The
java.util.concurrent package defines three executor
interfaces:
Executor, a simple interface that supports
launching new tasks.
ExecutorService, a subinterface of
Executor, which adds features that help manage the
lifecycle, both of the individual tasks and of the executor
itself.
ScheduledExecutorService, a subinterface of
ExecutorService, supports future and/or periodic
execution of tasks.
Typically, variables that refer to executor objects are declared as
one of these three interface types, not with an executor class type.
The Executor Interface
The
Executor
interface provides a single method,
execute, designed to
be a drop-in replacement for a common thread-creation idiom. If
r is a
Runnable object, and
e
is an
Executor object you can replace
with
However, the definition of
execute is less specific. The
low-level idiom creates a new thread and launches it immediately.
Depending on the
Executor implementation,
execute may do the same thing, but is more likely to use
an existing worker thread to run
r, or to place
r in a queue to wait for a worker thread to become
available. (We'll describe worker threads in the section on
Thread Pools.)
The executor implementations in java.util.concurrent are
designed to make full use of the more advanced
ExecutorService and ScheduledExecutorService
interfaces, although they also work with the base
Executor interface.
The ExecutorService Interface
The
ExecutorService
interface supplements
execute with a similar, but more
versatile
submit method. Like
execute,
submit accepts
Runnable objects, but also
accepts
Callable
objects, which allow the task to return a value. The
submit method returns a
Future
object, which is used to retrieve the
Callable return
value and to manage the status of both
Callable and
Runnable tasks.
ExecutorService also provides methods for submitting
large collections of Callable objects. Finally,
ExecutorService provides a number of methods for managing
the shutdown of the executor. To support immediate shutdown, tasks
should handle interrupts correctly.
The ScheduledExecutorService Interface
The
ScheduledExecutorService
interface supplements the methods of its parent
ExecutorService with
schedule, which
executes a
Runnable or
Callable task after a
specified delay. In addition, the interface defines
scheduleAtFixedRate and
scheduleWithFixedDelay, which executes specified tasks
repeatedly, at defined intervals.
