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How to implement a nonblocking future processor in Java in order to retrieve processed results later

I am using an external library that has the code from below. I am sending a lot of commands and am interesed in the result for statistics to check how many calls failed and how many succeeded
public Future<CommandResult> sendCommand(Command command) {
return command.execute();
}
CommandResult can be success or failure
However, if I use client.sendCommand(command).get(); then, I am waiting for the result synchronously, meanwhile the app is being blocked.
I would like to check only later (after 30 seconds which calls succeded and which failed). I am guaranteed to get an answer in 10 seconds.
The problem is that the app waits for the computation to complete, and then retrieves its result.
I was thinking about this approach based on the answers:
List<Future< CommandResult >> futures = new ArrayList<>();
for(Command command: commands) {
futures.add(client.sendCommand(command));
}
//in a scheduler, 30+ seconds later
for (Future<Boolean> future : futures) {
saveResult(future.get());
}

I would like to check only later (after 30 seconds which calls succeeded and which failed). I am guaranteed to get an answer in 10 seconds. The problem is that the app waits for the computation to complete, and then retrieves its result.
If you want check on the results at a later time then your solution with Future<Boolean> should be fine. The jobs will run in the background and you will get the results form then when you call future.get(). Each of those get() calls do block however.
If you want to get the results as they come in, I would use an ExecutorCompletionService which you can poll anytime to see if you have results. The poll is non-blocking.
// create your thread pool using fixed or other pool
Executor<Result> threadPool = Executors.newFixedThreadPool(5);
// wrap the Executor in a CompletionService
CompletionService<Boolean> completionService =
new ExecutorCompletionService<>(e);
// submit jobs to the pool through the ExecutorCompletionService
for (Job job : jobs) {
completionService.submit(job);
}
// after we have submitted all of the jobs we can shutdown the Executor
// the jobs submitted will continue to run
threadPool.shutdown();
...
// some point later you can do
int jobsRunning = jobs.size();
for (int jobsRunning = jobs.size(); jobsRunning > 0; ) {
// do some processing ...
// are any results available?
Boolean result = completionService.poll();
if (result != null) {
// process a result if available
jobsRunning--;
}
}
Note that you will need to track how many jobs you submitted to the CompletionService.

Future is a legacy java feature which does not allow for reactive non blocking functionalities. The CompletableFuture is a later enhancement in Java in order to allow such reactive non blocking functionalities.
You can based on this previous SO answer try to convert your Future into a CompletableFuture and then you will have methods exposed to take advantage of non blocking execution.
Check the following example and modify accordingly.
public class Application {
public static void main(String[] args) throws ParseException {
Future future = new SquareCalculator().calculate(10);
CompletableFuture<Integer> completableFuture = makeCompletableFuture(future);
System.out.println("before apply");
completableFuture.thenApply(s -> {
System.out.println(s);
return s;
});
System.out.println("after apply method");
}
public static <T> CompletableFuture<T> makeCompletableFuture(Future<T> future) {
if (future.isDone())
return transformDoneFuture(future);
return CompletableFuture.supplyAsync(() -> {
try {
if (!future.isDone())
awaitFutureIsDoneInForkJoinPool(future);
return future.get();
} catch (ExecutionException e) {
throw new RuntimeException(e);
} catch (InterruptedException e) {
// Normally, this should never happen inside ForkJoinPool
Thread.currentThread().interrupt();
// Add the following statement if the future doesn't have side effects
// future.cancel(true);
throw new RuntimeException(e);
}
});
}
private static <T> CompletableFuture<T> transformDoneFuture(Future<T> future) {
CompletableFuture<T> cf = new CompletableFuture<>();
T result;
try {
result = future.get();
} catch (Throwable ex) {
cf.completeExceptionally(ex);
return cf;
}
cf.complete(result);
return cf;
}
private static void awaitFutureIsDoneInForkJoinPool(Future<?> future)
throws InterruptedException {
ForkJoinPool.managedBlock(new ForkJoinPool.ManagedBlocker() {
#Override public boolean block() throws InterruptedException {
try {
future.get();
} catch (ExecutionException e) {
throw new RuntimeException(e);
}
return true;
}
#Override public boolean isReleasable() {
return future.isDone();
}
});
}
}
And then the class to create an example Future
public class SquareCalculator {
private ExecutorService executor
= Executors.newSingleThreadExecutor();
public Future<Integer> calculate(Integer input) {
return executor.submit(() -> {
Thread.sleep(1000);
return input * input;
});
}
}
Will result into

If converting the Future instances to CompletableFuture (see answer from Panagiotis Bougioukos) is an option, then you can implement a simple helper function for turning a Stream<CompletableFuture<T>> into a CompletableFuture<Stream<T>>:
public static <T> CompletableFuture<Stream<T>> collect(Stream<CompletableFuture<T>> futures) {
return futures
.map(future -> future.thenApply(Stream::of))
.reduce(
CompletableFuture.completedFuture(Stream.empty()),
(future1, future2) ->
future1
.thenCompose(stream1 ->
future2
.thenApply(stream2 ->
concat(stream1, stream2)))
);
}
Essentially this reduces the stream of futures in parallel to a future of a stream.
If you use this e.g. on a stream of futures of strings, it will return a future that completes once the last of the individual futures completed:
Stream<CompletableFuture<String>> streamOfFutures = ...
CompletableFuture<Stream<String>> futureOfStream = collect(streamOfFutures);
// Prints a list of strings once the "slowest" future completed
System.out.println(futureOfStream.get().toList());

Wait on any future in Java 8 without creating thread per future

I have a collection of futures and I would like to wait on any of them, meaning have a blocking call that will return once any future is done.
I saw CompletableFuture.anyOf() but if I understood its code correctly it creates a thread per future, I would like to use a less wasteful approach in terms of resources, if its possible in Java.

Straight answer is yes, here's a sample method
private <T> CompletableFuture<T> waitAny(List<CompletableFuture<T>> allFutures) throws InterruptedException {
Thread thread = Thread.currentThread();
while (!thread.isInterrupted()) {
for (CompletableFuture<T> future : allFutures) {
if (future.isDone()) {
return future;
}
}
Thread.sleep(50L);
}
throw new InterruptedException();
}
Second option
private <T> CompletableFuture<T> waitAny(List<CompletableFuture<T>> allFutures) throws InterruptedException {
CompletableFuture<CompletableFuture<T>> any = new CompletableFuture<>();
for (CompletableFuture<T> future : allFutures) {
future.handleAsync((t, throwable) -> {
any.complete(future);
return null;
});
}
try {
return any.get();
} catch (ExecutionException e) {
throw new IllegalStateException(e);
}
}
but the whole context of the task is unclear,
there's probably more optimal solution.

Cancel task on timeout in RxJava

I'm experimenting with RxJava and Java 8's CompletableFuture class
and do not quite get how to handle timeout conditions.
import static net.javacrumbs.futureconverter.java8rx.FutureConverter.toObservable;
// ...
Observable<String> doSomethingSlowly() {
CompletableFuture<PaymentResult> task = CompletableFuture.supplyAsync(() -> {
// this call may be very slow - if it takes too long,
// we want to time out and cancel it.
return processor.slowExternalCall();
});
return toObservable(task);
}
// ...
doSomethingSlowly()
.single()
.timeout(3, TimeUnit.SECONDS, Observable.just("timeout"));
This basically works (if the timeout of three seconds is reached, "timeout" is published). I would however additionally want to cancel the future task that I've wrapped in an Observable - is that possible with an RxJava-centric approach?
I know that one option would be to handle the timeout myself, using task.get(3, TimeUnit.SECONDS), but I wonder if it's possible to do all task handling stuff in RxJava.

Yes, you can do this. You would add a Subscription to the Subscriber.
This allows you to listen in on unsubscriptions, which will happen if you explicitly call subscribe().unsubscribe() or if the Observable completes successfully or with an error.
If you see an unsubscription before the future has completed, you can assume it's because of either an explicit unsubscribe or a timeout.
public class FutureTest {
public static void main(String[] args) throws IOException {
doSomethingSlowly()
.timeout(1, TimeUnit.SECONDS, Observable.just("timeout"))
.subscribe(System.out::println);
System.in.read(); // keep process alive
}
private static Observable<String> doSomethingSlowly() {
CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
}
return "Something";
});
return toObservable(future);
}
private static <T> Observable<T> toObservable(CompletableFuture<T> future) {
return Observable.create(subscriber -> {
subscriber.add(new Subscription() {
private boolean unsubscribed = false;
#Override
public void unsubscribe() {
if (!future.isDone()){
future.cancel(true);
}
unsubscribed = true;
}
#Override
public boolean isUnsubscribed() {
return unsubscribed;
}
});
future.thenAccept(value -> {
if (!subscriber.isUnsubscribed()){
subscriber.onNext(value);
subscriber.onCompleted();
}
}).exceptionally(throwable -> {
if (!subscriber.isUnsubscribed()) {
subscriber.onError(throwable);
}
return null;
});
});
}
}

Waiting on a list of Future

I have a method which returns a List of futures
List<Future<O>> futures = getFutures();
Now I want to wait until either all futures are done processing successfully or any of the tasks whose output is returned by a future throws an exception. Even if one task throws an exception, there is no point in waiting for the other futures.
Simple approach would be to
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;
}
}
}
But the problem here is if, for example, the 4th future throws an exception, then I will wait unnecessarily for the first 3 futures to be available.
How to solve this? Will count down latch help in any way? I'm unable to use Future isDone because the java doc says
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.

You can use a CompletionService to receive the futures as soon as they are ready and if one of them throws an exception cancel the processing. Something like this:
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;
}
}
I think you can further improve to cancel any still executing tasks if one of them throws an error.

If you are using Java 8 then you can do this easier with CompletableFuture and CompletableFuture.allOf, which applies the callback only after all supplied CompletableFutures are done.
// Waits for *all* futures to complete and returns a list of results.
// If *any* future completes exceptionally then the resulting future will also complete exceptionally.
public static <T> CompletableFuture<List<T>> all(List<CompletableFuture<T>> futures) {
CompletableFuture[] cfs = futures.toArray(new CompletableFuture[futures.size()]);
return CompletableFuture.allOf(cfs)
.thenApply(ignored -> futures.stream()
.map(CompletableFuture::join)
.collect(Collectors.toList())
);
}

Use a CompletableFuture in Java 8
// 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());

You can use an ExecutorCompletionService. The documentation even has an example for your exact use-case:
Suppose instead that you would like to use the first non-null result of the set of tasks, ignoring any that encounter exceptions, and cancelling all other tasks when the first one is ready:
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);
}
The important thing to notice here is that ecs.take() will get the first completed task, not just the first submitted one. Thus you should get them in the order of finishing the execution (or throwing an exception).

If you are using Java 8 and don't want to manipulate CompletableFutures, I have written a tool to retrieve results for a List<Future<T>> using streaming. The key is that you are forbidden to map(Future::get) as it throws.
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();
}
}
This needs an AggregateException that works like C#'s
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;
}
}
This component acts exactly as C#'s Task.WaitAll. I am working on a variant that does the same as CompletableFuture.allOf (equivalento to Task.WhenAll)
The reason why I did this is that I am using Spring's ListenableFuture and don't want to port to CompletableFuture despite it is a more standard way

In case that you want combine a List of CompletableFutures, you can do this :
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();
For more details on Future & CompletableFuture, useful links:
1. Future: https://www.baeldung.com/java-future
2. CompletableFuture: https://www.baeldung.com/java-completablefuture
3. CompletableFuture: https://www.callicoder.com/java-8-completablefuture-tutorial/

I've got a utility class that contains these:
#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);
}
};
}
Once you have that, using a static import, you can simple wait for all futures like this:
futures.stream().forEach(future -> uncheckedSupplier(future::get).get());
you can also collect all their results like this:
List<MyResultType> results = futures.stream()
.map(future -> uncheckedSupplier(future::get).get())
.collect(Collectors.toList());
Just revisiting my old post and noticing that you had another grief:
But the problem here is if, for example, the 4th future throws an exception, then I will wait unnecessarily for the first 3 futures to be available.
In this case, the simple solution is to do this in parallel:
futures.stream().parallel()
.forEach(future -> uncheckedSupplier(future::get).get());
This way the first exception, although it will not stop the future, will break the forEach-statement, like in the serial example, but since all wait in parallel, you won't have to wait for the first 3 to complete.

maybe this would help (nothing would replaced with raw thread, yeah!)
I suggest run each Future guy with a separated thread (they goes parallel), then when ever one of the got error, it just signal the manager(Handler class).
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...");}
}
}
I have to say the above code would error(didn't check), but I hope I could explain the solution. please have a try.

/**
* 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());
};

The CompletionService will take your Callables with the .submit() method and you can retrieve the computed futures with the .take() method.
One thing you must not forget is to terminate the ExecutorService by calling the .shutdown() method. Also you can only call this method when you have saved a reference to the executor service so make sure to keep one.
Example code - For a fixed number of work items to be worked on in parallel:
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();
Example code - For a dynamic number of work items to be worked on in parallel:
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;
}
}

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");
}
}

This is what i use to wait for a certain time on a list of futures. I think its cleaner.
CountDownLatch countDownLatch = new CountDownLatch(partitions.size());
// Some parallel work
for (Something tp : somethings) {
completionService.submit(() -> {
try {
work(something)
} catch (ConnectException e) {
} finally {
countDownLatch.countDown();
}
});
}
try {
if (!countDownLatch.await(secondsToWait, TimeUnit.SECONDS)){
}
} catch (InterruptedException e) {
}

A Guava-based solution can be implemented using Futures.FutureCombiner.
Here is the code example given in the javadoc:
final ListenableFuture<Instant> loginDateFuture =
loginService.findLastLoginDate(username);
final ListenableFuture<List<String>> recentCommandsFuture =
recentCommandsService.findRecentCommands(username);
ListenableFuture<UsageHistory> usageFuture =
Futures.whenAllSucceed(loginDateFuture, recentCommandsFuture)
.call(
() ->
new UsageHistory(
username,
Futures.getDone(loginDateFuture),
Futures.getDone(recentCommandsFuture)),
executor);
For more info, see the ListenableFutureExplained section of the user's guide.
If you're curious about how it works under the hood, I suggest looking at this part of the source code: AggregateFuture.java#L127-L186

Handling exceptions from Java ExecutorService tasks

I'm trying to use Java's ThreadPoolExecutor class to run a large number of heavy weight tasks with a fixed number of threads. Each of the tasks has many places during which it may fail due to exceptions.
I've subclassed ThreadPoolExecutor and I've overridden the afterExecute method which is supposed to provide any uncaught exceptions encountered while running a task. However, I can't seem to make it work.
For example:
public class ThreadPoolErrors extends ThreadPoolExecutor {
public ThreadPoolErrors() {
super( 1, // core threads
1, // max threads
1, // timeout
TimeUnit.MINUTES, // timeout units
new LinkedBlockingQueue<Runnable>() // work queue
);
}
protected void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
if(t != null) {
System.out.println("Got an error: " + t);
} else {
System.out.println("Everything's fine--situation normal!");
}
}
public static void main( String [] args) {
ThreadPoolErrors threadPool = new ThreadPoolErrors();
threadPool.submit(
new Runnable() {
public void run() {
throw new RuntimeException("Ouch! Got an error.");
}
}
);
threadPool.shutdown();
}
}
The output from this program is "Everything's fine--situation normal!" even though the only Runnable submitted to the thread pool throws an exception. Any clue to what's going on here?
Thanks!

WARNING: It should be noted that this solution will block the calling thread in future.get().
If you want to process exceptions thrown by the task, then it is generally better to use Callable rather than Runnable.
Callable.call() is permitted to throw checked exceptions, and these get propagated back to the calling thread:
Callable task = ...
Future future = executor.submit(task);
// do something else in the meantime, and then...
try {
future.get();
} catch (ExecutionException ex) {
ex.getCause().printStackTrace();
}
If Callable.call() throws an exception, this will be wrapped in an ExecutionException and thrown by Future.get().
This is likely to be much preferable to subclassing ThreadPoolExecutor. It also gives you the opportunity to re-submit the task if the exception is a recoverable one.

From the docs:
Note: When actions are enclosed in
tasks (such as FutureTask) either
explicitly or via methods such as
submit, these task objects catch and
maintain computational exceptions, and
so they do not cause abrupt
termination, and the internal
exceptions are not passed to this
method.
When you submit a Runnable, it'll get wrapped in a Future.
Your afterExecute should be something like this:
public final class ExtendedExecutor extends ThreadPoolExecutor {
// ...
protected void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
if (t == null && r instanceof Future<?>) {
try {
Future<?> future = (Future<?>) r;
if (future.isDone()) {
future.get();
}
} catch (CancellationException ce) {
t = ce;
} catch (ExecutionException ee) {
t = ee.getCause();
} catch (InterruptedException ie) {
Thread.currentThread().interrupt();
}
}
if (t != null) {
System.out.println(t);
}
}
}

The explanation for this behavior is right in the javadoc for afterExecute:
Note: When actions are enclosed in
tasks (such as FutureTask) either
explicitly or via methods such as
submit, these task objects catch and
maintain computational exceptions, and
so they do not cause abrupt
termination, and the internal
exceptions are not passed to this
method.

I got around it by wrapping the supplied runnable submitted to the executor.
CompletableFuture.runAsync(() -> {
try {
runnable.run();
} catch (Throwable e) {
Log.info(Concurrency.class, "runAsync", e);
}
}, executorService);

I'm using VerboseRunnable class from jcabi-log, which swallows all exceptions and logs them. Very convenient, for example:
import com.jcabi.log.VerboseRunnable;
scheduler.scheduleWithFixedDelay(
new VerboseRunnable(
Runnable() {
public void run() {
// the code, which may throw
}
},
true // it means that all exceptions will be swallowed and logged
),
1, 1, TimeUnit.MILLISECONDS
);

Another solution would be to use the ManagedTask and ManagedTaskListener.
You need a Callable or Runnable which implements the interface ManagedTask.
The method getManagedTaskListener returns the instance you want.
public ManagedTaskListener getManagedTaskListener() {
And you implement in ManagedTaskListener the taskDone method:
#Override
public void taskDone(Future<?> future, ManagedExecutorService executor, Object task, Throwable exception) {
if (exception != null) {
LOGGER.log(Level.SEVERE, exception.getMessage());
}
}
More details about managed task lifecycle and listener.

This works
It is derived from SingleThreadExecutor, but you can adapt it easily
Java 8 lamdas code, but easy to fix
It will create a Executor with a single thread, that can get a lot of tasks; and will wait for the current one to end execution to begin with the next
In case of uncaugth error or exception the uncaughtExceptionHandler will catch it
public final class SingleThreadExecutorWithExceptions {
public static ExecutorService newSingleThreadExecutorWithExceptions(final Thread.UncaughtExceptionHandler uncaughtExceptionHandler) {
ThreadFactory factory = (Runnable runnable) -> {
final Thread newThread = new Thread(runnable, "SingleThreadExecutorWithExceptions");
newThread.setUncaughtExceptionHandler( (final Thread caugthThread,final Throwable throwable) -> {
uncaughtExceptionHandler.uncaughtException(caugthThread, throwable);
});
return newThread;
};
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue(),
factory){
protected void afterExecute(Runnable runnable, Throwable throwable) {
super.afterExecute(runnable, throwable);
if (throwable == null && runnable instanceof Future) {
try {
Future future = (Future) runnable;
if (future.isDone()) {
future.get();
}
} catch (CancellationException ce) {
throwable = ce;
} catch (ExecutionException ee) {
throwable = ee.getCause();
} catch (InterruptedException ie) {
Thread.currentThread().interrupt(); // ignore/reset
}
}
if (throwable != null) {
uncaughtExceptionHandler.uncaughtException(Thread.currentThread(),throwable);
}
}
});
}
private static class FinalizableDelegatedExecutorService
extends DelegatedExecutorService {
FinalizableDelegatedExecutorService(ExecutorService executor) {
super(executor);
}
protected void finalize() {
super.shutdown();
}
}
/**
* A wrapper class that exposes only the ExecutorService methods
* of an ExecutorService implementation.
*/
private static class DelegatedExecutorService extends AbstractExecutorService {
private final ExecutorService e;
DelegatedExecutorService(ExecutorService executor) { e = executor; }
public void execute(Runnable command) { e.execute(command); }
public void shutdown() { e.shutdown(); }
public List shutdownNow() { return e.shutdownNow(); }
public boolean isShutdown() { return e.isShutdown(); }
public boolean isTerminated() { return e.isTerminated(); }
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
return e.awaitTermination(timeout, unit);
}
public Future submit(Runnable task) {
return e.submit(task);
}
public Future submit(Callable task) {
return e.submit(task);
}
public Future submit(Runnable task, T result) {
return e.submit(task, result);
}
public List> invokeAll(Collection> tasks)
throws InterruptedException {
return e.invokeAll(tasks);
}
public List> invokeAll(Collection> tasks,
long timeout, TimeUnit unit)
throws InterruptedException {
return e.invokeAll(tasks, timeout, unit);
}
public T invokeAny(Collection> tasks)
throws InterruptedException, ExecutionException {
return e.invokeAny(tasks);
}
public T invokeAny(Collection> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
return e.invokeAny(tasks, timeout, unit);
}
}
private SingleThreadExecutorWithExceptions() {}
}

This is because of AbstractExecutorService :: submit is wrapping your runnable into RunnableFuture (nothing but FutureTask) like below
AbstractExecutorService.java
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null); /////////HERE////////
execute(ftask);
return ftask;
}
Then execute will pass it to Worker and Worker.run() will call the below.
ThreadPoolExecutor.java
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run(); /////////HERE////////
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
Finally task.run(); in the above code call will call
FutureTask.run(). Here is the exception handler code, because of
this you are NOT getting the expected exception.
class FutureTask<V> implements RunnableFuture<V>
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) { /////////HERE////////
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}

If you want to monitor the execution of task, you could spin 1 or 2 threads (maybe more depending on the load) and use them to take tasks from an ExecutionCompletionService wrapper.

The doc's example wasn't giving me the results I wanted.
When a Thread process was abandoned (with explicit interput();s) Exceptions were appearing.
Also I wanted to keep the "System.exit" functionality that a normal main thread has with a typical throw, I wanted this so that the programmer was not forced to work on the code having to worry on it's context (... a thread), If any error appears, it must either be a programming error, or the case must be solved in place with a manual catch... no need for overcomplexities really.
So I changed the code to match my needs.
#Override
protected void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
if (t == null && r instanceof Future<?>) {
Future<?> future = (Future<?>) r;
boolean terminate = false;
try {
future.get();
} catch (ExecutionException e) {
terminate = true;
e.printStackTrace();
} catch (InterruptedException | CancellationException ie) {// ignore/reset
Thread.currentThread().interrupt();
} finally {
if (terminate) System.exit(0);
}
}
}
Be cautious though, this code basically transforms your threads into a main thread Exception-wise, while keeping all it's parallel properties... But let's be real, designing architectures in function of the system's parallel mechanism (extends Thread) is the wrong approach IMHO... unless an event driven design is strictly required....but then... if that is the requirement the question is: Is the ExecutorService even needed in this case?... maybe not.

If your ExecutorService comes from an external source (i. e. it's not possible to subclass ThreadPoolExecutor and override afterExecute()), you can use a dynamic proxy to achieve the desired behavior:
public static ExecutorService errorAware(final ExecutorService executor) {
return (ExecutorService) Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),
new Class[] {ExecutorService.class},
(proxy, method, args) -> {
if (method.getName().equals("submit")) {
final Object arg0 = args[0];
if (arg0 instanceof Runnable) {
args[0] = new Runnable() {
#Override
public void run() {
final Runnable task = (Runnable) arg0;
try {
task.run();
if (task instanceof Future<?>) {
final Future<?> future = (Future<?>) task;
if (future.isDone()) {
try {
future.get();
} catch (final CancellationException ce) {
// Your error-handling code here
ce.printStackTrace();
} catch (final ExecutionException ee) {
// Your error-handling code here
ee.getCause().printStackTrace();
} catch (final InterruptedException ie) {
Thread.currentThread().interrupt();
}
}
}
} catch (final RuntimeException re) {
// Your error-handling code here
re.printStackTrace();
throw re;
} catch (final Error e) {
// Your error-handling code here
e.printStackTrace();
throw e;
}
}
};
} else if (arg0 instanceof Callable<?>) {
args[0] = new Callable<Object>() {
#Override
public Object call() throws Exception {
final Callable<?> task = (Callable<?>) arg0;
try {
return task.call();
} catch (final Exception e) {
// Your error-handling code here
e.printStackTrace();
throw e;
} catch (final Error e) {
// Your error-handling code here
e.printStackTrace();
throw e;
}
}
};
}
}
return method.invoke(executor, args);
});
}

This is similar to mmm's solution, but a bit more understandable. Have your tasks extend an abstract class that wraps the run() method.
public abstract Task implements Runnable {
public abstract void execute();
public void run() {
try {
execute();
} catch (Throwable t) {
// handle it
}
}
}
public MySampleTask extends Task {
public void execute() {
// heavy, error-prone code here
}
}

Instead of subclassing ThreadPoolExecutor, I would provide it with a ThreadFactory instance that creates new Threads and provides them with an UncaughtExceptionHandler