java-design-patterns/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/EventManager.java

/*
 * The MIT License
 * Copyright © 2014-2019 Ilkka Seppälä
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

package com.iluwatar.event.asynchronous;

import java.util.Map;
import java.util.Random;
import java.util.concurrent.ConcurrentHashMap;

/**
 * EventManager handles and maintains a pool of event threads. {@link Event} threads are created
 * upon user request. Thre are two types of events; Asynchronous and Synchronous. There can be
 * multiple Asynchronous events running at once but only one Synchronous event running at a time.
 * Currently supported event operations are: start, stop, and getStatus. Once an event is complete,
 * it then notifies EventManager through a listener. The EventManager then takes the event out of
 * the pool.
 */
public class EventManager implements ThreadCompleteListener {

  public static final int MAX_RUNNING_EVENTS = 1000;
  // Just don't wanna have too many running events. :)
  public static final int MIN_ID = 1;
  public static final int MAX_ID = MAX_RUNNING_EVENTS;
  public static final int MAX_EVENT_TIME = 1800; // in seconds / 30 minutes.
  private int currentlyRunningSyncEvent = -1;
  private Random rand;
  private Map<Integer, Event> eventPool;

  private static final String DOES_NOT_EXIST = " does not exist.";

  /**
   * EventManager constructor.
   */
  public EventManager() {
    rand = new Random(1);
    eventPool = new ConcurrentHashMap<Integer, Event>(MAX_RUNNING_EVENTS);

  }

  /**
   * Create a Synchronous event.
   *
   * @param eventTime Time an event should run for.
   * @return eventId
   * @throws MaxNumOfEventsAllowedException When too many events are running at a time.
   * @throws InvalidOperationException      No new synchronous events can be created when one is
   *                                        already running.
   * @throws LongRunningEventException      Long running events are not allowed in the app.
   */
  public int create(int eventTime)
      throws MaxNumOfEventsAllowedException, InvalidOperationException, LongRunningEventException {
    if (currentlyRunningSyncEvent != -1) {
      throw new InvalidOperationException("Event [" + currentlyRunningSyncEvent + "] is still"
          + " running. Please wait until it finishes and try again.");
    }

    var eventId = createEvent(eventTime, true);
    currentlyRunningSyncEvent = eventId;

    return eventId;
  }

  /**
   * Create an Asynchronous event.
   *
   * @param eventTime Time an event should run for.
   * @return eventId
   * @throws MaxNumOfEventsAllowedException When too many events are running at a time.
   * @throws LongRunningEventException      Long running events are not allowed in the app.
   */
  public int createAsync(int eventTime) throws MaxNumOfEventsAllowedException,
      LongRunningEventException {
    return createEvent(eventTime, false);
  }

  private int createEvent(int eventTime, boolean isSynchronous)
      throws MaxNumOfEventsAllowedException, LongRunningEventException {
    if (eventPool.size() == MAX_RUNNING_EVENTS) {
      throw new MaxNumOfEventsAllowedException("Too many events are running at the moment."
          + " Please try again later.");
    }

    if (eventTime >= MAX_EVENT_TIME) {
      throw new LongRunningEventException(
          "Maximum event time allowed is " + MAX_EVENT_TIME + " seconds. Please try again.");
    }

    var newEventId = generateId();

    var newEvent = new Event(newEventId, eventTime, isSynchronous);
    newEvent.addListener(this);
    eventPool.put(newEventId, newEvent);

    return newEventId;
  }

  /**
   * Starts event.
   *
   * @param eventId The event that needs to be started.
   * @throws EventDoesNotExistException If event does not exist in our eventPool.
   */
  public void start(int eventId) throws EventDoesNotExistException {
    if (!eventPool.containsKey(eventId)) {
      throw new EventDoesNotExistException(eventId + DOES_NOT_EXIST);
    }

    eventPool.get(eventId).start();
  }

  /**
   * Stops event.
   *
   * @param eventId The event that needs to be stopped.
   * @throws EventDoesNotExistException If event does not exist in our eventPool.
   */
  public void cancel(int eventId) throws EventDoesNotExistException {
    if (!eventPool.containsKey(eventId)) {
      throw new EventDoesNotExistException(eventId + DOES_NOT_EXIST);
    }

    if (eventId == currentlyRunningSyncEvent) {
      currentlyRunningSyncEvent = -1;
    }

    eventPool.get(eventId).stop();
    eventPool.remove(eventId);
  }

  /**
   * Get status of a running event.
   *
   * @param eventId The event to inquire status of.
   * @throws EventDoesNotExistException If event does not exist in our eventPool.
   */
  public void status(int eventId) throws EventDoesNotExistException {
    if (!eventPool.containsKey(eventId)) {
      throw new EventDoesNotExistException(eventId + DOES_NOT_EXIST);
    }

    eventPool.get(eventId).status();
  }

  /**
   * Gets status of all running events.
   */
  @SuppressWarnings("rawtypes")
  public void statusOfAllEvents() {
    eventPool.entrySet().forEach(entry -> ((Event) ((Map.Entry) entry).getValue()).status());
  }

  /**
   * Stop all running events.
   */
  @SuppressWarnings("rawtypes")
  public void shutdown() {
    eventPool.entrySet().forEach(entry -> ((Event) ((Map.Entry) entry).getValue()).stop());
  }

  /**
   * Returns a pseudo-random number between min and max, inclusive. The difference between min and
   * max can be at most
   * <code>Integer.MAX_VALUE - 1</code>.
   */
  private int generateId() {
    // nextInt is normally exclusive of the top value,
    // so add 1 to make it inclusive
    var randomNum = rand.nextInt((MAX_ID - MIN_ID) + 1) + MIN_ID;
    while (eventPool.containsKey(randomNum)) {
      randomNum = rand.nextInt((MAX_ID - MIN_ID) + 1) + MIN_ID;
    }

    return randomNum;
  }

  /**
   * Callback from an {@link Event} (once it is complete). The Event is then removed from the pool.
   */
  @Override
  public void completedEventHandler(int eventId) {
    eventPool.get(eventId).status();
    if (eventPool.get(eventId).isSynchronous()) {
      currentlyRunningSyncEvent = -1;
    }
    eventPool.remove(eventId);
  }

  /**
   * Getter method for event pool.
   */
  public Map<Integer, Event> getEventPool() {
    return eventPool;
  }

  /**
   * Get number of currently running Synchronous events.
   */
  public int numOfCurrentlyRunningSyncEvent() {
    return currentlyRunningSyncEvent;
  }
}