Develop Reference

Does orTimeout method of CompletableFuture on Java-9 kill the long running task? [duplicate]

I have method that is checking the CompletableFuture execution time. If such CompletableFuture is executing for more than 2 seconds i want to kill this task. But how can I doit if i don't have control overy thread where CompletableFuture methods are executed ?
final CompletableFuture<List<List<Student>>> responseFuture = new CompletableFuture<>();
responseFuture.supplyAsync(this::createAllRandomGroups)
.thenAccept(this::printGroups)
.exceptionally(throwable -> {
throwable.printStackTrace();
return null;
});
createAllRandomGroups()
private List<List<Student>> createAllRandomGroups() {
System.out.println("XD");
List<Student> allStudents = ClassGroupUtils.getActiveUsers();
Controller controller = Controller.getInstance();
List<List<Student>> groups = new ArrayList<>();
int groupSize = Integer.valueOf(controller.getGroupSizeComboBox().getSelectionModel().getSelectedItem());
int numberOfGroupsToGenerate = allStudents.size() / groupSize;
int studentWithoutGroup = allStudents.size() % groupSize;
if (studentWithoutGroup != 0) groups.add(this.getListOfStudentsWithoutGroup(allStudents, groupSize));
for(int i = 0; i < numberOfGroupsToGenerate; i++) {
boolean isGroupCreated = false;
while (!isGroupCreated){
Collections.shuffle(allStudents);
List<Student> newGroup = this.createNewRandomGroupOfStudents(allStudents, groupSize);
groups.add(newGroup);
if (!DataManager.isNewGroupDuplicated(newGroup.toString())) {
isGroupCreated = true;
allStudents.removeAll(newGroup);
}
}
}
DataManager.saveGroupsToCache(groups);
return groups;
}
printGroups()
private void printGroups(List<List<Student>> lists) {
System.out.println(lists);
}
This statement responseFuture.cancel(true); does not kill thread where responseFuture is doing the methods. So what is the most elegant way to terminate CompletableFuture thread ?

When you create a chain of CompletableFuture stages like b = a.thenApply(function), this handy method creates a setup of different components. Basically, these components refer to each other as a → function → b, so the completion of a will trigger the evaluation of function which will first pre-check whether b still is not completed, then evaluate your function and attempt to complete b with the result.
But b itself has no knowledge of function or the thread that will evaluate it. In fact, function is not special to b, anyone could call complete, completeExceptionally or cancel on it from any thread, the first one winning. Hence, the completable in the class name.
The only way to get hands on the threads evaluating the functions, is to be in control of them right from the start, e.g.
ExecutorService myWorkers = Executors.newFixedThreadPool(2);
CompletableFuture<FinalResultType> future
= CompletableFuture.supplyAsync(() -> generateInitialValue(), myWorkers)
.thenApplyAsync(v -> nextCalculation(v), myWorkers)
.thenApplyAsync(v -> lastCalculation(v), myWorkers);
future.whenComplete((x,y) -> myWorkers.shutdownNow());
Now, the completion of future, e.g. via cancellation, will ensure that no new evaluation will be triggered by this chain and further makes an attempt to interrupt ongoing evaluations, if any.
So you can implement a timeout, e.g.
try {
try {
FinalResultType result = future.get(2, TimeUnit.SECONDS);
System.out.println("got "+result);
}
catch(TimeoutException ex) {
if(future.cancel(true)) System.out.println("cancelled");
else System.out.println("got "+future.get());
}
}
catch(ExecutionException|InterruptedException ex) {
ex.printStackTrace();
}
Not that the rejection of tasks due to the shutdown of the thread pool may cause some of the intermediate future to never complete, but for this chain of stages, this is irrelevant. All that matters, is, that the final stage future is completed, which is guaranteed, as it is its completion which triggers the shutdown.

The only way to terminate a thread is via interruption, which is a cooperative mechanism. This means the the thread must implement interruption logic, by handling the InterruptedException.
But it is a really bad practice to interrupt threads that you don't own, which I think is your case.

Chain CompletableFuture and stop on first success

I'm consuming an API that returns CompletableFutures for querying devices (similar to digitalpetri modbus).
I need to call this API with a couple of options to query a device and figure out what it is - this is basically trial and error until it succeeds. These are embedded device protocols that I cannot change, but you can think of the process as working similar to the following:
Are you an apple?
If not, then are you a pineapple?
If not, then are you a pen?
...
While the API uses futures, in reality, the communications are serial (going over the same physical piece of wire), so they will never be executed synchronously. Once I know what it is, I want to be able to stop trying and let the caller know what it is.
I already know that I can get the result of only one of the futures with any (see below), but that may result in additional attempts that should be avoided.
Is there a pattern for chaining futures where you stop once one of them succeeds?
Similar, but is wasteful of very limited resources.
List<CompletableFuture<String>> futures = Arrays.asList(
CompletableFuture.supplyAsync(() -> "attempt 1"),
CompletableFuture.supplyAsync(() -> "attempt 2"),
CompletableFuture.supplyAsync(() -> "attempt 3"));
CompletableFuture<String>[] futuresArray = (CompletableFuture<String>[]) futures.toArray();
CompletableFuture<Object> c = CompletableFuture.anyOf(futuresArray);

Suppose that you have a method that is "pseudo-asynchronous" as you describe, i.e. it has an asynchronous API but requires some locking to perform:
private final static Object lock = new Object();
private static CompletableFuture<Boolean> pseudoAsyncCall(int input) {
return CompletableFuture.supplyAsync(() -> {
synchronized (lock) {
System.out.println("Executing for " + input);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
return input > 3;
}
});
}
And a List<Integer> of inputs that you want to check against this method, you can check each of them in sequence with recursive composition:
public static CompletableFuture<Integer> findMatch(List<Integer> inputs) {
return findMatch(inputs, 0);
}
private static CompletableFuture<Integer> findMatch(List<Integer> inputs, int startIndex) {
if (startIndex >= inputs.size()) {
// no match found -- an exception could be thrown here if preferred
return CompletableFuture.completedFuture(null);
}
return pseudoAsyncCall(inputs.get(startIndex))
.thenCompose(result -> {
if (result) {
return CompletableFuture.completedFuture(inputs.get(startIndex));
} else {
return findMatch(inputs, startIndex + 1);
}
});
}
This would be used like this:
public static void main(String[] args) {
List<Integer> inputs = Arrays.asList(0, 1, 2, 3, 4, 5);
CompletableFuture<Integer> matching = findMatch(inputs);
System.out.println("Found match: " + matching.join());
}
Output:
Executing for 0
Executing for 1
Executing for 2
Executing for 3
Executing for 4
Found match: 4
As you can see, it is not called for input 5, while your API (findMatch()) remains asynchronous.

I think the best you can do is, after your retrieval of the result,
futures.forEach(f -> f.cancel(true));
This will not affect the one having produced the result, and tries its best to stop the others. Since IIUC you get them from an outside source, there's no guarantee it will actually interrupt their work.
However, since
this class has no direct control over the computation that causes it to be completed, cancellation is treated as just another form of exceptional completion
(from CompletableFuture doc), I doubt it will do what you actually want.

How to kill CompletableFuture related threads?

I have method that is checking the CompletableFuture execution time. If such CompletableFuture is executing for more than 2 seconds i want to kill this task. But how can I doit if i don't have control overy thread where CompletableFuture methods are executed ?
final CompletableFuture<List<List<Student>>> responseFuture = new CompletableFuture<>();
responseFuture.supplyAsync(this::createAllRandomGroups)
.thenAccept(this::printGroups)
.exceptionally(throwable -> {
throwable.printStackTrace();
return null;
});
createAllRandomGroups()
private List<List<Student>> createAllRandomGroups() {
System.out.println("XD");
List<Student> allStudents = ClassGroupUtils.getActiveUsers();
Controller controller = Controller.getInstance();
List<List<Student>> groups = new ArrayList<>();
int groupSize = Integer.valueOf(controller.getGroupSizeComboBox().getSelectionModel().getSelectedItem());
int numberOfGroupsToGenerate = allStudents.size() / groupSize;
int studentWithoutGroup = allStudents.size() % groupSize;
if (studentWithoutGroup != 0) groups.add(this.getListOfStudentsWithoutGroup(allStudents, groupSize));
for(int i = 0; i < numberOfGroupsToGenerate; i++) {
boolean isGroupCreated = false;
while (!isGroupCreated){
Collections.shuffle(allStudents);
List<Student> newGroup = this.createNewRandomGroupOfStudents(allStudents, groupSize);
groups.add(newGroup);
if (!DataManager.isNewGroupDuplicated(newGroup.toString())) {
isGroupCreated = true;
allStudents.removeAll(newGroup);
}
}
}
DataManager.saveGroupsToCache(groups);
return groups;
}
printGroups()
private void printGroups(List<List<Student>> lists) {
System.out.println(lists);
}
This statement responseFuture.cancel(true); does not kill thread where responseFuture is doing the methods. So what is the most elegant way to terminate CompletableFuture thread ?

When you create a chain of CompletableFuture stages like b = a.thenApply(function), this handy method creates a setup of different components. Basically, these components refer to each other as a → function → b, so the completion of a will trigger the evaluation of function which will first pre-check whether b still is not completed, then evaluate your function and attempt to complete b with the result.
But b itself has no knowledge of function or the thread that will evaluate it. In fact, function is not special to b, anyone could call complete, completeExceptionally or cancel on it from any thread, the first one winning. Hence, the completable in the class name.
The only way to get hands on the threads evaluating the functions, is to be in control of them right from the start, e.g.
ExecutorService myWorkers = Executors.newFixedThreadPool(2);
CompletableFuture<FinalResultType> future
= CompletableFuture.supplyAsync(() -> generateInitialValue(), myWorkers)
.thenApplyAsync(v -> nextCalculation(v), myWorkers)
.thenApplyAsync(v -> lastCalculation(v), myWorkers);
future.whenComplete((x,y) -> myWorkers.shutdownNow());
Now, the completion of future, e.g. via cancellation, will ensure that no new evaluation will be triggered by this chain and further makes an attempt to interrupt ongoing evaluations, if any.
So you can implement a timeout, e.g.
try {
try {
FinalResultType result = future.get(2, TimeUnit.SECONDS);
System.out.println("got "+result);
}
catch(TimeoutException ex) {
if(future.cancel(true)) System.out.println("cancelled");
else System.out.println("got "+future.get());
}
}
catch(ExecutionException|InterruptedException ex) {
ex.printStackTrace();
}
Not that the rejection of tasks due to the shutdown of the thread pool may cause some of the intermediate future to never complete, but for this chain of stages, this is irrelevant. All that matters, is, that the final stage future is completed, which is guaranteed, as it is its completion which triggers the shutdown.

The only way to terminate a thread is via interruption, which is a cooperative mechanism. This means the the thread must implement interruption logic, by handling the InterruptedException.
But it is a really bad practice to interrupt threads that you don't own, which I think is your case.

Limiting rate of requests with Reactor

I'm using project reactor to load data from a web service using rest. This is done in parallel with multiple threads. I'm starting to hit rate limits on the web service, so I would like to send at most 10 requests per second to avoid getting these errors. How would I do that using reactor?
Using zipWith(Mono.delayMillis(100))? Or is there some better way?
Thank you

You can use delayElements instead of the whole zipwith.

One could use Flux.delayElements to process a 10 requests batch at every 1s; be aware though that if the processing takes longer than 1s the next batch will still be started in parallel hence being processed together with the previous one (and potentially many other previous ones)!
That's why I propose another solution where a 10 requests batch is still processed at every 1s but, if its processing takes longer than 1s, the next batch will fail (see overflow IllegalStateException); one could deal with that failure such that to continue the overall processing but I won't show that here because I want to keep the example simple; see onErrorResume useful to handle overflow IllegalStateException.
The code below will do a GET on https://www.google.com/ at a rate of 10 requests per second. You'll have to do additional changes in order to support the situation where your server is not able to process in 1s all your 10 requests; you could just skip sending requests when those asked at previous second are still processed by your server.
#Test
void parallelHttpRequests() {
// this is just for limiting the test running period otherwise you don't need it
int COUNT = 2;
// use whatever (blocking) http client you desire;
// when using e.g. WebClient (Spring, non blocking client)
// the example will slightly change for no longer use
// subscribeOn(Schedulers.elastic())
RestTemplate client = new RestTemplate();
// exit, lock, condition are provided to allow one to run
// all this code in a #Test, otherwise they won't be needed
var exit = new AtomicBoolean(false);
var lock = new ReentrantLock();
var condition = lock.newCondition();
MessageFormat message = new MessageFormat("#batch: {0}, #req: {1}, resultLength: {2}");
Flux.interval(Duration.ofSeconds(1L))
.take(COUNT) // this is just for limiting the test running period otherwise you don't need it
.doOnNext(batch -> debug("#batch", batch)) // just for debugging
.flatMap(batch -> Flux.range(1, 10) // 10 requests per 1 second
.flatMap(i -> Mono.fromSupplier(() ->
client.getForEntity("https://www.google.com/", String.class).getBody()) // your request goes here (1 of 10)
.map(s -> message.format(new Object[]{batch, i, s.length()})) // here the request's result will be the output of message.format(...)
.doOnSubscribe(s -> debug("doOnSubscribe: #batch = " + batch + ", i = " + i)) // just for debugging
.subscribeOn(Schedulers.elastic()) // one I/O thread per request
)
)
// consider using onErrorResume to handle overflow IllegalStateException
.subscribe(
s -> debug("received", s) // do something with the above request's result
e -> {
// pay special attention to overflow IllegalStateException
debug("error", e.getMessage());
signalAll(exit, condition, lock);
},
() -> {
debug("done");
signalAll(exit, condition, lock);
}
);
await(exit, condition, lock);
}
// you won't need the "await" and "signalAll" methods below which
// I created only to be easier for one to run this in a test class
private void await(AtomicBoolean exit, Condition condition, Lock lock) {
lock.lock();
while (!exit.get()) {
try {
condition.await();
} catch (InterruptedException e) {
// maybe spurious wakeup
e.printStackTrace();
}
}
lock.unlock();
debug("exit");
}
private void signalAll(AtomicBoolean exit, Condition condition, Lock lock) {
exit.set(true);
try {
lock.lock();
condition.signalAll();
} finally {
lock.unlock();
}
}

Delegating to threads while preserving linear readability

I've been experimenting with different ways to handle blocking methods with disconnected results while maintaining state which might have been interrupted. I've found it to be frustrating having to deal with disparate classes and methods where sending and receiving are difficult to align.
In the following example, SomeBlockingMethod() normally returns void as a message is sent to some other process. But instead I've made it synchronized with a listener which receives the result. By spinning it off to a thread, I can wait() for the result with a timeout or indefinitely.
This is nice because once the result is returned, I can continue working with a particular state which I had to pause while waiting for the result of the threaded task.
This there anything wrong with my approach?
Although this question may seem generic, I am specifically looking for advice on threading in Java.
Example pseudocode:
public class SomeClass implements Command {
#Override
public void onCommand() {
Object stateObject = new SomeObjectWithState();
// Do things with stateObject
Runnable rasync = () -> {
Object r = SomeBlockingMethod();
// Blocking method timed out
if (r == null)
return;
Runnable rsync = () -> {
// Continue operation on r which must be done synchronously
// Also do things with stateObject
};
Scheduler().run(rsync);
};
Scheduler().run(rasync);
}
Update with CompletableFuture:
CompletableFuture<Object> f = CompletableFuture.supplyAsync(() -> {
return SomeBlockingMethod();
});
f.thenRun(() -> { () -> {
String r = null;
try {
r = f.get();
}
catch (Exception e) {
e.printStackTrace();
}
// Continue but done asynchronously
});
or better yet:
CompletableFuture.supplyAsync(() -> {
return SomeBlockingMethod();
}).thenAccept((
Object r) -> {
// Continue but done asynchronously
});
The problem with using strictly CompletableFuture is that CompletableFuture.thenAccept is run from the global thread pool and is not guaranteed to be synchronous with the calling thread.
Adding the scheduler back for the synchronous task fixes this:
CompletableFuture.supplyAsync(() -> {
return SomeBlockingMethod();
}).thenAccept((
Object r) -> {
Runnable rsync = () -> {
// Continue operation on r which must be done synchronously
};
Scheduler().run(rsync);
});
A caveat of using CompletableFuture compared to the complete scheduler method is that any previous state which exists outside must be final or effectively final.

You should check out RxJava, it uses stream manipulation and has threading support.
api.getPeople()
.observeOn(Schedulers.computation())
.filter(p -> return p.isEmployee();)
.map(p -> return String.format("%s %s - %s", p.firstName(), p.lastName(), p.payrollNumber());)
.toList()
.observerOn(<ui scheudler>)
.subscirbe(p -> screen.setEmployees(p);)