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java.lang.Object
java.util.concurrent.ExecutorCompletionService<V>
CompletionService
that uses a supplied Executor
to execute tasks. This class arranges that submitted tasks are,
upon completion, placed on a queue accessible using take.
The class is lightweight enough to be suitable for transient use
when processing groups of tasks.
Usage Examples.
Suppose you have a set of solvers for a certain problem, each
returning a value of some type Result, and would like to
run them concurrently, processing the results of each of them that
return a non-null value, in some method use(Result r). You
could write this as:
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException, ExecutionException { CompletionService<Result> ecs = new ExecutorCompletionService<Result>(e); for (Callable<Result> s : solvers) ecs.submit(s); int n = solvers.size(); for (int i = 0; i < n; ++i) { Result r = ecs.take().get(); if (r != null) use(r); } }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); }
Constructor Summary | |
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Method Summary | |
Future |
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Future | |
Future |
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Future | |
Future |
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Methods inherited from class java.lang.Object | |
clone , equals , extends Object> getClass , finalize , hashCode , notify , notifyAll , toString , wait , wait , wait |
public ExecutorCompletionService(Executor executor)
Creates an ExecutorCompletionService using the supplied executor for base task execution and aLinkedBlockingQueue
as a completion queue.
- Parameters:
executor
- the executor to use
- Throws:
NullPointerException
- if executor is null
public ExecutorCompletionService(Executor executor, BlockingQueue> completionQueue)
Creates an ExecutorCompletionService using the supplied executor for base task execution and the supplied queue as its completion queue.
- Parameters:
executor
- the executor to usecompletionQueue
- the queue to use as the completion queue normally one dedicated for use by this service
- Throws:
NullPointerException
- if executor or completionQueue are null
public Futurepoll()
Retrieves and removes the Future representing the next completed task or null if none are present.
- Specified by:
- poll in interface CompletionService<V>
- Returns:
- the Future representing the next completed task, or null if none are present
public Futurepoll(long timeout, TimeUnit unit) throws InterruptedException
Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.
- Specified by:
- poll in interface CompletionService<V>
- Parameters:
timeout
- how long to wait before giving up, in units of unitunit
- a TimeUnit determining how to interpret the timeout parameter
- Returns:
- the Future representing the next completed task or null if the specified waiting time elapses before one is present
- Throws:
InterruptedException
- if interrupted while waiting
public Futuresubmit(Callable task)
Submits a value-returning task for execution and returns a Future representing the pending results of the task. Upon completion, this task may be taken or polled.
- Specified by:
- submit in interface CompletionService<V>
- Parameters:
task
- the task to submit
- Returns:
- a Future representing pending completion of the task
- Throws:
RejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if the task is null
public Futuresubmit(Runnable task, V result)
Submits a Runnable task for execution and returns a Future representing that task. Upon completion, this task may be taken or polled.
- Specified by:
- submit in interface CompletionService<V>
- Parameters:
task
- the task to submitresult
- the result to return upon successful completion
- Returns:
- a Future representing pending completion of the task, and whose get() method will return the given result value upon completion
- Throws:
RejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if the task is null
public Futuretake() throws InterruptedException
Retrieves and removes the Future representing the next completed task, waiting if none are yet present.
- Specified by:
- take in interface CompletionService<V>
- Returns:
- the Future representing the next completed task
- Throws:
InterruptedException
- if interrupted while waiting
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