android/guava/src/com/google/common/util/concurrent/SequentialExecutor.java
@Override
  public void execute(final Runnable task) {
    checkNotNull(task);
    final Runnable submittedTask;
    final long oldRunCount;
    synchronized (queue) {
      // If the worker is already running (or execute() on the delegate returned successfully, and
      // the worker has yet to start) then we don't need to start the worker.
      if (workerRunningState == RUNNING || workerRunningState == QUEUED) {
        queue.add(task);
        return;
      }

      oldRunCount = workerRunCount;

      // If the worker is not yet running, the delegate Executor might reject our attempt to start
      // it. To preserve FIFO order and failure atomicity of rejected execution when the same
      // Runnable is executed more than once, allocate a wrapper that we know is safe to remove by
      // object identity.
      // A data structure that returned a removal handle from add() would allow eliminating this
      // allocation.
      submittedTask =
          new Runnable() {
            @Override
            public void run() {
              task.run();
            }
          };
      queue.add(submittedTask);
      workerRunningState = QUEUING;
    }

    try {
      executor.execute(worker);
    } catch (RuntimeException | Error t) {
      synchronized (queue) {
        boolean removed =
            (workerRunningState == IDLE || workerRunningState == QUEUING)
                && queue.removeLastOccurrence(submittedTask);
        // If the delegate is directExecutor(), the submitted runnable could have thrown a REE. But
        // that's handled by the log check that catches RuntimeExceptions in the queue worker.
        if (!(t instanceof RejectedExecutionException) || removed) {
          throw t;
        }
      }
      return;
    }

    /*
     * This is an unsynchronized read! After the read, the function returns immediately or acquires
     * the lock to check again. Since an IDLE state was observed inside the preceding synchronized
     * block, and reference field assignment is atomic, this may save reacquiring the lock when
     * another thread or the worker task has cleared the count and set the state.
     *
     * <p>When {@link #executor} is a directExecutor(), the value written to
     * {@code workerRunningState} will be available synchronously, and behaviour will be
     * deterministic.
     */
    @SuppressWarnings("GuardedBy")
    boolean alreadyMarkedQueued = workerRunningState != QUEUING;
    if (alreadyMarkedQueued) {
      return;
    }
    synchronized (queue) {
      if (workerRunCount == oldRunCount && workerRunningState == QUEUING) {
        workerRunningState = QUEUED;
      }
    }
  }
@Override
  public void execute(final Runnable task) {
    checkNotNull(task);
    final Runnable submittedTask;
    final long oldRunCount;
    synchronized (queue) {
      // If the worker is already running (or execute() on the delegate returned successfully, and
      // the worker has yet to start) then we don't need to start the worker.
      if (workerRunningState == RUNNING || workerRunningState == QUEUED) {
        queue.add(task);
        return;
      }

      oldRunCount = workerRunCount;

      // If the worker is not yet running, the delegate Executor might reject our attempt to start
      // it. To preserve FIFO order and failure atomicity of rejected execution when the same
      // Runnable is executed more than once, allocate a wrapper that we know is safe to remove by
      // object identity.
      // A data structure that returned a removal handle from add() would allow eliminating this
      // allocation.
      submittedTask =
          new Runnable() {
            @Override
            public void run() {
              task.run();
            }
          };
      queue.add(submittedTask);
      workerRunningState = QUEUING;
    }

    try {
      executor.execute(worker);
    } catch (RuntimeException | Error t) {
      synchronized (queue) {
        boolean removed =
            (workerRunningState == IDLE || workerRunningState == QUEUING)
                && queue.removeLastOccurrence(submittedTask);
        // If the delegate is directExecutor(), the submitted runnable could have thrown a REE. But
        // that's handled by the log check that catches RuntimeExceptions in the queue worker.
        if (!(t instanceof RejectedExecutionException) || removed) {
          throw t;
        }
      }
      return;
    }

    /*
     * This is an unsynchronized read! After the read, the function returns immediately or acquires
     * the lock to check again. Since an IDLE state was observed inside the preceding synchronized
     * block, and reference field assignment is atomic, this may save reacquiring the lock when
     * another thread or the worker task has cleared the count and set the state.
     *
     * <p>When {@link #executor} is a directExecutor(), the value written to
     * {@code workerRunningState} will be available synchronously, and behaviour will be
     * deterministic.
     */
    @SuppressWarnings("GuardedBy")
    boolean alreadyMarkedQueued = workerRunningState != QUEUING;
    if (alreadyMarkedQueued) {
      return;
    }
    synchronized (queue) {
      if (workerRunCount == oldRunCount && workerRunningState == QUEUING) {
        workerRunningState = QUEUED;
      }
    }
  }
Adds a task to the queue and makes sure a worker thread is running.

<p>If this method throws, e.g. a {@code RejectedExecutionException} from the delegate executor,
execution of tasks will stop until a call to this method or to {@link #resume()} is made.