我有一个返回List期货
List<Future<O>> futures = getFutures();
现在我要等到所有期货都成功完成处理，或者未来返回的任何任务抛出异常。即使一个任务抛出异常，等待其他期货也没有意义。
简单的方法是
wait() {
For(Future f : futures) {
try {
f.get();
} catch(Exception e) {
//TODO catch specific exception
// this future threw exception , means somone could not do its task
return;
}
}
}
但问题在于，例如，如果第四个未来会引发异常，那么我将不必要地等待前3个期货可用。
如何解决这个问题？会以任何方式倒数闩锁帮助吗？我无法使用Future isDone，因为java doc说
boolean isDone()
Returns true if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return true.
答案 0 :(得分：106)
您可以使用CompletionService在它们准备好后立即收到期货，如果其中一个抛出异常则取消处理。像这样：
Executor executor = Executors.newFixedThreadPool(4);
CompletionService<SomeResult> completionService =
new ExecutorCompletionService<SomeResult>(executor);
//4 tasks
for(int i = 0; i < 4; i++) {
completionService.submit(new Callable<SomeResult>() {
public SomeResult call() {
...
return result;
}
});
}
int received = 0;
boolean errors = false;
while(received < 4 && !errors) {
Future<SomeResult> resultFuture = completionService.take(); //blocks if none available
try {
SomeResult result = resultFuture.get();
received ++;
... // do something with the result
}
catch(Exception e) {
//log
errors = true;
}
}
我认为如果其中一个任务抛出错误，您可以进一步改进以取消任何仍在执行的任务。
编辑：我在这里找到了一个更全面的例子：http://blog.teamlazerbeez.com/2009/04/29/java-completionservice/
答案 1 :(得分：80)
如果您使用的是 Java 8 ，那么您可以使用CompletableFuture和CompletableFuture.allOf更轻松地完成此操作，仅在完成所有提供的CompletableFutures后才应用回调。
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答案 2 :(得分：15)
您可以使用ExecutorCompletionService。文档甚至还有一个确切用例的例子：
假设你想要使用任务集的第一个非空结果，忽略任何遇到的异常，并在第一个任务就绪时取消所有其他任务：
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException {
CompletionService<Result> ecs = new ExecutorCompletionService<Result>(e);
int n = solvers.size();
List<Future<Result>> futures = new ArrayList<Future<Result>>(n);
Result result = null;
try {
for (Callable<Result> s : solvers)
futures.add(ecs.submit(s));
for (int i = 0; i < n; ++i) {
try {
Result r = ecs.take().get();
if (r != null) {
result = r;
break;
}
} catch (ExecutionException ignore) {
}
}
} finally {
for (Future<Result> f : futures)
f.cancel(true);
}
if (result != null)
use(result);
}
这里要注意的重要一点是，ecs.take（）将获得第一个已完成的任务，而不仅仅是第一个提交的任务。因此，您应该按照完成执行（或抛出异常）的顺序获取它们。
答案 3 :(得分：4)
在Java 8中使用CompletableFuture
// Kick of multiple, asynchronous lookups
CompletableFuture<User> page1 = gitHubLookupService.findUser("Test1");
CompletableFuture<User> page2 = gitHubLookupService.findUser("Test2");
CompletableFuture<User> page3 = gitHubLookupService.findUser("Test3");
// Wait until they are all done
CompletableFuture.allOf(page1,page2,page3).join();
logger.info("--> " + page1.get());
答案 4 :(得分：1)
如果您使用的是Java 8并且不想操纵CompletableFuture，我已经编写了一个工具来检索使用流式传输的List<Future<T>>的结果。关键是你被禁止map(Future::get)抛出。
public final class Futures
{
private Futures()
{}
public static <E> Collector<Future<E>, Collection<E>, List<E>> present()
{
return new FutureCollector<>();
}
private static class FutureCollector<T> implements Collector<Future<T>, Collection<T>, List<T>>
{
private final List<Throwable> exceptions = new LinkedList<>();
@Override
public Supplier<Collection<T>> supplier()
{
return LinkedList::new;
}
@Override
public BiConsumer<Collection<T>, Future<T>> accumulator()
{
return (r, f) -> {
try
{
r.add(f.get());
}
catch (InterruptedException e)
{}
catch (ExecutionException e)
{
exceptions.add(e.getCause());
}
};
}
@Override
public BinaryOperator<Collection<T>> combiner()
{
return (l1, l2) -> {
l1.addAll(l2);
return l1;
};
}
@Override
public Function<Collection<T>, List<T>> finisher()
{
return l -> {
List<T> ret = new ArrayList<>(l);
if (!exceptions.isEmpty())
throw new AggregateException(exceptions, ret);
return ret;
};
}
@Override
public Set<java.util.stream.Collector.Characteristics> characteristics()
{
return java.util.Collections.emptySet();
}
}
这需要一个像C＃＆＃39;
一样的AggregateException
public class AggregateException extends RuntimeException
{
/**
*
*/
private static final long serialVersionUID = -4477649337710077094L;
private final List<Throwable> causes;
private List<?> successfulElements;
public AggregateException(List<Throwable> causes, List<?> l)
{
this.causes = causes;
successfulElements = l;
}
public AggregateException(List<Throwable> causes)
{
this.causes = causes;
}
@Override
public synchronized Throwable getCause()
{
return this;
}
public List<Throwable> getCauses()
{
return causes;
}
public List<?> getSuccessfulElements()
{
return successfulElements;
}
public void setSuccessfulElements(List<?> successfulElements)
{
this.successfulElements = successfulElements;
}
}
此组件的行为与C＃Task.WaitAll完全相同。我正在处理与CompletableFuture.allOf相同的变体（等效于Task.WhenAll）
我这样做的原因是我使用的是Spring ListenableFuture并且不希望移植到CompletableFuture，尽管它是更标准的方式
答案 5 :(得分：1)
如果要合并CompletableFuture列表，可以执行以下操作：
List<CompletableFuture<Void>> futures = new ArrayList<>();
// ... Add futures to this ArrayList of CompletableFutures
// CompletableFuture.allOf() method demand a variadic arguments
// You can use this syntax to pass a List instead
CompletableFuture<Void> allFutures = CompletableFuture.allOf(
futures.toArray(new CompletableFuture[futures.size()]));
// Wait for all individual CompletableFuture to complete
// All individual CompletableFutures are executed in parallel
allFutures.get();
有关Future＆CompletableFuture的更多详细信息，有用的链接：
1.未来：https://www.baeldung.com/java-future
2. CompletableFuture：https://www.baeldung.com/java-completablefuture
3. CompletableFuture：https://www.callicoder.com/java-8-completablefuture-tutorial/
答案 6 :(得分：0)
也许这会有所帮助（没有什么会被原始线程取代，是的！）
我建议用一个单独的线程运行每个Future的人（它们并行），然后当遇到一个错误时，它只是发出一个管理器（Handler类）的信号。
class Handler{
//...
private Thread thisThread;
private boolean failed=false;
private Thread[] trds;
public void waitFor(){
thisThread=Thread.currentThread();
List<Future<Object>> futures = getFutures();
trds=new Thread[futures.size()];
for (int i = 0; i < trds.length; i++) {
RunTask rt=new RunTask(futures.get(i), this);
trds[i]=new Thread(rt);
}
synchronized (this) {
for(Thread tx:trds){
tx.start();
}
}
for(Thread tx:trds){
try {tx.join();
} catch (InterruptedException e) {
System.out.println("Job failed!");break;
}
}if(!failed){System.out.println("Job Done");}
}
private List<Future<Object>> getFutures() {
return null;
}
public synchronized void cancelOther(){if(failed){return;}
failed=true;
for(Thread tx:trds){
tx.stop();//Deprecated but works here like a boss
}thisThread.interrupt();
}
//...
}
class RunTask implements Runnable{
private Future f;private Handler h;
public RunTask(Future f,Handler h){this.f=f;this.h=h;}
public void run(){
try{
f.get();//beware about state of working, the stop() method throws ThreadDeath Error at any thread state (unless it blocked by some operation)
}catch(Exception e){System.out.println("Error, stopping other guys...");h.cancelOther();}
catch(Throwable t){System.out.println("Oops, some other guy has stopped working...");}
}
}
我必须说上面的代码会出错（没有检查），但我希望我能解释一下这个解决方案。请试一试。
答案 7 :(得分：0)
CompletionService将使用.submit（）方法获取您的Callables，您可以使用.take（）方法检索计算的期货。
您不能忘记的一件事是通过调用.shutdown（）方法来终止ExecutorService。此外，只有在保存对执行程序服务的引用时才能调用此方法，因此请确保保留一个。
示例代码 - 对于要并行处理的固定数量的工作项：
ExecutorService service = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
CompletionService<YourCallableImplementor> completionService =
new ExecutorCompletionService<YourCallableImplementor>(service);
ArrayList<Future<YourCallableImplementor>> futures = new ArrayList<Future<YourCallableImplementor>>();
for (String computeMe : elementsToCompute) {
futures.add(completionService.submit(new YourCallableImplementor(computeMe)));
}
//now retrieve the futures after computation (auto wait for it)
int received = 0;
while(received < elementsToCompute.size()) {
Future<YourCallableImplementor> resultFuture = completionService.take();
YourCallableImplementor result = resultFuture.get();
received ++;
}
//important: shutdown your ExecutorService
service.shutdown();
示例代码 - 对于要并行处理的动态工作项数：
public void runIt(){
ExecutorService service = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
CompletionService<CallableImplementor> completionService = new ExecutorCompletionService<CallableImplementor>(service);
ArrayList<Future<CallableImplementor>> futures = new ArrayList<Future<CallableImplementor>>();
//Initial workload is 8 threads
for (int i = 0; i < 9; i++) {
futures.add(completionService.submit(write.new CallableImplementor()));
}
boolean finished = false;
while (!finished) {
try {
Future<CallableImplementor> resultFuture;
resultFuture = completionService.take();
CallableImplementor result = resultFuture.get();
finished = doSomethingWith(result.getResult());
result.setResult(null);
result = null;
resultFuture = null;
//After work package has been finished create new work package and add it to futures
futures.add(completionService.submit(write.new CallableImplementor()));
} catch (InterruptedException | ExecutionException e) {
//handle interrupted and assert correct thread / work packet count
}
}
//important: shutdown your ExecutorService
service.shutdown();
}
public class CallableImplementor implements Callable{
boolean result;
@Override
public CallableImplementor call() throws Exception {
//business logic goes here
return this;
}
public boolean getResult() {
return result;
}
public void setResult(boolean result) {
this.result = result;
}
}
答案 8 :(得分：0)
/**
* execute suppliers as future tasks then wait / join for getting results
* @param functors a supplier(s) to execute
* @return a list of results
*/
private List getResultsInFuture(Supplier<?>... functors) {
CompletableFuture[] futures = stream(functors)
.map(CompletableFuture::supplyAsync)
.collect(Collectors.toList())
.toArray(new CompletableFuture[functors.length]);
CompletableFuture.allOf(futures).join();
return stream(futures).map(a-> {
try {
return a.get();
} catch (InterruptedException | ExecutionException e) {
//logger.error("an error occurred during runtime execution a function",e);
return null;
}
}).collect(Collectors.toList());
};
答案 9 :(得分：0)
我有一个包含以下内容的实用程序类：
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
let cell = collectionView.cellForItem(at: indexPath)
if indexPath.section == 0 {
if (indexPath.row == 0) {cell!.contentView.layer.backgroundColor = colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)}
if (indexPath.row == 1) {cell!.contentView.layer.backgroundColor = colorLiteral(red: 0, green: 0.4784313725, blue: 1, alpha: 1)}
}
}
func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {
let cell = collectionView.cellForItem(at: indexPath)
if indexPath.section == 0 {
if (indexPath.row == 0) { cell!.contentView.layer.backgroundColor = colorLiteral(red: 1, green: 1, blue: 1, alpha: 1)}
if (indexPath.row == 1) { cell!.contentView.layer.backgroundColor = colorLiteral(red: 1, green: 1, blue: 1, alpha: 1)}
}
}
func collectionView(_ collectionView: UICollectionView, didHighlightItemAt indexPath: IndexPath) {
}
func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
}
一旦有了，就可以使用静态导入，简单地等待所有这样的期货：
@FunctionalInterface
public interface CheckedSupplier<X> {
X get() throws Throwable;
}
public static <X> Supplier<X> uncheckedSupplier(final CheckedSupplier<X> supplier) {
return () -> {
try {
return supplier.get();
} catch (final Throwable checkedException) {
throw new IllegalStateException(checkedException);
}
};
}
您还可以像这样收集所有结果：
futures.stream().forEach(future -> uncheckedSupplier(future::get).get());
只需回顾一下我的旧帖子，并注意到您还遇到另一种悲伤：
但是这里的问题是，例如，如果第4个期货抛出异常，那么我将不必要地等待前3个期货可用。
在这种情况下，简单的解决方案是并行执行此操作：
List<MyResultType> results = futures.stream()
.map(future -> uncheckedSupplier(future::get).get())
.collect(Collectors.toList());
这样，第一个异常虽然不会停止将来，但会破坏forEach语句，就像在串行示例中一样，但是由于所有并行，所以您不必等待前三个完成
答案 10 :(得分：0)
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Stack2 {
public static void waitFor(List<Future<?>> futures) {
List<Future<?>> futureCopies = new ArrayList<Future<?>>(futures);//contains features for which status has not been completed
while (!futureCopies.isEmpty()) {//worst case :all task worked without exception, then this method should wait for all tasks
Iterator<Future<?>> futureCopiesIterator = futureCopies.iterator();
while (futureCopiesIterator.hasNext()) {
Future<?> future = futureCopiesIterator.next();
if (future.isDone()) {//already done
futureCopiesIterator.remove();
try {
future.get();// no longer waiting
} catch (InterruptedException e) {
//ignore
//only happen when current Thread interrupted
} catch (ExecutionException e) {
Throwable throwable = e.getCause();// real cause of exception
futureCopies.forEach(f -> f.cancel(true));//cancel other tasks that not completed
return;
}
}
}
}
}
public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(3);
Runnable runnable1 = new Runnable (){
public void run(){
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
}
}
};
Runnable runnable2 = new Runnable (){
public void run(){
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
}
}
};
Runnable fail = new Runnable (){
public void run(){
try {
Thread.sleep(1000);
throw new RuntimeException("bla bla bla");
} catch (InterruptedException e) {
}
}
};
List<Future<?>> futures = Stream.of(runnable1,fail,runnable2)
.map(executorService::submit)
.collect(Collectors.toList());
double start = System.nanoTime();
waitFor(futures);
double end = (System.nanoTime()-start)/1e9;
System.out.println(end +" seconds");
}
}