An object that executes submitted {@link Runnable} tasks. This
interface provides a way of decoupling task submission from the
mechanics of how each task will be run, including details of thread
use, scheduling, etc. An Executor is normally used
instead of explicitly creating threads. For example, rather than
invoking new Thread(new(RunnableTask())).start() for each
of a set of tasks, you might use:
Executor executor = anExecutor;
executor.execute(new RunnableTask1());
executor.execute(new RunnableTask2());
...
However, the Executor interface does not strictly
require that execution be asynchronous. In the simplest case, an
executor can run the submitted task immediately in the caller's
thread:
class DirectExecutor implements Executor {
public void execute(Runnable r) {
r.run();
}
}
More typically, tasks are executed in some thread other
than the caller's thread. The executor below spawns a new thread
for each task.
class ThreadPerTaskExecutor implements Executor {
public void execute(Runnable r) {
new Thread(r).start();
}
}
Many Executor implementations impose some sort of
limitation on how and when tasks are scheduled. The executor below
serializes the submission of tasks to a second executor,
illustrating a composite executor.
class SerialExecutor implements Executor {
final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
final Executor executor;
Runnable active;
SerialExecutor(Executor executor) {
this.executor = executor;
}
public synchronized void execute(final Runnable r) {
tasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (active == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((active = tasks.poll()) != null) {
executor.execute(active);
}
}
}
The Executor implementations provided in this package
implement {@link ExecutorService}, which is a more extensive
interface. The {@link ThreadPoolExecutor} class provides an
extensible thread pool implementation. The {@link Executors} class
provides convenient factory methods for these Executors.
Memory consistency effects: Actions in a thread prior to
submitting a {@code Runnable} object to an {@code Executor}
happen-before
its execution begins, perhaps in another thread. |
