# Interface Executor

All Known Subinterfaces:
  `ExecutorService, ScheduledExecutorService`

All Known Implementing Classes:
  `AbstractExecutorService, ForkJoinPool, ScheduledThreadPoolExecutor, ThreadPoolExecutor`

public 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.

Since:
  1.5

## Method Summary

Modifier and Type
Method
Description
`void`
`execute(Runnable command)`
Executes the given command at some time in the future.

## Method Details

### execute

void execute(Runnable command)
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, at the discretion of the `Executor` implementation.

Parameters:
  `command` - the runnable task
Throws:
  `RejectedExecutionException` - if this task cannot be accepted for execution
  `NullPointerException` - if command is null