Executor

AI-generated Key Takeaways

• The Executor interface provides a way to decouple task submission from the mechanics of how each task will be run, often using thread pools instead of explicitly creating threads.

• Executor implementations have flexibility in how they execute tasks, including running them immediately, in a new thread, or in a pooled thread.

• ExecutorService extends Executor with more advanced features, such as managing termination and tracking task progress with Future objects.

• Several implementations like ThreadPoolExecutor and ScheduledThreadPoolExecutor are available in the java.util.concurrent package, along with factory methods in Executors for easier creation.

• Memory consistency is guaranteed: actions in a thread before submitting a task happen-before its execution begins.

public interface Executor

Known Indirect Subclasses AbstractExecutorService, ExecutorService, ForkJoinPool, ScheduledExecutorService, ScheduledThreadPoolExecutor, ThreadPoolExecutor AbstractExecutorService Provides default implementations of ExecutorService execution methods.  ExecutorService An Executor that provides methods to manage termination and methods that can produce a Future for tracking progress of one or more asynchronous tasks.  ForkJoinPool An ExecutorService for running ForkJoinTasks.  ScheduledExecutorService An ExecutorService that can schedule commands to run after a given delay, or to execute periodically.  ScheduledThreadPoolExecutor A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically.  ThreadPoolExecutor An ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Executors factory methods.

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(final Runnable r) {
     tasks.add(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 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.

Public Method Summary

abstract void

execute(Runnable command) Executes the given command at some time in the future.

Public Methods

public abstract 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