FutureTask
public
class
FutureTask
extends Object
implements
RunnableFuture<V>
java.lang.Object | |
↳ | java.util.concurrent.FutureTask<V> |
A cancellable asynchronous computation. This class provides a base
implementation of Future
, with methods to start and cancel
a computation, query to see if the computation is complete, and
retrieve the result of the computation. The result can only be
retrieved when the computation has completed; the get
methods will block if the computation has not yet completed. Once
the computation has completed, the computation cannot be restarted
or cancelled (unless the computation is invoked using
runAndReset()
).
A FutureTask
can be used to wrap a Callable
or
Runnable
object. Because FutureTask
implements
Runnable
, a FutureTask
can be submitted to an
Executor
for execution.
In addition to serving as a standalone class, this class provides
protected
functionality that may be useful when creating
customized task classes.
Summary
Public constructors | |
---|---|
FutureTask(Callable<V> callable)
Creates a |
|
FutureTask(Runnable runnable, V result)
Creates a |
Public methods | |
---|---|
boolean
|
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task. |
V
|
get(long timeout, TimeUnit unit)
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available. |
V
|
get()
Waits if necessary for the computation to complete, and then retrieves its result. |
boolean
|
isCancelled()
Returns |
boolean
|
isDone()
Returns |
void
|
run()
Sets this Future to the result of its computation unless it has been cancelled. |
String
|
toString()
Returns a string representation of this FutureTask. |
Protected methods | |
---|---|
void
|
done()
Protected method invoked when this task transitions to state
|
boolean
|
runAndReset()
Executes the computation without setting its result, and then resets this future to initial state, failing to do so if the computation encounters an exception or is cancelled. |
void
|
set(V v)
Sets the result of this future to the given value unless this future has already been set or has been cancelled. |
void
|
setException(Throwable t)
Causes this future to report an |
Inherited methods | |
---|---|
Public constructors
FutureTask
public FutureTask (Callable<V> callable)
Creates a FutureTask
that will, upon running, execute the
given Callable
.
Parameters | |
---|---|
callable |
Callable : the callable task |
Throws | |
---|---|
NullPointerException |
if the callable is null |
FutureTask
public FutureTask (Runnable runnable, V result)
Creates a FutureTask
that will, upon running, execute the
given Runnable
, and arrange that get
will return the
given result on successful completion.
Parameters | |
---|---|
runnable |
Runnable : the runnable task |
result |
V : the result to return on successful completion. If
you don't need a particular result, consider using
constructions of the form:
Future<?> f = new FutureTask<Void>(runnable, null) |
Throws | |
---|---|
NullPointerException |
if the runnable is null |
Public methods
cancel
public boolean cancel (boolean mayInterruptIfRunning)
Attempts to cancel execution of this task. This method has no
effect if the task is already completed or cancelled, or could
not be cancelled for some other reason. Otherwise, if this
task has not started when cancel
is called, this task
should never run. If the task has already started, then the
mayInterruptIfRunning
parameter determines whether the
thread executing this task (when known by the implementation)
is interrupted in an attempt to stop the task.
The return value from this method does not necessarily
indicate whether the task is now cancelled; use isCancelled()
.
Parameters | |
---|---|
mayInterruptIfRunning |
boolean : true if the thread
executing this task should be interrupted (if the thread is
known to the implementation); otherwise, in-progress tasks are
allowed to complete |
Returns | |
---|---|
boolean |
false if the task could not be cancelled,
typically because it has already completed; true
otherwise. If two or more threads cause a task to be cancelled,
then at least one of them returns true . Implementations
may provide stronger guarantees. |
get
public V get (long timeout, TimeUnit unit)
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
Parameters | |
---|---|
timeout |
long : the maximum time to wait |
unit |
TimeUnit : the time unit of the timeout argument |
Returns | |
---|---|
V |
the computed result |
Throws | |
---|---|
CancellationException |
|
ExecutionException |
|
InterruptedException |
|
TimeoutException |
get
public V get ()
Waits if necessary for the computation to complete, and then retrieves its result.
Returns | |
---|---|
V |
the computed result |
Throws | |
---|---|
CancellationException |
|
ExecutionException |
|
InterruptedException |
isCancelled
public boolean isCancelled ()
Returns true
if this task was cancelled before it completed
normally.
Returns | |
---|---|
boolean |
true if this task was cancelled before it completed |
isDone
public 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
.
Returns | |
---|---|
boolean |
true if this task completed |
run
public void run ()
Sets this Future to the result of its computation unless it has been cancelled.
toString
public String toString ()
Returns a string representation of this FutureTask.
Implementation Requirements:
- The default implementation returns a string identifying this
FutureTask, as well as its completion state. The state, in
brackets, contains one of the strings
"Completed Normally"
,"Completed Exceptionally"
,"Cancelled"
, or"Not completed"
.
Returns | |
---|---|
String |
a string representation of this FutureTask |
Protected methods
done
protected void done ()
Protected method invoked when this task transitions to state
isDone
(whether normally or via cancellation). The
default implementation does nothing. Subclasses may override
this method to invoke completion callbacks or perform
bookkeeping. Note that you can query status inside the
implementation of this method to determine whether this task
has been cancelled.
runAndReset
protected boolean runAndReset ()
Executes the computation without setting its result, and then resets this future to initial state, failing to do so if the computation encounters an exception or is cancelled. This is designed for use with tasks that intrinsically execute more than once.
Returns | |
---|---|
boolean |
true if successfully run and reset |
set
protected void set (V v)
Sets the result of this future to the given value unless this future has already been set or has been cancelled.
This method is invoked internally by the run()
method
upon successful completion of the computation.
Parameters | |
---|---|
v |
V : the value |
setException
protected void setException (Throwable t)
Causes this future to report an ExecutionException
with the given throwable as its cause, unless this future has
already been set or has been cancelled.
This method is invoked internally by the run()
method
upon failure of the computation.
Parameters | |
---|---|
t |
Throwable : the cause of failure |