I created a java thread pool using
dbChangeService = Executors.newSingleThreadExecutor();
I have implemented a runnable interface and added it to
Future future = dbChangeService.submit(dbChgProcessor);
when I do future.isDone() it returns false which is understandable because I am checking immediately after submitting.
What is the correct way of using Future.isDone() method so that it can produce meaningful results.
PS: I don't want to use Future.get() which is a blocking call.
Instead of simply doing future.isDone(), you should poll for it to be true. E.g.
while(!future.isDone())
{
log.info("future not done yet, waiting before retrying...");
Thread.sleep(300);
}
Object result = future.get();
This still is still kind of blocking, so you probably wanna do it on another thread that will notify your main tread using a lock object. Or even better why not use CompletableFuture? See resources here & here.
So first of all let me clarify that:
Future.isDone() tells us if the executor has finished processing the task.
If the task is complete, it will return true otherwise, it returns false.
I would suggest you to use CompletableFuture, something like:
CompletableFuture.supplyAsync(() -> {
...
});
This will return a CompletableFuture as well.
As I said in the comments, a CompletableFuture provides a broad set of methods for creating multiple Futures, chaining, and combining. It also has comprehensive exception handling support
Check a full guide with CompletableFuture examples here
Related
I am trying to implement multi threading in my Spring Boot app. I am just beginner on multi threading in Java and after making some search and reading articles on various pages, I need to be clarified about the following points. So;
As far as I see, I can use Thread, Runnable or CompletableFuture in order to implement multi threading in a Java app. CompletableFuture seems a newer and cleaner way, but Thread may have more advantages. So, should I stick to CompletableFuture or use all of them based on the scenario?
Basically I want to send 2 concurrent requests to the same service method by using CompletableFuture:
CompletableFuture<Integer> future1 = fetchAsync(1);
CompletableFuture<Integer> future2 = fetchAsync(2);
Integer result1 = future1.get();
Integer result2 = future2.get();
How can I send these request concurrently and then return result based on the following condition:
if the first result is not null, return result and stop process
if the first result is null, return the second result and stop process
How can I do this? Should I use CompletableFuture.anyOf() for that?
CompletableFuture is a tool which settles atop the Executor/ExecutorService abstraction, which has implementations dealing with Runnable and Thread. You usually have no reason to deal with Thread creation manually. If you find CompletableFuture unsuitable for a particular task you may try the other tools/abstractions first.
If you want to proceed with the first (in the sense of faster) non‑null result, you can use something like
CompletableFuture<Integer> future1 = fetchAsync(1);
CompletableFuture<Integer> future2 = fetchAsync(2);
Integer result = CompletableFuture.anyOf(future1, future2)
.thenCompose(i -> i != null?
CompletableFuture.completedFuture((Integer)i):
future1.thenCombine(future2, (a, b) -> a != null? a: b))
.join();
anyOf allows you to proceed with the first result, but regardless of its actual value. So to use the first non‑null result we need to chain another operation which will resort to thenCombine if the first result is null. This will only complete when both futures have been completed but at this point we already know that the faster result was null and the second is needed. The overall code will still result in null when both results were null.
Note that anyOf accepts arbitrarily typed futures and results in a CompletableFuture<Object>. Hence, i is of type Object and a type cast needed. An alternative with full type safety would be
CompletableFuture<Integer> future1 = fetchAsync(1);
CompletableFuture<Integer> future2 = fetchAsync(2);
Integer result = future1.applyToEither(future2, Function.identity())
.thenCompose(i -> i != null?
CompletableFuture.completedFuture(i):
future1.thenCombine(future2, (a, b) -> a != null? a: b))
.join();
which requires us to specify a function which we do not need here, so this code resorts to Function.identity(). You could also just use i -> i to denote an identity function; that’s mostly a stylistic choice.
Note that most complications stem from the design that tries to avoid blocking threads by always chaining a dependent operation to be executed when the previous stage has been completed. The examples above follow this principle as the final join() call is only for demonstration purposes; you can easily remove it and return the future, if the caller expects a future rather than being blocked.
If you are going to perform the final blocking join() anyway, because you need the result value immediately, you can also use
Integer result = future1.applyToEither(future2, Function.identity()).join();
if(result == null) {
Integer a = future1.join(), b = future2.join();
result = a != null? a: b;
}
which might be easier to read and debug. This ease of use is the motivation behind the upcoming Virtual Threads feature. When an action is running on a virtual thread, you don’t need to avoid blocking calls. So with this feature, if you still need to return a CompletableFuture without blocking the your caller thread, you can use
CompletableFuture<Integer> resultFuture = future1.applyToEitherAsync(future2, r-> {
if(r != null) return r;
Integer a = future1.join(), b = future2.join();
return a != null? a: b;
}, Executors.newVirtualThreadPerTaskExecutor());
By requesting a virtual thread for the dependent action, we can use blocking join() calls within the function without hesitation which makes the code simpler, in fact, similar to the previous non-asynchronous variant.
In all cases, the code will provide the faster result if it is non‑null, without waiting for the completion of the second future. But it does not stop the evaluation of the unnecessary future. Stopping an already ongoing evaluation is not supported by CompletableFuture at all. You can call cancel(…) on it, but this will will only set the completion state (result) of the future to “exceptionally completed with a CancellationException”
So whether you call cancel or not, the already ongoing evaluation will continue in the background and only its final result will be ignored.
This might be acceptable for some operations. If not, you would have to change the implementation of fetchAsync significantly. You could use an ExecutorService directly and submit an operation to get a Future which support cancellation with interruption.
But it also requires the operation’s code to be sensitive to interruption, to have an actual effect:
When calling blocking operations, use those methods that may abort and throw an InterruptedException and do not catch-and-continue.
When performing a long running computational intense task, poll Thread.interrupted() occasionally and bail out when true.
So, should I stick to CompletableFuture or use all of them based on the scenario?
Use the one that is most appropriate to the scenario. Obviously, we can't be more specific unless you explain the scenario.
There are various factors to take into account. For example:
Thread + Runnable doesn't have a natural way to wait for / return a result. (But it is not hard to implement.)
Repeatedly creating bare Thread objects is inefficient because thread creation is expensive. Thread pooling is better but you shouldn't implement a thread pool yourself.
Solutions that use an ExecutorService take care of thread pooling and allow you to use Callable and return a Future. But for a once-off async computation this might be over-kill.
Solutions that involve ComputableFuture allow you to compose and combine asynchronous tasks. But if you don't need to do that, using ComputableFuture may be overkill.
As you can see ... there is no single correct answer for all scenarios.
Should I use CompletableFuture.anyOf() for that?
No. The logic of your example requires that you must have the result for future1 to determine whether or not you need the result for future2. So the solution is something like this:
Integer i1 = future1.get();
if (i1 == null) {
return future2.get();
} else {
future2.cancel(true);
return i1;
}
Note that the above works with plain Future as well as CompletableFuture. If you were using CompletableFuture because you thought that anyOf was the solution, then you didn't need to do that. Calling ExecutorService.submit(Callable) will give you a Future ...
It will be more complicated if you need to deal with exceptions thrown by the tasks and/or timeouts. In the former case, you need to catch ExecutionException and the extract its cause exception to get the exception thrown by the task.
There is also the caveat that the second computation may ignore the interrupt and continue on regardless.
So, should I stick to CompletableFuture or use all of them based on the scenario?
Well, they all have different purposes and you'll probably use them all either directly or indirectly:
Thread represents a thread and while it can be subclassed in most cases you shouldn't do so. Many frameworks maintain thread pools, i.e. they spin up several threads that then can take tasks from a task pool. This is done to reduce the overhead that thread creation brings as well as to reduce the amount of contention (many threads and few cpu cores mean a lot of context switches so you'd normally try to have fewer threads that just work on one task after another).
Runnable was one of the first interfaces to represent tasks that a thread can work on. Another is Callable which has 2 major differences to Runnable: 1) it can return a value while Runnable has void and 2) it can throw checked exceptions. Depending on your case you can use either but since you want to get a result, you'll more likely use Callable.
CompletableFuture and Future are basically a way for cross-thread communication, i.e. you can use those to check whether the task is done already (non-blocking) or to wait for completion (blocking).
So in many cases it's like this:
you submit a Runnable or Callable to some executor
the executor maintains a pool of Threads to execute the tasks you submitted
the executor returns a Future (one implementation being CompletableFuture) for you to check on the status and results of the task without having to synchronize yourself.
However, there may be other cases where you directly provide a Runnable to a Thread or even subclass Thread but nowadays those are far less common.
How can I do this? Should I use CompletableFuture.anyOf() for that?
CompletableFuture.anyOf() wouldn't work since you'd not be able to determine which of the 2 you'd pass in was successful first.
Since you're interested in result1 first (which btw can't be null if the type is int) you basically want to do the following:
Integer result1 = future1.get(); //block until result 1 is ready
if( result1 != null ) {
return result1;
} else {
return future2.get(); //result1 was null so wait for result2 and return it
}
You'd not want to call future2.get() right away since that would block until both are done but instead you're first interested in future1 only so if that produces a result you wouldn't have for future2 to ever finish.
Note that the code above doesn't handle exceptional completions and there's also probably a more elegant way of composing the futures like you want but I don't remember it atm (if I do I'll add it as an edit).
Another note: you could call future2.cancel() if result1 isn't null but I'd suggest you first check whether cancelling would even work (e.g. you'd have a hard time really cancelling a webservice request) and what the results of interrupting the service would be. If it's ok to just let it complete and ignore the result that's probably the easier way to go.
I have a list of futures over which i keep looping checking Future.isDone() and if yes, then i use Future.get(). But, there are cases where the thread goes on forever and to avoid this i want to timeout.
Please help me timing out a future without using Future.get() as its a blocking operation. Most of the questions on timing out here on stackoverflow point to Future.get() with a timeout inside it.
If you can use CompletableFuture instead of plain Future you can use CompletableFuture.anyOf() method:
Returns a new CompletableFuture that is completed when any of the given CompletableFutures complete, with the same result.
CompletableFuture<Void> combinedFuture =
CompletableFuture.anyOf(future1, future2, future3);
combinedFuture.get();
if (future1.isDone()) { ... }
if (future2.isDone()) { ... }
if (future3.isDone()) { ... }
In Java 9 you have extra methods for handling timeouts e.g. CompletableFuture.completeOnTimeout().
I've got a question about CompletableFuture and its possible usage for lazy computations.
It seems like it is a great substitute for RunnableFuture for this task since it is possible to easily create task chains and to have total control of each chain link. Still I found that it is very hard to control when exactly does the computation take place.
If I just create a CompletableFuture with supplyAssync method or something like that, it is OK. It waits patiently for me to call get or join method to compute. But if I try to make an actual chain with whenCompose, handle or any other method, the evaluation starts immediately, which is very frustrating.
Of course, I can always place some blocker task at the start of the chain and release the block when I am ready to begin calculation, but it seems a bit ugly solution. Does anybody know how to control when does CompletableFuture actually run.
CompletableFuture is a push-design, i.e. results are pushed down to dependent tasks as soon as they become available. This also means side-chains that are not in themselves consumed still get executed, which can have side-effects.
What you want is a pull-design where ancestors would only be pulled in as their data is consumed.
This would be a fundamentally different design because side-effects of non-consumed trees would never happen.
Of course with enough contortions CF could be made to do what you want, but you should look into the fork-join framework instead which allows you to only run the computations you depend on instead of pushing down results.
There's a conceptual difference between RunnableFuture and CompletableFuture that you're missing here.
RunnableFuture implementations take a task as input and hold onto it. It runs the task when you call the run method.
A CompletableFuture does not hold onto a task. It only knows about the result of a task. It has three states: complete, incomplete, and completed exceptionally (failed).
CompletableFuture.supplyAsync is a factory method that gives you an incomplete CompletableFuture. It also schedules a task which, when it completes, will pass its result to the CompletableFuture's complete method. In other words, the future that supplyAsync hands you doesn't know anything about the task, and can't control when the task runs.
To use a CompletableFuture in the way you describe, you would need to create a subclass:
public class RunnableCompletableFuture<T> extends CompletableFuture<T> implements RunnableFuture<T> {
private final Callable<T> task;
public RunnableCompletableFuture(Callable<T> task) {
this.task = task;
}
#Override
public void run() {
try {
complete(task.call());
} catch (Exception e) {
completeExceptionally(e);
}
}
}
A simple way of dealing with your problem is wrapping your CompletableFuture in something with a lazy nature. You could use a Supplier or even Java 8 Stream.
it is late, but how about using constructor for first CompletableFuture in the chain?
CompletableFuture<Object> cf = new CompletableFuture<>();
// compose the chain
cf.thenCompose(sometask_here);
// later starts the chain with
cf.complete(anInputObject);
I am playing with Java 8 completable futures. I have the following code:
CountDownLatch waitLatch = new CountDownLatch(1);
CompletableFuture<?> future = CompletableFuture.runAsync(() -> {
try {
System.out.println("Wait");
waitLatch.await(); //cancel should interrupt
System.out.println("Done");
} catch (InterruptedException e) {
System.out.println("Interrupted");
throw new RuntimeException(e);
}
});
sleep(10); //give it some time to start (ugly, but works)
future.cancel(true);
System.out.println("Cancel called");
assertTrue(future.isCancelled());
assertTrue(future.isDone());
sleep(100); //give it some time to finish
Using runAsync I schedule execution of a code that waits on a latch. Next I cancel the future, expecting an interrupted exception to be thrown inside. But it seems that the thread remains blocked on the await call and the InterruptedException is never thrown even though the future is canceled (assertions pass). An equivalent code using ExecutorService works as expected. Is it a bug in the CompletableFuture or in my example?
When you call CompletableFuture#cancel, you only stop the downstream part of the chain. Upstream part, i. e. something that will eventually call complete(...) or completeExceptionally(...), doesn't get any signal that the result is no more needed.
What are those 'upstream' and 'downstream' things?
Let's consider the following code:
CompletableFuture
.supplyAsync(() -> "hello") //1
.thenApply(s -> s + " world!") //2
.thenAccept(s -> System.out.println(s)); //3
Here, the data flows from top to bottom - from being created by supplier, through being modified by function, to being consumed by println. The part above particular step is called upstream, and the part below is downstream. E. g. steps 1 and 2 are upstream for step 3.
Here's what happens behind the scenes. This is not precise, rather it's a convenient mind model of what's going on.
Supplier (step 1) is being executed (inside the JVM's common ForkJoinPool).
The result of the supplier is then being passed by complete(...) to the next CompletableFuture downstream.
Upon receiving the result, that CompletableFuture invokes next step - a function (step 2) which takes in previous step result and returns something that will be passed further, to the downstream CompletableFuture's complete(...).
Upon receiving the step 2 result, step 3 CompletableFuture invokes the consumer, System.out.println(s). After consumer is finished, the downstream CompletableFuture will receive it's value, (Void) null
As we can see, each CompletableFuture in this chain has to know who are there downstream waiting for the value to be passed to their's complete(...) (or completeExceptionally(...)). But the CompletableFuture don't have to know anything about it's upstream (or upstreams - there might be several).
Thus, calling cancel() upon step 3 doesn't abort steps 1 and 2, because there's no link from step 3 to step 2.
It is supposed that if you're using CompletableFuture then your steps are small enough so that there's no harm if a couple of extra steps will get executed.
If you want cancellation to be propagated upstream, you have two options:
Implement this yourself - create a dedicated CompletableFuture (name it like cancelled) which is checked after every step (something like step.applyToEither(cancelled, Function.identity()))
Use reactive stack like RxJava 2, ProjectReactor/Flux or Akka Streams
Apparently, it's intentional. The Javadoc for the method CompletableFuture::cancel states:
[Parameters:] mayInterruptIfRunning - this value has no effect in this implementation because interrupts are not used to control processing.
Interestingly, the method ForkJoinTask::cancel uses almost the same wording for the parameter mayInterruptIfRunning.
I have a guess on this issue:
interruption is intended to be used with blocking operations, like sleep, wait or I/O operations,
but neither CompletableFuture nor ForkJoinTask are intended to be used with blocking operations.
Instead of blocking, a CompletableFuture should create a new CompletionStage, and cpu-bound tasks are a prerequisite for the fork-join model. So, using interruption with either of them would defeat their purpose. And on the other hand, it might increase complexity, that's not required if used as intended.
If you actually want to be able to cancel a task, then you have to use Future itself (e.g. as returned by ExecutorService.submit(Callable<T>), not CompletableFuture. As pointed out in the answer by nosid, CompletableFuture completely ignores any call to cancel(true).
My suspicion is that the JDK team did not implement interruption because:
Interruption was always hacky, difficult for people to understand, and difficult to work with. The Java I/O system is not even interruptible, despite calls to InputStream.read() being blocking calls! (And the JDK team have no plans to make the standard I/O system interruptible again, like it was in the very early Java days.)
The JDK team have been trying very hard to phase out old broken APIs from the early Java days, such as Object.finalize(), Object.wait(), Thread.stop(), etc. I believe Thread.interrupt() is considered to be in the category of things that must be eventually deprecated and replaced. Therefore, newer APIs (like ForkJoinPool and CompletableFuture) are already not supporting it.
CompletableFuture was designed for building DAG-structured pipelines of operations, similar to the Java Stream API. It's very dificult to succinctly describe how interruption of one node of a dataflow DAG should affect execution in the rest of the DAG. (Should all concurrent tasks be canceled immediately, when any node is interrupted?)
I suspect the JDK team just didn't want to deal with getting interruption right, given the levels of internal complexity that the JDK and libraries have reached these days. (The internals of the lambda system -- ugh.)
One very hacky way around this would be to have each CompletableFuture export a reference to itself to an externally-visible AtomicReference, then the Thread reference could be interrupted directly when needed from another external thread. Or if you start all the tasks using your own ExecutorService, in your own ThreadPool, you can manually interrupt any or all the threads that were started, even if CompletableFuture refuses to trigger interruption via cancel(true). (Note though that CompletableFuture lambdas cannot throw checked exceptions, so if you have an interruptible wait in a CompletableFuture, you'll have to re-throw as an unchecked exception.)
More simply, you could just declare an AtomicReference<Boolean> cancel = new AtomicReference<>() in an external scope, and periodically check this flag from inside each CompletableFuture task's lambda.
You could also try setting up a DAG of Future instances rather than a DAG of CompletableFuture instances, that way you can exactly specify how exceptions and interruption/cancellation in any one task should affect the other currently-running tasks. I show how to do this in my example code in my question here, and it works well, but it's a lot of boilerplate.
You need an alternative implementation of CompletionStage to accomplish true thread interruption. I've just released a small library that serves exactly this purpose - https://github.com/vsilaev/tascalate-concurrent
The call to wait will still block even if Future.cancel(..) is called. As mentioned by others the CompletableFuture will not use interrupts to cancel the task.
According to the javadoc of CompletableFuture.cancel(..):
mayInterruptIfRunning this value has no effect in this implementation because interrupts are not used to control processing.
Even if the implementation would cause an interrupt, you would still need a blocking operation in order to cancel the task or check the status via Thread.interrupted().
Instead of interrupting the Thread, which might not be always easy to do, you may have check points in your operation where you can gracefully terminate the current task. This can be done in a loop over some elements that will be processed or you check before each step of the operation for the cancel status and throw an CancellationException yourself.
The tricky part is to get a reference of the CompletableFuture within the task in order to call Future.isCancelled(). Here is an example of how it can be done:
public abstract class CancelableTask<T> {
private CompletableFuture<T> task;
private T run() {
try {
return compute();
} catch (Throwable e) {
task.completeExceptionally(e);
}
return null;
}
protected abstract T compute() throws Exception;
protected boolean isCancelled() {
Future<T> future = task;
return future != null && future.isCancelled();
}
public Future<T> start() {
synchronized (this) {
if (task != null) throw new IllegalStateException("Task already started.");
task = new CompletableFuture<>();
}
return task.completeAsync(this::run);
}
}
Edit: Here the improved CancelableTask version as a static factory:
public static <T> CompletableFuture<T> supplyAsync(Function<Future<T>, T> operation) {
CompletableFuture<T> future = new CompletableFuture<>();
return future.completeAsync(() -> operation.apply(future));
}
here is the test method:
#Test
void testFuture() throws InterruptedException {
CountDownLatch started = new CountDownLatch(1);
CountDownLatch done = new CountDownLatch(1);
AtomicInteger counter = new AtomicInteger();
Future<Object> future = supplyAsync(task -> {
started.countDown();
while (!task.isCancelled()) {
System.out.println("Count: " + counter.getAndIncrement());
}
System.out.println("Task cancelled");
done.countDown();
return null;
});
// wait until the task is started
assertTrue(started.await(5, TimeUnit.SECONDS));
future.cancel(true);
System.out.println("Cancel called");
assertTrue(future.isCancelled());
assertTrue(future.isDone());
assertTrue(done.await(5, TimeUnit.SECONDS));
}
If you really want to use interrupts in addition to the CompletableFuture, then you can pass a custom Executor to CompletableFuture.completeAsync(..) where you create your own Thread, override cancel(..) in the CompletableFuture and interrupt your Thread.
The CancellationException is part of the internal ForkJoin cancel routine. The exception will come out when you retrieve the result of future:
try { future.get(); }
catch (Exception e){
System.out.println(e.toString());
}
Took a while to see this in a debugger. The JavaDoc is not that clear on what is happening or what you should expect.
Below is the piece of code that submits a job.. Let's say I have 3 threads running. how does the get method wait and obtain the appropriate thread results.
Future<?> result = threadPool.submitTasks(new Callable<T>() {
public T call() throws Exception {
// do something
}
});
anyType = (T) result.get();
Or Lets say I have Task A resulted 1 and Task B resulted 2.. When it comes to get method, what is the guarantee that it returns the correct values?
Your submitted task (in this case the Callable) is wrapped into the instance of the returned Future. In essence, the Future is directly related to the task it was created for, and not any other task.
Internally, when calling get, the future will attempt to acquire a lock that it shares in common with its wrapped task. Once acquired, it then queries the status of the task in order to determine what to do next:
Throw an exception if the Future was cancelled, or if the underlying task generated an exception
Otherwise, return the result that was generated by the task.
This is broadly how it works, there are several implementations of Future and they all have different internal logic.
You're assuming there is a guarantee they will receive the correct output. This is a question of implementing Thread safe code.
Often, to make an implementation thread safe, you will lock it from use with some sort of flag. This will indicate to other threads that they can not use it.