Java Reference
In-Depth Information
The concurrency utilities provide executors as a high-level alternative to low-level
ThreadingAPIexpressionsforexecutingrunnabletasks.An
executor
isanobjectwhose
class directly or indirectly implements the
java.util.concurrent.Executor
interface, which decouples task submission from task-execution mechanics.
Note
Theexecutorframework'suseofinterfacestodecoupletasksubmissionfrom
task-executionmechanicsisanalogoustotheCollectionsFramework'suseofcorein-
terfaces todecouplelists,sets,queues,deques,andmapsfromtheirimplementations.
Decoupling results in flexible code that is easier to maintain.
Executor
declaresasolitary
void execute(Runnable runnable)
method
that executes the runnable task named
runnable
at some point in the future.
ex-
ecute()
throws
java.lang.NullPointerException
when
runnable
is
null
, and
java.util.concurrent.RejectedExecutionException
when it cannot execute
runnable
.
Note
RejectedExecutionException
can be thrown when an executor is
shutting down and does not want to accept new tasks. Also, this exception can be
thrown when the executor does not have enough room to store the task (perhaps the
executorusesaboundedblockingqueuetostoretasksandthequeueisfull—Idiscuss
blocking queues later in this chapter).
The following example presents the
Executor
equivalent of the aforementioned
new Thread(new RunnableTask()).start();
expression:
Executor executor = ...; // ... represents some executor
creation
executor.execute(new RunnableTask());
Although
Executor
is easy to use, this interface is limited in various ways:
•
Executor
focusesexclusivelyon
Runnable
.Because
Runnable
's
run()
methoddoesnotreturnavalue,thereisnoconvenientwayforarunnabletask
to return a value to its caller.
•
Executor
doesnotprovideawaytotracktheprogressofexecutingrunnable
tasks, cancel an executing runnable task, or determine when the runnable task
finishes execution.
•
Executor
cannot execute a collection of runnable tasks.
•
Executor
doesnotprovideawayforanapplicationtoshutdownanexecutor
(much less to properly shut down an executor).
