/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.util.List; import java.util.Collection; // BEGIN android-note // removed security manager docs // END android-note /** * 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. * *

An ExecutorService can be shut down, which will cause * it to reject new tasks. Two different methods are provided for * shutting down an 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 ExecutorService should be shut down to allow * reclamation of its resources. * *

Method submit extends base method {@link * Executor#execute} by creating and returning a {@link Future} that * can be used to cancel execution and/or wait for completion. * Methods invokeAny and 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 ExecutorService in two phases, * first by calling shutdown to reject incoming tasks, and then * calling shutdownNow, if necessary, to cancel any lingering tasks: * * 
 {@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. */ 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. * *

This method does not wait for actively executing tasks to * terminate. Use {@link #awaitTermination awaitTermination} to * do that. * *

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. * * @return list of tasks that never commenced execution */ List shutdownNow(); /** * Returns true if this executor has been shut down. * * @return true if this executor has been shut down */ boolean isShutdown(); /** * Returns true if all tasks have completed following shut down. * Note that isTerminated is never true unless * either shutdown or shutdownNow was called first. * * @return 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. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return true if this executor terminated and * 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 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 * 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 * @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 get method will * return the given result upon successful completion. * * @param task the task to submit * @param result the result to return * @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. The Future's get method will * return 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. * {@link Future#isDone} is 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 * @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 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. * {@link Future#isDone} is 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 * @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 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 * @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 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 * @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 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; }