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, at the discretion of the {@code Executor} implementation.*/void execute(Runnable command);
}

Executor是一个接口， 这样的设计用来执行Runnable任务，把任务提交过程与具体的执行机制解耦开。有了Executor你不必再费心如何控制任务的执行，包括线程、调度等。当然这些都依赖于如何实现Executor接口，你可以使用单线程、多线程、线程池等来执行具体的任务， 甚至也可以不启用额外的线程，而在调用者所在的线程中完成任务。比如：

1，在调用者线程中完成任务：

class DirectExecutor implements Executor {public void execute(Runnable r) {r.run();}}

2，多线程，每个任务一个线程：

class ThreadPerTaskExecutor implements Executor {public void execute(Runnable r) {new Thread(r).start();}}

3, 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.

许多Executor的实现会对任务执行的时机和调度方法做一些定制。下面的SerialExecutor在任务的调度上做了定制， 使所有提交给serial executor的任务串行的执行， 而任务的具体执行过程交给另一个 executor 负责，改executor通过构造方法传入。

class SerialExecutor implements Executor {final Queue tasks = new ArrayDeque();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);}}}}

实现了Executor接口的子类：

AbstractExecutorService,

ExecutorService,

ForkJoinPool,

ScheduledExecutorService,

ScheduledThreadPoolExecutor,

ThreadPoolExecutor