Excecutor的实现通常会创建线程来执行任务。但JVM只有在所有线程全部终止后才会退出。因此，如果无法正确地关闭Executor,那么JVM将无法结束。
为了解决执行服务的生命周期问题，Excecutor扩展了ExecutorService接口，添加了一些用于生命周期管理的方法。
/**
* 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.
提供用于管理终止的方法以及可以产生Future来跟踪一个或多个异步任务的进度的方法的执行器
*
* An {@code ExecutorService} can be shut down, which will cause
* it to reject new tasks.
ExecutorService可以关闭，这将会导致它拒绝新的任务
* Two different methods are provided for
* shutting down an {@code ExecutorService}.
ExecutorService提供了两种不同的关闭方法
* The {@link #shutdown}
* method will allow previously submitted tasks to execute before
* terminating,
shutdown方法将允许先前提交的任务在终止之前执行
* while the {@link #shutdownNow} method prevents waiting
* tasks from starting and attempts to stop currently executing tasks.
而shutdownNow方法阻止等待的任务启动并尝试停止当前正在执行的任务
* Upon termination, an executor has no tasks actively executing, no
* tasks awaiting execution, and no new tasks can be submitted.
终止后，执行者将没有正在执行的任务，没有正在等待执行的任务，并且无法提交新任务
* An unused {@code ExecutorService} should be shut down to allow
* reclamation of its resources.
应该关闭未使用的ExecutorService以便回收其资源
*
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.)
*
*
The {@link Executors} class provides factory methods for the
* executor services provided in this package.
*
*
Usage Examples
*
* 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:
*
* {@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
* }
* }}
*
* 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:
以下方法分两个阶段关闭ExecutorService：
首先通过调用shutdown来拒绝传入的任务，然后
如有必要，调用shutdownNow取消所有延迟的任务
*
* {@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();
* }
* }}
*
* Memory consistency effects: Actions in a thread prior to the
* submission of a {@code Runnable} or {@code Callable} task to an
* {@code ExecutorService}
* happen-before
* any actions taken by that task, which in turn happen-before 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.
*
*
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完成后执行。
使用awaitTermination awaitTermination可以做到这一点。
*
* @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，
暂停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
*
*
This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
此方法不等待主动执行的任务终止。
使用awaitTermination可以做到这一点
*
*
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.
除了尽最大努力尝试停止处理正在执行的任务之外，没有任何保证。
例如，典型的实现将通过线程中断取消，因此任何无法响应中断的任务都可能永远不会终止。
*
* @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 shutdownNow();
/**
* Returns {@code true} if this executor has been shut down.
*
* @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.
*
* @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.
阻塞直到所有的任务在执行关闭请求或者超时发生或者当前线程中断后完成
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if this executor terminated and
* {@code false} if the timeout elapsed before termination
* @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.
*
*
* If you would like to immediately block waiting
* for a task, you can use constructions of the form
* {@code result = exec.submit(aCallable).get();}
*
*
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.
*
* @param task the task to submit
* @param 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
*/
Future submit(Callable 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.
*
* @param task the task to submit
* @param result the result to return
* @param 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
*/
Future submit(Runnable task, T result);
/**
* Submits a Runnable task for execution and returns a Future
* representing that task.
提交一个Runnable任务以执行并返回一个代表该任务的Future
The Future's {@code get} method will
* return {@code null} upon successful completion.
*
* @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.
执行给定的任务，当所有任务完成时，返回保存其状态和结果的列表
* {@link Future#isDone} is {@code true} for each
* element of the returned list.
* Note that a completed 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.
*
* @param tasks the collection of tasks
* @param 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
*/
List> invokeAll(Collection> 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.
执行给定的任务，返回持有它们的状态和结果（当全部完成或超时到期时，以先
发生者为准）的列表。
* {@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 completed 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.
*
* @param tasks the collection of tasks
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @param 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
*/
List> invokeAll(Collection> 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.
*
* @param tasks the collection of tasks
* @param 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 invokeAny(Collection> 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.
*
* @param tasks the collection of tasks
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @param 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 invokeAny(Collection> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}