/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * 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.*;

import java.util.concurrent.atomic.AtomicInteger;

import java.security.AccessControlContext;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.security.PrivilegedExceptionAction;

import java.security.PrivilegedActionException;

import java.security.AccessControlException;

import sun.security.util.SecurityConstants;

/**

 * Factory and utility methods for {@link Executor}, {@link

 * ExecutorService}, {@link ScheduledExecutorService}, {@link

 * ThreadFactory}, and {@link Callable} classes defined in this

 * package. This class supports the following kinds of methods:

 *

 * <ul>

 *   <li> Methods that create and return an {@link ExecutorService}

 *        set up with commonly useful configuration settings.

 *   <li> Methods that create and return a {@link ScheduledExecutorService}

 *        set up with commonly useful configuration settings.

 *   <li> Methods that create and return a "wrapped" ExecutorService, that

 *        disables reconfiguration by making implementation-specific methods

 *        inaccessible.

 *   <li> Methods that create and return a {@link ThreadFactory}

 *        that sets newly created threads to a known state.

 *   <li> Methods that create and return a {@link Callable}

 *        out of other closure-like forms, so they can be used

 *        in execution methods requiring <tt>Callable</tt>.

 * </ul>

 *

 * @since 1.5

 * @author Doug Lea

 */

public class Executors {

    /**

     * Creates a thread pool that reuses a fixed number of threads

     * operating off a shared unbounded queue.  At any point, at most

     * <tt>nThreads</tt> threads will be active processing tasks.

     * If additional tasks are submitted when all threads are active,

     * they will wait in the queue until a thread is available.

     * If any thread terminates due to a failure during execution

     * prior to shutdown, a new one will take its place if needed to

     * execute subsequent tasks.  The threads in the pool will exist

     * until it is explicitly {@link ExecutorService#shutdown shutdown}.

     *

     * @param nThreads the number of threads in the pool

     * @return the newly created thread pool

     * @throws IllegalArgumentException if {@code nThreads <= 0}

     */

    public static ExecutorService newFixedThreadPool(int nThreads) {

        return new ThreadPoolExecutor(nThreads, nThreads,

                                      0L, TimeUnit.MILLISECONDS,

                                      new LinkedBlockingQueue<Runnable>());

    }

    /**

     * Creates a thread pool that reuses a fixed number of threads

     * operating off a shared unbounded queue, using the provided

     * ThreadFactory to create new threads when needed.  At any point,

     * at most <tt>nThreads</tt> threads will be active processing

     * tasks.  If additional tasks are submitted when all threads are

     * active, they will wait in the queue until a thread is

     * available.  If any thread terminates due to a failure during

     * execution prior to shutdown, a new one will take its place if

     * needed to execute subsequent tasks.  The threads in the pool will

     * exist until it is explicitly {@link ExecutorService#shutdown

     * shutdown}.

     *

     * @param nThreads the number of threads in the pool

     * @param threadFactory the factory to use when creating new threads

     * @return the newly created thread pool

     * @throws NullPointerException if threadFactory is null

     * @throws IllegalArgumentException if {@code nThreads <= 0}

     */

    public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {

        return new ThreadPoolExecutor(nThreads, nThreads,

                                      0L, TimeUnit.MILLISECONDS,

                                      new LinkedBlockingQueue<Runnable>(),

                                      threadFactory);

    }

    /**

     * Creates an Executor that uses a single worker thread operating

     * off an unbounded queue. (Note however that if this single

     * thread terminates due to a failure during execution prior to

     * shutdown, a new one will take its place if needed to execute

     * subsequent tasks.)  Tasks are guaranteed to execute

     * sequentially, and no more than one task will be active at any

     * given time. Unlike the otherwise equivalent

     * <tt>newFixedThreadPool(1)</tt> the returned executor is

     * guaranteed not to be reconfigurable to use additional threads.

     *

     * @return the newly created single-threaded Executor

     */

    public static ExecutorService newSingleThreadExecutor() {

        return new FinalizableDelegatedExecutorService

            (new ThreadPoolExecutor(1, 1,

                                    0L, TimeUnit.MILLISECONDS,

                                    new LinkedBlockingQueue<Runnable>()));

    }

    /**

     * Creates an Executor that uses a single worker thread operating

     * off an unbounded queue, and uses the provided ThreadFactory to

     * create a new thread when needed. Unlike the otherwise

     * equivalent <tt>newFixedThreadPool(1, threadFactory)</tt> the

     * returned executor is guaranteed not to be reconfigurable to use

     * additional threads.

     *

     * @param threadFactory the factory to use when creating new

     * threads

     *

     * @return the newly created single-threaded Executor

     * @throws NullPointerException if threadFactory is null

     */

    public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {

        return new FinalizableDelegatedExecutorService

            (new ThreadPoolExecutor(1, 1,

                                    0L, TimeUnit.MILLISECONDS,

                                    new LinkedBlockingQueue<Runnable>(),

                                    threadFactory));

    }

    /**

     * Creates a thread pool that creates new threads as needed, but

     * will reuse previously constructed threads when they are

     * available.  These pools will typically improve the performance

     * of programs that execute many short-lived asynchronous tasks.

     * Calls to <tt>execute</tt> will reuse previously constructed

     * threads if available. If no existing thread is available, a new

     * thread will be created and added to the pool. Threads that have

     * not been used for sixty seconds are terminated and removed from

     * the cache. Thus, a pool that remains idle for long enough will

     * not consume any resources. Note that pools with similar

     * properties but different details (for example, timeout parameters)

     * may be created using {@link ThreadPoolExecutor} constructors.

     *

     * @return the newly created thread pool

     */

    public static ExecutorService newCachedThreadPool() {

        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,

                                      60L, TimeUnit.SECONDS,

                                      new SynchronousQueue<Runnable>());

    }

    /**

     * Creates a thread pool that creates new threads as needed, but

     * will reuse previously constructed threads when they are

     * available, and uses the provided

     * ThreadFactory to create new threads when needed.

     * @param threadFactory the factory to use when creating new threads

     * @return the newly created thread pool

     * @throws NullPointerException if threadFactory is null

     */

    public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {

        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,

                                      60L, TimeUnit.SECONDS,

                                      new SynchronousQueue<Runnable>(),

                                      threadFactory);

    }

    /**

     * Creates a single-threaded executor that can schedule commands

     * to run after a given delay, or to execute periodically.

     * (Note however that if this single

     * thread terminates due to a failure during execution prior to

     * shutdown, a new one will take its place if needed to execute

     * subsequent tasks.)  Tasks are guaranteed to execute

     * sequentially, and no more than one task will be active at any

     * given time. Unlike the otherwise equivalent

     * <tt>newScheduledThreadPool(1)</tt> the returned executor is

     * guaranteed not to be reconfigurable to use additional threads.

     * @return the newly created scheduled executor

     */

    public static ScheduledExecutorService newSingleThreadScheduledExecutor() {

        return new DelegatedScheduledExecutorService

            (new ScheduledThreadPoolExecutor(1));

    }

    /**

     * Creates a single-threaded executor that can schedule commands

     * to run after a given delay, or to execute periodically.  (Note

     * however that if this single thread terminates due to a failure

     * during execution prior to shutdown, a new one will take its

     * place if needed to execute subsequent tasks.)  Tasks are

     * guaranteed to execute sequentially, and no more than one task

     * will be active at any given time. Unlike the otherwise

     * equivalent <tt>newScheduledThreadPool(1, threadFactory)</tt>

     * the returned executor is guaranteed not to be reconfigurable to

     * use additional threads.

     * @param threadFactory the factory to use when creating new

     * threads

     * @return a newly created scheduled executor

     * @throws NullPointerException if threadFactory is null

     */

    public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {

        return new DelegatedScheduledExecutorService

            (new ScheduledThreadPoolExecutor(1, threadFactory));

    }

    /**

     * Creates a thread pool that can schedule commands to run after a

     * given delay, or to execute periodically.

     * @param corePoolSize the number of threads to keep in the pool,

     * even if they are idle.

     * @return a newly created scheduled thread pool

     * @throws IllegalArgumentException if {@code corePoolSize < 0}

     */

    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {

        return new ScheduledThreadPoolExecutor(corePoolSize);

    }

    /**

     * Creates a thread pool that can schedule commands to run after a

     * given delay, or to execute periodically.

     * @param corePoolSize the number of threads to keep in the pool,

     * even if they are idle.

     * @param threadFactory the factory to use when the executor

     * creates a new thread.

     * @return a newly created scheduled thread pool

     * @throws IllegalArgumentException if {@code corePoolSize < 0}

     * @throws NullPointerException if threadFactory is null

     */

    public static ScheduledExecutorService newScheduledThreadPool(

            int corePoolSize, ThreadFactory threadFactory) {

        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);

    }

    /**

     * Returns an object that delegates all defined {@link

     * ExecutorService} methods to the given executor, but not any

     * other methods that might otherwise be accessible using

     * casts. This provides a way to safely "freeze" configuration and

     * disallow tuning of a given concrete implementation.

     * @param executor the underlying implementation

     * @return an <tt>ExecutorService</tt> instance

     * @throws NullPointerException if executor null

     */

    public static ExecutorService unconfigurableExecutorService(ExecutorService executor) {

        if (executor == null)

            throw new NullPointerException();

        return new DelegatedExecutorService(executor);

    }

    /**

     * Returns an object that delegates all defined {@link

     * ScheduledExecutorService} methods to the given executor, but

     * not any other methods that might otherwise be accessible using

     * casts. This provides a way to safely "freeze" configuration and

     * disallow tuning of a given concrete implementation.

     * @param executor the underlying implementation

     * @return a <tt>ScheduledExecutorService</tt> instance

     * @throws NullPointerException if executor null

     */

    public static ScheduledExecutorService unconfigurableScheduledExecutorService(ScheduledExecutorService executor) {

        if (executor == null)

            throw new NullPointerException();

        return new DelegatedScheduledExecutorService(executor);

    }

    /**

     * Returns a default thread factory used to create new threads.

     * This factory creates all new threads used by an Executor in the

     * same {@link ThreadGroup}. If there is a {@link

     * java.lang.SecurityManager}, it uses the group of {@link

     * System#getSecurityManager}, else the group of the thread

     * invoking this <tt>defaultThreadFactory</tt> method. Each new

     * thread is created as a non-daemon thread with priority set to

     * the smaller of <tt>Thread.NORM_PRIORITY</tt> and the maximum

     * priority permitted in the thread group.  New threads have names

     * accessible via {@link Thread#getName} of

     * <em>pool-N-thread-M</em>, where <em>N</em> is the sequence

     * number of this factory, and <em>M</em> is the sequence number

     * of the thread created by this factory.

     * @return a thread factory

     */

    public static ThreadFactory defaultThreadFactory() {

        return new DefaultThreadFactory();

    }

    /**

     * Returns a thread factory used to create new threads that

     * have the same permissions as the current thread.

     * This factory creates threads with the same settings as {@link

     * Executors#defaultThreadFactory}, additionally setting the

     * AccessControlContext and contextClassLoader of new threads to

     * be the same as the thread invoking this

     * <tt>privilegedThreadFactory</tt> method.  A new

     * <tt>privilegedThreadFactory</tt> can be created within an

     * {@link AccessController#doPrivileged} action setting the

     * current thread's access control context to create threads with

     * the selected permission settings holding within that action.

     *

     * <p> Note that while tasks running within such threads will have

     * the same access control and class loader settings as the

     * current thread, they need not have the same {@link

     * java.lang.ThreadLocal} or {@link

     * java.lang.InheritableThreadLocal} values. If necessary,

     * particular values of thread locals can be set or reset before

     * any task runs in {@link ThreadPoolExecutor} subclasses using

     * {@link ThreadPoolExecutor#beforeExecute}. Also, if it is

     * necessary to initialize worker threads to have the same

     * InheritableThreadLocal settings as some other designated

     * thread, you can create a custom ThreadFactory in which that

     * thread waits for and services requests to create others that

     * will inherit its values.

     *

     * @return a thread factory

     * @throws AccessControlException if the current access control

     * context does not have permission to both get and set context

     * class loader.

     */

    public static ThreadFactory privilegedThreadFactory() {

        return new PrivilegedThreadFactory();

    }

    /**

     * Returns a {@link Callable} object that, when

     * called, runs the given task and returns the given result.  This

     * can be useful when applying methods requiring a

     * <tt>Callable</tt> to an otherwise resultless action.

     * @param task the task to run

     * @param result the result to return

     * @return a callable object

     * @throws NullPointerException if task null

     */

    public static <T> Callable<T> callable(Runnable task, T result) {

        if (task == null)

            throw new NullPointerException();

        return new RunnableAdapter<T>(task, result);

    }

    /**

     * Returns a {@link Callable} object that, when

     * called, runs the given task and returns <tt>null</tt>.

     * @param task the task to run

     * @return a callable object

     * @throws NullPointerException if task null

     */

    public static Callable<Object> callable(Runnable task) {

        if (task == null)

            throw new NullPointerException();

        return new RunnableAdapter<Object>(task, null);

    }

    /**

     * Returns a {@link Callable} object that, when

     * called, runs the given privileged action and returns its result.

     * @param action the privileged action to run

     * @return a callable object

     * @throws NullPointerException if action null

     */

    public static Callable<Object> callable(final PrivilegedAction<?> action) {

        if (action == null)

            throw new NullPointerException();

        return new Callable<Object>() {

            public Object call() { return action.run(); }};

    }

    /**

     * Returns a {@link Callable} object that, when

     * called, runs the given privileged exception action and returns

     * its result.

     * @param action the privileged exception action to run

     * @return a callable object

     * @throws NullPointerException if action null

     */

    public static Callable<Object> callable(final PrivilegedExceptionAction<?> action) {

        if (action == null)

            throw new NullPointerException();

        return new Callable<Object>() {

            public Object call() throws Exception { return action.run(); }};

    }

    /**

     * Returns a {@link Callable} object that will, when

     * called, execute the given <tt>callable</tt> under the current

     * access control context. This method should normally be

     * invoked within an {@link AccessController#doPrivileged} action

     * to create callables that will, if possible, execute under the

     * selected permission settings holding within that action; or if

     * not possible, throw an associated {@link

     * AccessControlException}.

     * @param callable the underlying task

     * @return a callable object

     * @throws NullPointerException if callable null

     *

     */

    public static <T> Callable<T> privilegedCallable(Callable<T> callable) {

        if (callable == null)

            throw new NullPointerException();

        return new PrivilegedCallable<T>(callable);

    }

    /**

     * Returns a {@link Callable} object that will, when

     * called, execute the given <tt>callable</tt> under the current

     * access control context, with the current context class loader

     * as the context class loader. This method should normally be

     * invoked within an {@link AccessController#doPrivileged} action

     * to create callables that will, if possible, execute under the

     * selected permission settings holding within that action; or if

     * not possible, throw an associated {@link

     * AccessControlException}.

     * @param callable the underlying task

     *

     * @return a callable object

     * @throws NullPointerException if callable null

     * @throws AccessControlException if the current access control

     * context does not have permission to both set and get context

     * class loader.

     */

    public static <T> Callable<T> privilegedCallableUsingCurrentClassLoader(Callable<T> callable) {

        if (callable == null)

            throw new NullPointerException();

        return new PrivilegedCallableUsingCurrentClassLoader<T>(callable);

    }

    // Non-public classes supporting the public methods

    /**

     * A callable that runs given task and returns given result

     */

    static final class RunnableAdapter<T> implements Callable<T> {

        final Runnable task;

        final T result;

        RunnableAdapter(Runnable task, T result) {

            this.task = task;

            this.result = result;

        }

        public T call() {

            task.run();

            return result;

        }

    }

    /**

     * A callable that runs under established access control settings

     */

    static final class PrivilegedCallable<T> implements Callable<T> {

        private final Callable<T> task;

        private final AccessControlContext acc;

        PrivilegedCallable(Callable<T> task) {

            this.task = task;

            this.acc = AccessController.getContext();

        }

        public T call() throws Exception {

            try {

                return AccessController.doPrivileged(

                    new PrivilegedExceptionAction<T>() {

                        public T run() throws Exception {

                            return task.call();

                        }

                    }, acc);

            } catch (PrivilegedActionException e) {

                throw e.getException();

            }

        }

    }

    /**

     * A callable that runs under established access control settings and

     * current ClassLoader

     */

    static final class PrivilegedCallableUsingCurrentClassLoader<T> implements Callable<T> {

        private final Callable<T> task;

        private final AccessControlContext acc;

        private final ClassLoader ccl;

        PrivilegedCallableUsingCurrentClassLoader(Callable<T> task) {

            SecurityManager sm = System.getSecurityManager();

            if (sm != null) {

                // Calls to getContextClassLoader from this class

                // never trigger a security check, but we check

                // whether our callers have this permission anyways.

                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

                // Whether setContextClassLoader turns out to be necessary

                // or not, we fail fast if permission is not available.

                sm.checkPermission(new RuntimePermission("setContextClassLoader"));

            }

            this.task = task;

            this.acc = AccessController.getContext();

            this.ccl = Thread.currentThread().getContextClassLoader();

        }

        public T call() throws Exception {

            try {

                return AccessController.doPrivileged(

                    new PrivilegedExceptionAction<T>() {

                        public T run() throws Exception {

                            ClassLoader savedcl = null;

                            Thread t = Thread.currentThread();

                            try {

                                ClassLoader cl = t.getContextClassLoader();

                                if (ccl != cl) {

                                    t.setContextClassLoader(ccl);

                                    savedcl = cl;

                                }

                                return task.call();

                            } finally {

                                if (savedcl != null)

                                    t.setContextClassLoader(savedcl);

                            }

                        }

                    }, acc);

            } catch (PrivilegedActionException e) {

                throw e.getException();

            }

        }

    }

    /**

     * The default thread factory

     */

    static class DefaultThreadFactory implements ThreadFactory {

        private static final AtomicInteger poolNumber = new AtomicInteger(1);

        private final ThreadGroup group;

        private final AtomicInteger threadNumber = new AtomicInteger(1);

        private final String namePrefix;

        DefaultThreadFactory() {

            SecurityManager s = System.getSecurityManager();

            group = (s != null) ? s.getThreadGroup() :

                                  Thread.currentThread().getThreadGroup();

            namePrefix = "pool-" +

                          poolNumber.getAndIncrement() +

                         "-thread-";

        }

        public Thread newThread(Runnable r) {

            Thread t = new Thread(group, r,

                                  namePrefix + threadNumber.getAndIncrement(),

                                  0);

            if (t.isDaemon())

                t.setDaemon(false);

            if (t.getPriority() != Thread.NORM_PRIORITY)

                t.setPriority(Thread.NORM_PRIORITY);

            return t;

        }

    }

    /**

     * Thread factory capturing access control context and class loader

     */

    static class PrivilegedThreadFactory extends DefaultThreadFactory {

        private final AccessControlContext acc;

        private final ClassLoader ccl;

        PrivilegedThreadFactory() {

            super();

            SecurityManager sm = System.getSecurityManager();

            if (sm != null) {

                // Calls to getContextClassLoader from this class

                // never trigger a security check, but we check

                // whether our callers have this permission anyways.

                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

                // Fail fast

                sm.checkPermission(new RuntimePermission("setContextClassLoader"));

            }

            this.acc = AccessController.getContext();

            this.ccl = Thread.currentThread().getContextClassLoader();

        }

        public Thread newThread(final Runnable r) {

            return super.newThread(new Runnable() {

                public void run() {

                    AccessController.doPrivileged(new PrivilegedAction<Void>() {

                        public Void run() {

                            Thread.currentThread().setContextClassLoader(ccl);

                            r.run();

                            return null;

                        }

                    }, acc);

                }

            });

        }

    }

    /**

     * A wrapper class that exposes only the ExecutorService methods

     * of an ExecutorService implementation.

     */

    static class DelegatedExecutorService extends AbstractExecutorService {

        private final ExecutorService e;

        DelegatedExecutorService(ExecutorService executor) { e = executor; }

        public void execute(Runnable command) { e.execute(command); }

        public void shutdown() { e.shutdown(); }

        public List<Runnable> shutdownNow() { return e.shutdownNow(); }

        public boolean isShutdown() { return e.isShutdown(); }

        public boolean isTerminated() { return e.isTerminated(); }

        public boolean awaitTermination(long timeout, TimeUnit unit)

            throws InterruptedException {

            return e.awaitTermination(timeout, unit);

        }

        public Future<?> submit(Runnable task) {

            return e.submit(task);

        }

        public <T> Future<T> submit(Callable<T> task) {

            return e.submit(task);

        }

        public <T> Future<T> submit(Runnable task, T result) {

            return e.submit(task, result);

        }

        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)

            throws InterruptedException {

            return e.invokeAll(tasks);

        }

        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,

                                             long timeout, TimeUnit unit)

            throws InterruptedException {

            return e.invokeAll(tasks, timeout, unit);

        }

        public <T> T invokeAny(Collection<? extends Callable<T>> tasks)

            throws InterruptedException, ExecutionException {

            return e.invokeAny(tasks);

        }

        public <T> T invokeAny(Collection<? extends Callable<T>> tasks,

                               long timeout, TimeUnit unit)

            throws InterruptedException, ExecutionException, TimeoutException {

            return e.invokeAny(tasks, timeout, unit);

        }

    }

    static class FinalizableDelegatedExecutorService

        extends DelegatedExecutorService {

        FinalizableDelegatedExecutorService(ExecutorService executor) {

            super(executor);

        }

        protected void finalize() {

            super.shutdown();

        }

    }

    /**

     * A wrapper class that exposes only the ScheduledExecutorService

     * methods of a ScheduledExecutorService implementation.

     */

    static class DelegatedScheduledExecutorService

            extends DelegatedExecutorService

            implements ScheduledExecutorService {

        private final ScheduledExecutorService e;

        DelegatedScheduledExecutorService(ScheduledExecutorService executor) {

            super(executor);

            e = executor;

        }

        public ScheduledFuture<?> schedule(Runnable command, long delay,  TimeUnit unit) {

            return e.schedule(command, delay, unit);

        }

        public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {

            return e.schedule(callable, delay, unit);

        }

        public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay,  long period, TimeUnit unit) {

            return e.scheduleAtFixedRate(command, initialDelay, period, unit);

        }

        public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay,  long delay, TimeUnit unit) {

            return e.scheduleWithFixedDelay(command, initialDelay, delay, unit);

        }

    }

    /** Cannot instantiate. */

    private Executors() {}

}