一： 接口Executor
ThreadPoolExecutor的顶层接口，看文档注释。
/**
* An object that executes submitted {@link 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 {@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:
*
* 一个执行提交Runnable任务的对象。
* 这个接口提供了一种将任务提交与如何运行每个任务的机制分离的方法，包括线程使用、调度等细节。
* 通常使用Executor，而不是显式地创建线程。
* 例如：不会为了一组任务中的每一个任务调用new Thread(new(RunnableTask())).start()，
* 你可以使用下面的方法：
*
* <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. In the simplest case, an
* executor can run the submitted task immediately in the caller's
* thread:
*
* 但是，Executor接口并不严格要求执行是异步的。
* 最简单的情况下，执行器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.
* 更典型的是，任务在调用者以外的线程中执行。
* 下面的执行器为每一个提交的任务生成一个新的线程。
*
* <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. The executor below
* serializes the submission of tasks to a second executor,
* illustrating a composite executor.
* 许多Executor的实现对任务的调度方式和时间加了某种限制。
* 下面的executor将提交的任务序列化到第二个executor，解释了一个组合的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. The {@link ThreadPoolExecutor} class provides an
* extensible thread pool implementation. The {@link Executors} class
* provides convenient factory methods for these Executors.
* 这个包中提供的Executor的实现，其实现了ExecutorService，这是一个更广泛的接口。
* ThreadPoolExecutor类提供了一个可扩展的线程池实现。
* 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.
* 内存一致性影响：在将可运行对象提交给执行器之前，线程中的操作发生在其执行开始之前，可能发生在另一个线程中。
* 查看：https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/package-summary.html#MemoryVisibility
*
* @since 1.5
* @author Doug Lea
*/
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.
* 在将来的某个时间执行给定的命令。命令可以在一个新线程、一个线程池或者调用者的线程中执行，
* 具体由Executor的实现来决定
* @param command the runnable task 可运行的任务
* @throws RejectedExecutionException if this task cannot be
* accepted for execution 如果无法接受此任务执行
* @throws NullPointerException if command is null
*/
void execute(Runnable command);
}
Executor接口总结：就是将任务的提交和执行细节给封装起来了，上层不需要知道了解。
常见的执行方式有以下几种：
- 直接在调用者的线程中执行。（没啥意义，还不如调用者线程直接调用呢）
- 每一个任务都在一个新的线程中执行，可能会导致线程数过多。
- 任务在线程池中执行。ThreadPoolExecutor就是这种实现
二： 接口ExecutorService
看文档注释
/**
* An {@link Executor} that provides methods to manage termination and
* methods that can produce a {@link Future} for tracking progress of
* one or more asynchronous tasks.
*
* 一个Executor，它提供了管理终止的方法和生成Future的方法，Future用于追踪一个或多个异步任务。
*
* <p>An {@code ExecutorService} can be shut down, which will cause
* it to reject new tasks. Two different methods are provided for
* shutting down an {@code ExecutorService}. The {@link #shutdown}
* method will allow previously submitted tasks to execute before
* terminating, while the {@link #shutdownNow} method prevents waiting
* tasks from starting and attempts to stop currently executing tasks.
* Upon termination, an executor has no tasks actively executing, no
* tasks awaiting execution, and no new tasks can be submitted. An
* unused {@code ExecutorService} should be shut down to allow
* reclamation of its resources.
*
* ExecutorService可以关闭，这将导致拒绝新任务。提供两种方法来关闭ExecutorService。
* shutdown方法允许在终止之前执行先前提交的任务。
* shutdownNow方法阻止等待的任务启动并且尝试停止正在执行中的任务。
* 终止后，executor没有正在执行的任务，没有等待执行的任务，并且不能提交新的任务。
* 一个没用的ExecutorService应该关闭，以回收其资源。
*
* <p>Method {@code submit} extends base method {@link
* Executor#execute(Runnable)} by creating and returning a {@link Future}
* that can be used to cancel execution and/or wait for completion.
* Methods {@code invokeAny} and {@code invokeAll} perform the most
* commonly useful forms of bulk execution, executing a collection of
* tasks and then waiting for at least one, or all, to
* complete. (Class {@link ExecutorCompletionService} can be used to
* write customized variants of these methods.)
*
* submit方法通过创建并返回一个Future来扩展基础方法Executor.execute(Runnable)，
* Future可以用来取消执行 和/或 等待完成。
* invokeAny和invokeAll用来做最常用的批量执行，执行一组任务并且等待至少一个或全部任务完成。
* （ExecutorCompletionService类可以用来编写这些方法的自定义变体）
*
* <p>The {@link Executors} class provides factory methods for the
* executor services provided in this package.
*
* Executors类为这个包中提供的executor services提供工厂方法。
*
* <h3>Usage Examples</h3>
* 示例用法
*
* Here is a sketch of a network service in which threads in a thread
* pool service incoming requests. It uses the preconfigured {@link
* Executors#newFixedThreadPool} factory method:
*
* 下面是一个网络服务的简述，其中线程池中的线程为传入的请求提供服务。
* 它使用预定义的Executors.newFixedThreadPool工厂方法：
*
* <pre> {@code
* class NetworkService implements Runnable {
* private final ServerSocket serverSocket;
* private final ExecutorService pool;
*
* public NetworkService(int port, int poolSize)
* throws IOException {
* serverSocket = new ServerSocket(port);
* pool = Executors.newFixedThreadPool(poolSize);
* }
*
* public void run() { // run the service
* try {
* for (;;) {
* pool.execute(new Handler(serverSocket.accept()));
* }
* } catch (IOException ex) {
* pool.shutdown();
* }
* }
* }
*
* class Handler implements Runnable {
* private final Socket socket;
* Handler(Socket socket) { this.socket = socket; }
* public void run() {
* // read and service request on socket
* }
* }}</pre>
*
* The following method shuts down an {@code ExecutorService} in two phases,
* first by calling {@code shutdown} to reject incoming tasks, and then
* calling {@code shutdownNow}, if necessary, to cancel any lingering tasks:
*
* 下面的方法分两步来关闭ExecutorService，首先调用shutdown来拒绝接受新任务，
* 然后调用shutdownNow，如果必须的话，取消任何继续存留的任务。
*
* <pre> {@code
* void shutdownAndAwaitTermination(ExecutorService pool) {
* pool.shutdown(); // Disable new tasks from being submitted
* try {
* // Wait a while for existing tasks to terminate
* if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
* pool.shutdownNow(); // Cancel currently executing tasks
* // Wait a while for tasks to respond to being cancelled
* if (!pool.awaitTermination(60, TimeUnit.SECONDS))
* System.err.println("Pool did not terminate");
* }
* } catch (InterruptedException ie) {
* // (Re-)Cancel if current thread also interrupted
* pool.shutdownNow();
* // Preserve interrupt status
* Thread.currentThread().interrupt();
* }
* }}</pre>
*
* <p>Memory consistency effects: Actions in a thread prior to the
* submission of a {@code Runnable} or {@code Callable} task to an
* {@code ExecutorService}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* any actions taken by that task, which in turn <i>happen-before</i> the
* result is retrieved via {@code Future.get()}.
*
* @since 1.5
* @author Doug Lea
*/
public interface ExecutorService extends Executor {
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
*
* 启动有序的关闭，执行以前提交的任务，但是不再接受新任务。
* 如果已经关闭了再次调用没有其他效果。
*
* <p>This method does not wait for previously submitted tasks to
* complete execution. Use {@link #awaitTermination awaitTermination}
* to do that.
* 此方法不会等待先前提交的任务执行完成，就是说会立即返回。
* 可以使用awaitTermination来等待。
*
* @throws SecurityException if a security manager exists and
* shutting down this ExecutorService may manipulate
* threads that the caller is not permitted to modify
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")},
* or the security manager's {@code checkAccess} method
* denies access.
*/
void shutdown();
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks
* that were awaiting execution.
* 尝试停止所有正在执行的任务，停止正在等待的任务的处理，并返回正在等待执行的任务的列表。
*
* <p>This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
* 此方法不会等待活跃的任务终止。使用awaitTermination来等待。
*
* <p>There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks. For example, typical
* implementations will cancel via {@link Thread#interrupt}, so any
* task that fails to respond to interrupts may never terminate.
* 除了尽最大努力停止正在执行的任务之外，没有其他保证。
* 例如，典型的实现将使用Thread.interrupt方法来取消，
* 因此任何未能响应中断的任务可能永远不会终止。
*
* @return list of tasks that never commenced execution
* @throws SecurityException if a security manager exists and
* shutting down this ExecutorService may manipulate
* threads that the caller is not permitted to modify
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")},
* or the security manager's {@code checkAccess} method
* denies access.
*/
List<Runnable> shutdownNow();
/**
* Returns {@code true} if this executor has been shut down.
* 如果已经关闭掉了则返回true
* @return {@code true} if this executor has been shut down
*/
boolean isShutdown();
/**
* Returns {@code true} if all tasks have completed following shut down.
* Note that {@code isTerminated} is never {@code true} unless
* either {@code shutdown} or {@code shutdownNow} was called first.
*
* 如果关闭后所有任务都已完成，则返回true。
* 注意：除非先调用了shutdown或shutdownNow，否则永远不会返回true。
*
* @return {@code true} if all tasks have completed following shut down
*/
boolean isTerminated();
/**
* Blocks until all tasks have completed execution after a shutdown
* request, or the timeout occurs, or the current thread is
* interrupted, whichever happens first.
*
* shutdown请求后进行阻塞，直到所有任务执行完成，或者超时发生，或者当前线程被中断（以先发生的为准）
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if this executor terminated and 执行器已终止
* {@code false} if the timeout elapsed before termination 超时发生在终止之前
* @throws InterruptedException if interrupted while waiting
*/
boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Submits a value-returning task for execution and returns a
* Future representing the pending results of the task. The
* Future's {@code get} method will return the task's result upon
* successful completion.
*
* 提供一个返回值的任务以供执行，并返回一个表示任务待定结果的Future。
* Future的get方法将在任务成功完成时返回任务的结果。
*
* <p>
* If you would like to immediately block waiting
* for a task, you can use constructions of the form
* {@code result = exec.submit(aCallable).get();}
*
* 如果你想要立即阻塞等待一个任务，你可以使用result=exec.submit(aCallable).get()形式的构造
*
* <p>Note: The {@link Executors} class includes a set of methods
* that can convert some other common closure-like objects,
* for example, {@link java.security.PrivilegedAction} to
* {@link Callable} form so they can be submitted.
*
* 注意：Executors类包含一些方法可以将一些常用的闭包类对象转（例如 java.security.PrivilegedAction）
* 换为Callable形式，以便提交它们。
*
* @param task the task to submit
* @param <T> the type of the task's result
* @return a Future representing pending completion of the task
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
* @throws NullPointerException if the task is null
*/
<T> Future<T> submit(Callable<T> task);
/**
* Submits a Runnable task for execution and returns a Future
* representing that task. The Future's {@code get} method will
* return the given result upon successful completion.
*
* 提交一个Runnable任务以执行，并且返回一个表示该任务的Future。
* Future的get方法将在成功完成后返回给定的结果。
*
* @param task the task to submit
* @param result the result to return
* @param <T> the type of the result
* @return a Future representing pending completion of the task
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
* @throws NullPointerException if the task is null
*/
<T> Future<T> submit(Runnable task, T result);
/**
* Submits a Runnable task for execution and returns a Future
* representing that task. The Future's {@code get} method will
* return {@code null} upon <em>successful</em> completion.
*
* 提交一个Runnable任务以执行，并且返回一个表示该任务的Future。
* Future的get方法将在成功完成后返回null。
*
* @param task the task to submit
* @return a Future representing pending completion of the task
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
* @throws NullPointerException if the task is null
*/
Future<?> submit(Runnable task);
/**
* Executes the given tasks, returning a list of Futures holding
* their status and results when all complete.
* {@link Future#isDone} is {@code true} for each
* element of the returned list.
* Note that a <em>completed</em> task could have
* terminated either normally or by throwing an exception.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* 执行给定的多个任务，当所有任务都完成时，返回一个保存其状态和结果的Future列表。
* 对于返回列表中的每一个元素，Future.isDone为true。
* 注意，已完成的任务可以是正常终止，也可以是抛出异常终止。
* 如果在执行此操作时修改了给定的集合，则此方法的是未定义的。？？啥意思
*
* @param tasks the collection of tasks
* @param <T> the type of the values returned from the tasks
* @return a list of Futures representing the tasks, in the same
* sequential order as produced by the iterator for the
* given task list, each of which has completed
* @throws InterruptedException if interrupted while waiting, in
* which case unfinished tasks are cancelled
* @throws NullPointerException if tasks or any of its elements are {@code null}
* @throws RejectedExecutionException if any task cannot be
* scheduled for execution
*/
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException;
/**
* Executes the given tasks, returning a list of Futures holding
* their status and results
* when all complete or the timeout expires, whichever happens first.
* {@link Future#isDone} is {@code true} for each
* element of the returned list.
* Upon return, tasks that have not completed are cancelled.
* Note that a <em>completed</em> task could have
* terminated either normally or by throwing an exception.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* 执行给定的多个任务，当所有任务都完成时或者过期超时（无论哪种先发生），返回一个保存其状态和结果的Future列表。
* 对于返回列表中的每一个元素，Future.isDone为true。
* 返回后，未完成的任务将被取消。
* 注意，已完成的任务可以是正常终止，也可以是抛出异常终止。
* 如果在执行此操作时修改了给定的集合，则此方法的是未定义的。
*
* @param tasks the collection of tasks
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @param <T> the type of the values returned from the tasks
* @return a list of Futures representing the tasks, in the same
* sequential order as produced by the iterator for the
* given task list. If the operation did not time out,
* each task will have completed. If it did time out, some
* of these tasks will not have completed.
* @throws InterruptedException if interrupted while waiting, in
* which case unfinished tasks are cancelled
* @throws NullPointerException if tasks, any of its elements, or
* unit are {@code null}
* @throws RejectedExecutionException if any task cannot be scheduled
* for execution
*/
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Executes the given tasks, returning the result
* of one that has completed successfully (i.e., without throwing
* an exception), if any do. Upon normal or exceptional return,
* tasks that have not completed are cancelled.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* 执行给定的多个任务，返回一个已成功完成的任务的结果（即没有抛出异常），如果有的话。
* 在正常或异常返回时，没完成的任务将被取消。
* 如果在执行此操作时修改了给定的集合，则此方法的是未定义的。
*
* @param tasks the collection of tasks
* @param <T> the type of the values returned from the tasks
* @return the result returned by one of the tasks
* @throws InterruptedException if interrupted while waiting
* @throws NullPointerException if tasks or any element task
* subject to execution is {@code null}
* @throws IllegalArgumentException if tasks is empty
* @throws ExecutionException if no task successfully completes
* @throws RejectedExecutionException if tasks cannot be scheduled
* for execution
*/
<T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException;
/**
* Executes the given tasks, returning the result
* of one that has completed successfully (i.e., without throwing
* an exception), if any do before the given timeout elapses.
* Upon normal or exceptional return, tasks that have not
* completed are cancelled.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* 执行给定的多个任务，返回一个已成功完成的任务的结果（即没有抛出异常），如果在超时之前有的话。
* 在正常或异常返回时，没完成的任务将被取消。
* 如果在执行此操作时修改了给定的集合，则此方法的是未定义的。
*
* @param tasks the collection of tasks
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @param <T> the type of the values returned from the tasks
* @return the result returned by one of the tasks
* @throws InterruptedException if interrupted while waiting
* @throws NullPointerException if tasks, or unit, or any element
* task subject to execution is {@code null}
* @throws TimeoutException if the given timeout elapses before
* any task successfully completes
* @throws ExecutionException if no task successfully completes
* @throws RejectedExecutionException if tasks cannot be scheduled
* for execution
*/
<T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
接口ExecutorService总结：
- ExecutorService 继承与Executor接口，也是将任务的提交和执行细节封装了起来，但是又进行了扩展。
- 扩展提供了终止的方法：shutdown、shutdownNow、isShutdown、isTerminated、awaitTermination
- shutdown：不再接受新任务，会等待已提交的任务都执行完，方法会立即返回。
- shutdownNow：不再接受新任务，停止正在执行的任务和等待执行的任务，方法会立即放回，返回结果为等待执行的任务列表。
- awaitTermination：shutdown后进行阻塞，直到超时或者所有任务执行完成。
- 扩展了Executor.execute(Runnable)方法，返回了一个Future，Future可以取消任务、返回任务结果以及等待完成。
- Future submit(task) ：提交一个任务（Callable、Runnable）并立即返回一个Future
- List<Future> invokeAll(Collection tasks)：执行给定的多个任务，当所有任务完成时返回一个Future集合
- T invokeAny(Collection tasks)：执行给定的多个任务，当其中一个任务完成就返回其Future。
