希望取消该任务，它就会在所有IO资源上调用cancel()，然后使用IOException终止任务，然后完成线程。
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，您可以修改类似的工厂类 -
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));
确保您传递了在CancellableTask中维护的相同可取消资源列表。
现在您需要修改cancel()中的CancellableTask方法，如下所示 -
synchronized (lock) {
for (Object cancellableResource : cancellableResources) {
if (cancellableResource instanceof HttpURLConnection) {
((HttpURLConnection) cancellableResource).disconnect();
success = true;
}
}
}