Interface ListenableFuture<V>
- All Superinterfaces:
Future<V>
- All Known Subinterfaces:
CheckedFuture<V,
,X> ListenableScheduledFuture<V>
- All Known Implementing Classes:
AbstractCheckedFuture
,AbstractFuture
,ForwardingCheckedFuture
,ForwardingCheckedFuture.SimpleForwardingCheckedFuture
,ForwardingListenableFuture
,ForwardingListenableFuture.SimpleForwardingListenableFuture
,ListenableFutureTask
,SettableFuture
Future
that accepts completion listeners. Each listener has an associated executor, and
it is invoked using this executor once the future's computation is complete. If the computation has already completed when the listener is added, the listener will
execute immediately.
See the Guava User Guide article on
ListenableFuture
.
Purpose
The main purpose of ListenableFuture
is to help you chain together a graph of
asynchronous operations. You can chain them together manually with calls to methods like
Futures.transform
, but you will
often find it easier to use a framework. Frameworks automate the process, often adding features
like monitoring, debugging, and cancellation. Examples of frameworks include:
The main purpose of addListener
is to support this chaining. You will
rarely use it directly, in part because it does not provide direct access to the Future
result. (If you want such access, you may prefer Futures.addCallback
.) Still, direct addListener
calls are occasionally useful:
final String name = ...;
inFlight.add(name);
ListenableFuture<Result> future = service.query(name);
future.addListener(new Runnable() {
public void run() {
processedCount.incrementAndGet();
inFlight.remove(name);
lastProcessed.set(name);
logger.info("Done with {0}", name);
}
}, executor);
How to get an instance
We encourage you to return ListenableFuture
from your methods so that your users can
take advantage of the utilities built atop the class. The way that you will
create ListenableFuture
instances depends on how you currently create Future
instances:
- If you receive them from an
java.util.concurrent.ExecutorService
, convert that service to aListeningExecutorService
, usually by callingMoreExecutors.listeningDecorator
. - If you manually call
FutureTask.set(V)
or a similar method, create aSettableFuture
instead. (If your needs are more complex, you may preferAbstractFuture
.)
Test doubles: If you need a ListenableFuture
for your test, try a SettableFuture
or one of the methods in the Futures.immediate*
family. Avoid creating a mock or stub Future
. Mock and stub implementations are
fragile because they assume that only certain methods will be called and because they often
implement subtleties of the API improperly.
Custom implementation: Avoid implementing ListenableFuture
from scratch. If you
can't get by with the standard implementations, prefer to derive a new Future
instance
with the methods in Futures
or, if necessary, to extend AbstractFuture
.
Occasionally, an API will return a plain Future
and it will be impossible to change
the return type. For this case, we provide a more expensive workaround in
JdkFutureAdapters
. However, when possible, it is more efficient and reliable to create a
ListenableFuture
directly.
- Since:
- 1.0
-
Nested Class Summary
Nested classes/interfaces inherited from interface java.util.concurrent.Future
Future.State
-
Method Summary
Modifier and TypeMethodDescriptionvoid
addListener
(Runnable listener, Executor executor) Registers a listener to be run on the given executor.Methods inherited from interface java.util.concurrent.Future
cancel, exceptionNow, get, get, isCancelled, isDone, resultNow, state
-
Method Details
-
addListener
Registers a listener to be run on the given executor. The listener will run when theFuture
's computation is complete or, if the computation is already complete, immediately.There is no guaranteed ordering of execution of listeners, but any listener added through this method is guaranteed to be called once the computation is complete.
Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown during
Executor.execute
(e.g., aRejectedExecutionException
or an exception thrown by direct execution) will be caught and logged.Note: For fast, lightweight listeners that would be safe to execute in any thread, consider
MoreExecutors.directExecutor()
. Otherwise, avoid it. HeavyweightdirectExecutor
listeners can cause problems, and these problems can be difficult to reproduce because they depend on timing. For example:- The listener may be executed by the caller of
addListener
. That caller may be a UI thread or other latency-sensitive thread. This can harm UI responsiveness. - The listener may be executed by the thread that completes this
Future
. That thread may be an internal system thread such as an RPC network thread. Blocking that thread may stall progress of the whole system. It may even cause a deadlock. - The listener may delay other listeners, even listeners that are not themselves
directExecutor
listeners.
This is the most general listener interface. For common operations performed using listeners, see
Futures
. For a simplified but general listener interface, seeaddCallback()
.Memory consistency effects: Actions in a thread prior to adding a listener happen-before its execution begins, perhaps in another thread.
- Parameters:
listener
- the listener to run when the computation is completeexecutor
- the executor to run the listener in- Throws:
RejectedExecutionException
- if we tried to execute the listener immediately but the executor rejected it.
- The listener may be executed by the caller of
-