/**
* An object that executes submitted {@link Runnable} tasks.
Exector是一个执行已提交的可运行任务的对象
* 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.
从线程使用和调度等相关的运行机制里面解耦任务执行的一种方式
* An {@code Executor} is normally used instead of explicitly creating threads.
* For example, rather than invoking {@code new Thread(new(RunnableTask())).start()} for each
of a set of tasks, you might use:
通常使用Executor代替显式创建线程,而不是为每个组任务调用线程的start方法
*
* <pre>
* Executor executor = <em>anExecutor</em>;
* executor.execute(new RunnableTask1());
* executor.execute(new RunnableTask2());
* ...
* </pre>
*
* However, the {@code Executor} interface does not strictly require
* that execution be asynchronous.
但是，Executor接口并不严格要求执行是异步的。
* In the simplest case, an executor can run the submitted task immediately
* in the caller's thread:
最简单的情况，Executor可以在调用者的线程中立即运行提交的任务
*
* <pre> {@code
* class DirectExecutor implements Executor {
* public void execute(Runnable r) {
* r.run();
* }
* }}</pre>
*
* More typically, tasks are executed in some thread other
* than the caller's thread. The executor below spawns a new thread
* for each task.
更典型地，任务在其他线程中而不是在调用者的线程中执行。
*
* <pre> {@code
* class ThreadPerTaskExecutor implements Executor {
* public void execute(Runnable r) {
* new Thread(r).start();
* }
* }}</pre>
*
* Many {@code Executor} implementations impose some sort of
* limitation on how and when tasks are scheduled.
许多Exector的实现者在任务怎样Scheduled以及何时Scheduled中强加了一些限制。
* The executor below
* serializes the submission of tasks to a second executor,
* illustrating a composite executor.
* <pre> {@code
* class SerialExecutor implements Executor {
* final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
* 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);
* }
* }
* }}</pre>
*
* The {@code Executor} implementations provided in this package
* implement {@link ExecutorService}, which is a more extensive interface.
实现了ExecutorService的Executor实现者是使用更广泛的
*The {@link ThreadPoolExecutor} class provides an
* extensible thread pool implementation.
ThreadPoolExecutor类提供可扩展的线程池实现
* The {@link Executors} class
* provides convenient factory methods for these Executors.
Executors类为这些执行器提供了方便的工厂方法
*
* <p>Memory consistency effects: Actions in a thread prior to
* submitting a {@code Runnable} object to an {@code Executor}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* its execution begins, perhaps in another thread.
*
* @since 1.5
* @author Doug Lea
*/
public interface Executor {
/**
* Executes the given command at some time in the future.
在将来的某个时间执行给定的命令。
* 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.
这个命令将会根据Executor的不同实现执行在一个新线程，或者一个线程池中、或者调用的线程中。
*
* @param command the runnable task
* @throws RejectedExecutionException if this task cannot be
* accepted for execution
* @throws NullPointerException if command is null
*/
void execute(Runnable command);
}
结论
执行器的作用如下：
1、从线程的使用和调度机制中解耦任务的执行
2、代替线程的显示创建
3、执行器没有严格要求一定是异步的，这个完全取决于它的实现类。
