我有一个返回期货列表的方法
List<Future<O>> futures = getFutures();
现在，我想要等待，直到所有的future都成功地完成处理，或者由future返回输出的任何任务抛出异常。即使一个任务抛出异常，等待其他未来也没有意义。
简单的方法就是
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;
}
}
}
但这里的问题是，例如，如果第4个future抛出异常，那么我将不必要地等待前3个future可用。
如何解决这个问题?倒计时插销有什么帮助吗?我无法使用未来的isDone，因为java文档说
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.
如果你想合并一个列表的CompletableFutures，你可以这样做:
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和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/
您可以使用ExecutorCompletionService。文档甚至为您的确切用例提供了一个示例:
相反，假设你想使用任务集的第一个非空结果，忽略任何遇到异常的任务，并在第一个任务准备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);
}
这里需要注意的重要一点是，ec .take()将获得第一个完成的任务，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。
也许这会有帮助(没有什么会被原始的线取代，耶!)
我建议用一个分开的线程运行每个Future家伙(它们并行)，然后当其中一个得到错误时，它只是信号管理器(Handler类)。
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...");}
}
}
我不得不说上面的代码会出错(没有检查)，但我希望我可以解释解决方案。请试一试。
CompletionService将使用.submit()方法获取你的Callables，你可以使用.take()方法检索计算出的期货。
您一定不能忘记的一件事是通过调用.shutdown()方法来终止ExecutorService。此外，只有在保存了对执行器服务的引用时才能调用此方法，因此请确保保留一个引用。
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数量的工作项:
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;
}
}
