Interface Executor

An object that executes submitted 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<>();
   final Executor executor;
   Runnable active;

   SerialExecutor(Executor executor) {
     this.executor = executor;
   }

   public synchronized void execute(Runnable r) {
     tasks.add(() -> {
       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 ExecutorService, which is a more extensive interface. The ThreadPoolExecutor class provides an extensible thread pool implementation. The Executors class provides convenient factory methods for these Executors.

Memory consistency effects: Actions in a thread prior to submitting a Runnable object to an Executor happen-before its execution begins, perhaps in another thread.