/*
 * 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 static java.lang.ref.Reference.reachabilityFence;
import dalvik.annotation.optimization.ReachabilitySensitive;
import java.security.AccessControlContext;
import java.security.AccessControlException;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import sun.security.util.SecurityConstants;

// BEGIN android-note
// removed security manager docs
// END android-note

/**
 * 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 {@code Callable}.
 * </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
     * {@code nThreads} 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 maintains enough threads to support
     * the given parallelism level, and may use multiple queues to
     * reduce contention. The parallelism level corresponds to the
     * maximum number of threads actively engaged in, or available to
     * engage in, task processing. The actual number of threads may
     * grow and shrink dynamically. A work-stealing pool makes no
     * guarantees about the order in which submitted tasks are
     * executed.
     *
     * @param parallelism the targeted parallelism level
     * @return the newly created thread pool
     * @throws IllegalArgumentException if {@code parallelism <= 0}
     * @since 1.8
     */
    public static ExecutorService newWorkStealingPool(int parallelism) {
        return new ForkJoinPool
            (parallelism,
             ForkJoinPool.defaultForkJoinWorkerThreadFactory,
             null, true);
    }

    /**
     * Creates a work-stealing thread pool using the number of
     * {@linkplain Runtime#availableProcessors available processors}
     * as its target parallelism level.
     *
     * @return the newly created thread pool
     * @see #newWorkStealingPool(int)
     * @since 1.8
     */
    public static ExecutorService newWorkStealingPool() {
        return new ForkJoinPool
            (Runtime.getRuntime().availableProcessors(),
             ForkJoinPool.defaultForkJoinWorkerThreadFactory,
             null, true);
    }

    /**
     * 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 {@code nThreads} 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
     * {@code newFixedThreadPool(1)} 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 {@code newFixedThreadPool(1, threadFactory)} 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 {@code execute} 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
     * {@code newScheduledThreadPool(1)} 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 {@code newScheduledThreadPool(1, threadFactory)}
     * 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 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 the 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 the 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 {@code ExecutorService} 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 {@code ScheduledExecutorService} instance
     * @throws NullPointerException if executor null
     */
    public static ScheduledExecutorService unconfigurableScheduledExecutorService(ScheduledExecutorService executor) {
        if (executor == null)
            throw new NullPointerException();
        return new DelegatedScheduledExecutorService(executor);
    }

    // Android-changed: Removed references to SecurityManager from javadoc.
    /**
     * 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}. Each new
     * thread is created as a non-daemon thread with priority set to
     * the smaller of {@code Thread.NORM_PRIORITY} 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();
    }

    // Android-changed: Dropped documentation for legacy security code.
    /**
     * Legacy security code; do not use.
     */
    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
     * {@code Callable} to an otherwise resultless action.
     * @param task the task to run
     * @param result the result to return
     * @param <T> the type of the result
     * @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 {@code null}.
     * @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(); }};
    }

    // Android-changed: Dropped documentation for legacy security code.
    /**
     * Legacy security code; do not use.
     */
    public static <T> Callable<T> privilegedCallable(Callable<T> callable) {
        if (callable == null)
            throw new NullPointerException();
        return new PrivilegedCallable<T>(callable);
    }

    // Android-changed: Dropped documentation for legacy security code.
    /**
     * Legacy security code; do not use.
     */
    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.
     */
    private static final class RunnableAdapter<T> implements Callable<T> {
        private final Runnable task;
        private final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
        public String toString() {
            return super.toString() + "[Wrapped task = " + task + "]";
        }
    }

    /**
     * A callable that runs under established access control settings.
     */
    private static final class PrivilegedCallable<T> implements Callable<T> {
        final Callable<T> task;
        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();
            }
        }

        public String toString() {
            return super.toString() + "[Wrapped task = " + task + "]";
        }
    }

    /**
     * A callable that runs under established access control settings and
     * current ClassLoader.
     */
    private static final class PrivilegedCallableUsingCurrentClassLoader<T>
            implements Callable<T> {
        final Callable<T> task;
        final AccessControlContext acc;
        final ClassLoader ccl;

        PrivilegedCallableUsingCurrentClassLoader(Callable<T> task) {
            // Android-removed: System.getSecurityManager always returns null.
            /*
            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 {
                            Thread t = Thread.currentThread();
                            ClassLoader cl = t.getContextClassLoader();
                            if (ccl == cl) {
                                return task.call();
                            } else {
                                t.setContextClassLoader(ccl);
                                try {
                                    return task.call();
                                } finally {
                                    t.setContextClassLoader(cl);
                                }
                            }
                        }
                    }, acc);
            } catch (PrivilegedActionException e) {
                throw e.getException();
            }
        }

        public String toString() {
            return super.toString() + "[Wrapped task = " + task + "]";
        }
    }

    /**
     * The default thread factory.
     */
    private 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.
     */
    private static class PrivilegedThreadFactory extends DefaultThreadFactory {
        final AccessControlContext acc;
        final ClassLoader ccl;

        PrivilegedThreadFactory() {
            super();
            // Android-removed: System.getSecurityManager always returns null.
            /*
            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<>() {
                        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.
     */
    private static class DelegatedExecutorService
            implements ExecutorService {
        // Android-added: @ReachabilitySensitive
        // Needed for FinalizableDelegatedExecutorService below.
        @ReachabilitySensitive
        private final ExecutorService e;
        DelegatedExecutorService(ExecutorService executor) { e = executor; }
        public void execute(Runnable command) {
            try {
                e.execute(command);
            } finally { reachabilityFence(this); }
        }
        public void shutdown() { e.shutdown(); }
        public List<Runnable> shutdownNow() {
            try {
                return e.shutdownNow();
            } finally { reachabilityFence(this); }
        }
        public boolean isShutdown() {
            try {
                return e.isShutdown();
            } finally { reachabilityFence(this); }
        }
        public boolean isTerminated() {
            try {
                return e.isTerminated();
            } finally { reachabilityFence(this); }
        }
        public boolean awaitTermination(long timeout, TimeUnit unit)
            throws InterruptedException {
            try {
                return e.awaitTermination(timeout, unit);
            } finally { reachabilityFence(this); }
        }
        public Future<?> submit(Runnable task) {
            try {
                return e.submit(task);
            } finally { reachabilityFence(this); }
        }
        public <T> Future<T> submit(Callable<T> task) {
            try {
                return e.submit(task);
            } finally { reachabilityFence(this); }
        }
        public <T> Future<T> submit(Runnable task, T result) {
            try {
                return e.submit(task, result);
            } finally { reachabilityFence(this); }
        }
        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
            throws InterruptedException {
            try {
                return e.invokeAll(tasks);
            } finally { reachabilityFence(this); }
        }
        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
                                             long timeout, TimeUnit unit)
            throws InterruptedException {
            try {
                return e.invokeAll(tasks, timeout, unit);
            } finally { reachabilityFence(this); }
        }
        public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
            throws InterruptedException, ExecutionException {
            try {
                return e.invokeAny(tasks);
            } finally { reachabilityFence(this); }
        }
        public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
                               long timeout, TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException {
            try {
                return e.invokeAny(tasks, timeout, unit);
            } finally { reachabilityFence(this); }
        }
    }

    private static class FinalizableDelegatedExecutorService
            extends DelegatedExecutorService {
        FinalizableDelegatedExecutorService(ExecutorService executor) {
            super(executor);
        }
        @SuppressWarnings("deprecation")
        protected void finalize() {
            super.shutdown();
        }
    }

    /**
     * A wrapper class that exposes only the ScheduledExecutorService
     * methods of a ScheduledExecutorService implementation.
     */
    private 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() {}
}
