Executor - a simple interface that supports launching new tasks:

public interface Executor {
    /**
     * Executes the given command at some time in the future.  The command
     * may execute in a new thread, in a pooled thread, or in the calling
     * thread.
     */
    void execute(Runnable command);
}
								

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:

(new Thread(r)).start();
								

with

e.execute(r);
								

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.

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.

ExecutorService - a subinterface of Executor, which adds features that help manage the lifecycle, both of the individual tasks and of the executor itself:

public interface ExecutorService extends Executor {

    void shutdown();
    List<Runnable> shutdownNow();
    boolean isShutdown();
    boolean isTerminated();
    boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException;

    <T> Future<T> submit(Callable<T> task);
    <T> Future<T> submit(Runnable task, T result);
    Future<?> submit(Runnable task);

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException;
    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException;

    <T> T invokeAny(Collection<? extends Callable<T>> tasks) throws InterruptedException, ExecutionException;
    <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;

}

								

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:

public interface Callable<V> {

    /**
     * Computes a result, or throws an exception if unable to do so.
     */
    V call() throws Exception;
}

								

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.

ScheduledExecutorService - a subinterface of ExecutorService, supports future and/or periodic execution of tasks:

public interface ScheduledExecutorService extends ExecutorService {

    public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit);

    public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit);

    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit);

    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit);
}

								

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.