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/*
* 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/licenses/publicdomain
*/
package java.util.concurrent;
/**
* 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 <tt>Executor</tt> is normally used
* instead of explicitly creating threads. For example, rather than
* invoking <tt>new Thread(new(RunnableTask())).start()</tt> for each
* of a set of tasks, you might use:
*
* <pre>
* Executor executor = <em>anExecutor</em>;
* executor.execute(new RunnableTask1());
* executor.execute(new RunnableTask2());
* ...
* </pre>
*
* However, the <tt>Executor</tt> 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:
*
* <pre>
* 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.
*
* <pre>
* class ThreadPerTaskExecutor implements Executor {
* public void execute(Runnable r) {
* new Thread(r).start();
* }
* }</pre>
*
* Many <tt>Executor</tt> 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.
*
* <pre>
* 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 <tt>Executor</tt> 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.
*
* <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. The command
* may execute in a new thread, in a pooled thread, or in the calling
* thread, at the discretion of the <tt>Executor</tt> implementation.
*
* @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);
}
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