code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
   * compliance with the License. You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software distributed under the License is
   * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
   * the License for the specific language governing permissions and limitations under the License.
   */
  
  package io.reactivex.rxjava3.internal.schedulers;
  
  import java.util.concurrent.*;
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.core.Scheduler;
  import io.reactivex.rxjava3.disposables.*;
  import io.reactivex.rxjava3.internal.disposables.EmptyDisposable;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * A scheduler with a shared, single threaded underlying ScheduledExecutorService.
   * @since 2.0
   */
  public final class SingleScheduler extends Scheduler {
  
      final ThreadFactory threadFactory;
      final AtomicReference<ScheduledExecutorService> executor = new AtomicReference<>();
  
      /** The name of the system property for setting the thread priority for this Scheduler. */
      private static final String KEY_SINGLE_PRIORITY = "rx3.single-priority";
  
      private static final String THREAD_NAME_PREFIX = "RxSingleScheduler";
  
      static final RxThreadFactory SINGLE_THREAD_FACTORY;
  
      static final ScheduledExecutorService SHUTDOWN;
      static {
          SHUTDOWN = Executors.newScheduledThreadPool(0);
          SHUTDOWN.shutdown();
  
          int priority = Math.max(Thread.MIN_PRIORITY, Math.min(Thread.MAX_PRIORITY,
                  Integer.getInteger(KEY_SINGLE_PRIORITY, Thread.NORM_PRIORITY)));
  
          SINGLE_THREAD_FACTORY = new RxThreadFactory(THREAD_NAME_PREFIX, priority, true);
      }
  
      public SingleScheduler() {
          this(SINGLE_THREAD_FACTORY);
      }
  
      /**
       * Constructs a SingleScheduler with the given ThreadFactory and prepares the
       * single scheduler thread.
       * @param threadFactory thread factory to use for creating worker threads. Note that this takes precedence over any
       *                      system properties for configuring new thread creation. Cannot be null.
       */
      public SingleScheduler(ThreadFactory threadFactory) {
          this.threadFactory = threadFactory;
          executor.lazySet(createExecutor(threadFactory));
      }
  
      static ScheduledExecutorService createExecutor(ThreadFactory threadFactory) {
          return SchedulerPoolFactory.create(threadFactory);
      }
  
      @Override
      public void start() {
          ScheduledExecutorService next = null;
          for (;;) {
              ScheduledExecutorService current = executor.get();
              if (current != SHUTDOWN) {
                  if (next != null) {
                      next.shutdown();
                  }
                  return;
              }
              if (next == null) {
                  next = createExecutor(threadFactory);
              }
              if (executor.compareAndSet(current, next)) {
                  return;
              }
  
          }
      }
  
      @Override
      public void shutdown() {
          ScheduledExecutorService current =  executor.getAndSet(SHUTDOWN);
          if (current != SHUTDOWN) {
              current.shutdownNow();
          }
      }
  
      @NonNull
      @Override
      public Worker createWorker() {
          return new ScheduledWorker(executor.get());
      }
  
      @NonNull
      @Override
      public Disposable scheduleDirect(@NonNull Runnable run, long delay, TimeUnit unit) {
          ScheduledDirectTask task = new ScheduledDirectTask(RxJavaPlugins.onSchedule(run), true);
          try {
              Future<?> f;
              if (delay <= 0L) {
                  f = executor.get().submit(task);
              } else {
                  f = executor.get().schedule(task, delay, unit);
              }
              task.setFuture(f);
              return task;
          } catch (RejectedExecutionException ex) {
              RxJavaPlugins.onError(ex);
              return EmptyDisposable.INSTANCE;
          }
      }
  
      @NonNull
      @Override
      public Disposable schedulePeriodicallyDirect(@NonNull Runnable run, long initialDelay, long period, TimeUnit unit) {
          final Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
          if (period <= 0L) {
  
              ScheduledExecutorService exec = executor.get();
  
              InstantPeriodicTask periodicWrapper = new InstantPeriodicTask(decoratedRun, exec);
              Future<?> f;
              try {
                  if (initialDelay <= 0L) {
                      f = exec.submit(periodicWrapper);
                  } else {
                      f = exec.schedule(periodicWrapper, initialDelay, unit);
                  }
                  periodicWrapper.setFirst(f);
              } catch (RejectedExecutionException ex) {
                  RxJavaPlugins.onError(ex);
                  return EmptyDisposable.INSTANCE;
              }
  
              return periodicWrapper;
          }
          ScheduledDirectPeriodicTask task = new ScheduledDirectPeriodicTask(decoratedRun, true);
          try {
              Future<?> f = executor.get().scheduleAtFixedRate(task, initialDelay, period, unit);
              task.setFuture(f);
              return task;
          } catch (RejectedExecutionException ex) {
              RxJavaPlugins.onError(ex);
              return EmptyDisposable.INSTANCE;
          }
      }
  
      static final class ScheduledWorker extends Scheduler.Worker {
  
          final ScheduledExecutorService executor;
  
          final CompositeDisposable tasks;
  
          volatile boolean disposed;
  
          ScheduledWorker(ScheduledExecutorService executor) {
              this.executor = executor;
              this.tasks = new CompositeDisposable();
          }
  
          @NonNull
          @Override
          public Disposable schedule(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
              if (disposed) {
                  return EmptyDisposable.INSTANCE;
              }
  
              Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
  
              ScheduledRunnable sr = new ScheduledRunnable(decoratedRun, tasks);
              tasks.add(sr);
  
              try {
                  Future<?> f;
                  if (delay <= 0L) {
                      f = executor.submit((Callable<Object>)sr);
                  } else {
                      f = executor.schedule((Callable<Object>)sr, delay, unit);
                  }
  
                  sr.setFuture(f);
              } catch (RejectedExecutionException ex) {
                  dispose();
                  RxJavaPlugins.onError(ex);
                  return EmptyDisposable.INSTANCE;
              }
  
              return sr;
          }
  
          @Override
          public void dispose() {
              if (!disposed) {
                  disposed = true;
                  tasks.dispose();
              }
          }
  
          @Override
          public boolean isDisposed() {
              return disposed;
          }
      }
  }
  