@Override
  public void notifyOperationCompleteExceptionally(final Throwable ex) {
    LOGGER.log(
        Level.WARNING,
        String.format("Timer %s raised exception: %s", this.getTimerName(), ex.getMessage()),
        ex);

    this.handleOperationCompletion();
    ++this.numFailures;
  }
/** An async timer visitor implementation used by NIS. */
public class NemAsyncTimerVisitor implements AsyncTimerVisitor, SerializableEntity {
  private static final Logger LOGGER = Logger.getLogger(NemAsyncTimerVisitor.class.getName());

  private final String timerName;
  private final TimeProvider timeProvider;

  private int numExecutions;
  private int numSuccesses;
  private int numFailures;

  private TimeInstant lastOperationStartTime = TimeInstant.ZERO;
  private int lastOperationTime;
  private int lastDelayTime;
  private int totalTime;

  /**
   * Creates a new visitor.
   *
   * @param timerName The friendly name of the timer.
   * @param timeProvider The time provider.
   */
  public NemAsyncTimerVisitor(final String timerName, final TimeProvider timeProvider) {
    this.timerName = timerName;
    this.timeProvider = timeProvider;
  }

  // region getters

  /**
   * Gets the friendly name of the timer.
   *
   * @return The friendly name of the timer.
   */
  public String getTimerName() {
    return this.timerName;
  }

  /**
   * Gets the number of executions.
   *
   * @return The number of executions.
   */
  public int getNumExecutions() {
    return this.numExecutions;
  }

  /**
   * Gets the number of successful completions.
   *
   * @return The number of successful completions.
   */
  public int getNumSuccesses() {
    return this.numSuccesses;
  }

  /**
   * Gets the number of exceptional completions.
   *
   * @return The number of exceptional completions.
   */
  public int getNumFailures() {
    return this.numFailures;
  }

  /**
   * Gets the start time of the last operation.
   *
   * @return The start time of the last operation.
   */
  public TimeInstant getLastOperationStartTime() {
    return this.lastOperationStartTime;
  }

  /**
   * Gets the total time of the last completed operation.
   *
   * @return The total time of the last completed operation.
   */
  public int getLastOperationTime() {
    return this.lastOperationTime;
  }

  /**
   * Gets the last delay time.
   *
   * @return The last delay time.
   */
  public int getLastDelayTime() {
    return this.lastDelayTime;
  }

  /**
   * Gets the average time of all completed operations.
   *
   * @return The average time of all completed operations.
   */
  public int getAverageOperationTime() {
    return this.getNumCompletions() > 0 ? this.totalTime / this.getNumCompletions() : 0;
  }

  /**
   * Gets the a value indicating whether or not the timer is currently executing.
   *
   * @return true if the timer is currently executing.
   */
  public boolean isExecuting() {
    return this.numExecutions > this.getNumCompletions();
  }

  private int getNumCompletions() {
    return this.numSuccesses + this.numFailures;
  }

  // endregion

  // region AsyncTimerVisitor

  @Override
  public void notifyOperationStart() {
    this.log("executing");

    this.lastOperationStartTime = this.timeProvider.getCurrentTime();
    ++this.numExecutions;
  }

  @Override
  public void notifyOperationComplete() {
    this.handleOperationCompletion();
    ++this.numSuccesses;
  }

  @Override
  public void notifyOperationCompleteExceptionally(final Throwable ex) {
    LOGGER.log(
        Level.WARNING,
        String.format("Timer %s raised exception: %s", this.getTimerName(), ex.getMessage()),
        ex);

    this.handleOperationCompletion();
    ++this.numFailures;
  }

  @Override
  public void notifyDelay(final int delay) {
    this.log(String.format("sleeping for %d ms", delay));
    this.lastDelayTime = delay;
  }

  @Override
  public void notifyStop() {
    this.log("stopping");
  }

  private void handleOperationCompletion() {
    final TimeInstant stopTime = this.timeProvider.getCurrentTime();
    this.lastOperationTime = stopTime.subtract(this.lastOperationStartTime);
    this.totalTime += this.lastOperationTime;
  }

  private void log(final String message) {
    LOGGER.fine(
        String.format(
            "[%d] Timer %s: %s", Thread.currentThread().getId(), this.timerName, message));
  }

  // endregion

  // region SerializableEntity

  @Override
  public void serialize(final Serializer serializer) {
    serializer.writeString("name", this.getTimerName());
    serializer.writeInt("executions", this.getNumExecutions());
    serializer.writeInt("successes", this.getNumSuccesses());
    serializer.writeInt("failures", this.getNumFailures());
    serializer.writeInt("last-delay-time", this.getLastDelayTime());
    TimeInstant.writeTo(serializer, "last-operation-start-time", this.getLastOperationStartTime());
    serializer.writeInt("last-operation-time", this.getLastOperationTime());
    serializer.writeInt("average-operation-time", this.getAverageOperationTime());
    serializer.writeInt("is-executing", this.isExecuting() ? 1 : 0);
  }

  // endregion
}
 private void log(final String message) {
   LOGGER.fine(
       String.format(
           "[%d] Timer %s: %s", Thread.currentThread().getId(), this.timerName, message));
 }