Intent

The lockable object design pattern ensures that there is only one user using the target object. Compared to the built-in synchronization mechanisms such as using the synchronized keyword, this pattern can lock objects for an undetermined time and is not tied to the duration of the request.

Explanation

Real-world example

The Sword Of Aragorn is a legendary object that only one creature can possess at the time. Every creature in the middle earth wants to possess is, so as long as it's not locked, every creature will fight for it.

Under the hood

In this particular module, the SwordOfAragorn.java is a class that implements the Lockable interface. It reaches the goal of the Lockable-Object pattern by implementing unlock() and unlock() methods using thread-safety logic. The thread-safety logic is implemented with the built-in monitor mechanism of Java. The SwordOfAaragorn.java has an Object property called "synchronizer". In every crucial concurrency code block, it's synchronizing the block by using the synchronizer.

Programmatic Example

/** This interface describes the methods to be supported by a lockable object. */
public interface Lockable {

  /**
   * Checks if the object is locked.
   *
   * @return true if it is locked.
   */
  boolean isLocked();

  /**
   * locks the object with the creature as the locker.
   *
   * @param creature as the locker.
   * @return true if the object was locked successfully.
   */
  boolean lock(Creature creature);

  /**
   * Unlocks the object.
   *
   * @param creature as the locker.
   */
  void unlock(Creature creature);

  /**
   * Gets the locker.
   *
   * @return the Creature that holds the object. Returns null if no one is locking.
   */
  Creature getLocker();

  /**
   * Returns the name of the object.
   *
   * @return the name of the object.
   */
  String getName();
}

We have defined that according to our context, the object must implement the Lockable interface.

For example, the SwordOfAragorn class:

public class SwordOfAragorn implements Lockable {

  private Creature locker;
  private final Object synchronizer;
  private static final String NAME = "The Sword of Aragorn";

  public SwordOfAragorn() {
    this.locker = null;
    this.synchronizer = new Object();
  }

  @Override
  public boolean isLocked() {
    return this.locker != null;
  }

  @Override
  public boolean lock(@NonNull Creature creature) {
    synchronized (synchronizer) {
      LOGGER.info("{} is now trying to acquire {}!", creature.getName(), this.getName());
      if (!isLocked()) {
        locker = creature;
        return true;
      } else {
        if (!locker.getName().equals(creature.getName())) {
          return false;
        }
      }
    }
    return false;
  }

  @Override
  public void unlock(@NonNull Creature creature) {
    synchronized (synchronizer) {
      if (locker != null && locker.getName().equals(creature.getName())) {
        locker = null;
        LOGGER.info("{} is now free!", this.getName());
      }
      if (locker != null) {
        throw new LockingException("You cannot unlock an object you are not the owner of.");
      }
    }
  }

  @Override
  public Creature getLocker() {
    return this.locker;
  }

  @Override
  public String getName() {
    return NAME;
  }
}

According to our context, there are creatures that are looking for the sword, so must define the parent class:

public abstract class Creature {

  private String name;
  private CreatureType type;
  private int health;
  private int damage;
  Set<Lockable> instruments;

  protected Creature(@NonNull String name) {
    this.name = name;
    this.instruments = new HashSet<>();
  }

  /**
   * Reaches for the Lockable and tried to hold it.
   *
   * @param lockable as the Lockable to lock.
   * @return true of Lockable was locked by this creature.
   */
  public boolean acquire(@NonNull Lockable lockable) {
    if (lockable.lock(this)) {
      instruments.add(lockable);
      return true;
    }
    return false;
  }

  /** Terminates the Creature and unlocks all of the Lockable that it posses. */
  public synchronized void kill() {
    LOGGER.info("{} {} has been slayed!", type, name);
    for (Lockable lockable : instruments) {
      lockable.unlock(this);
    }
    this.instruments.clear();
  }

  /**
   * Attacks a foe.
   *
   * @param creature as the foe to be attacked.
   */
  public synchronized void attack(@NonNull Creature creature) {
    creature.hit(getDamage());
  }

  /**
   * When a creature gets hit it removed the amount of damage from the creature's life.
   *
   * @param damage as the damage that was taken.
   */
  public synchronized void hit(int damage) {
    if (damage < 0) {
      throw new IllegalArgumentException("Damage cannot be a negative number");
    }
    if (isAlive()) {
      setHealth(getHealth() - damage);
      if (!isAlive()) {
        kill();
      }
    }
  }

  /**
   * Checks if the creature is still alive.
   *
   * @return true of creature is alive.
   */
  public synchronized boolean isAlive() {
    return getHealth() > 0;
  }

}

As mentioned before, we have classes that extend the Creature class, such as Elf, Orc, and Human.

Finally, the following program will simulate a battle for the sword:

public class App implements Runnable {

  private static final int WAIT_TIME = 3;
  private static final int WORKERS = 2;
  private static final int MULTIPLICATION_FACTOR = 3;

  /**
   * main method.
   *
   * @param args as arguments for the main method.
   */
  public static void main(String[] args) {
    var app = new App();
    app.run();
  }

  @Override
  public void run() {
    // The target object for this example.
    var sword = new SwordOfAragorn();
    // Creation of creatures.
    List<Creature> creatures = new ArrayList<>();
    for (var i = 0; i < WORKERS; i++) {
      creatures.add(new Elf(String.format("Elf %s", i)));
      creatures.add(new Orc(String.format("Orc %s", i)));
      creatures.add(new Human(String.format("Human %s", i)));
    }
    int totalFiends = WORKERS * MULTIPLICATION_FACTOR;
    ExecutorService service = Executors.newFixedThreadPool(totalFiends);
    // Attach every creature and the sword is a Fiend to fight for the sword.
    for (var i = 0; i < totalFiends; i = i + MULTIPLICATION_FACTOR) {
      service.submit(new Feind(creatures.get(i), sword));
      service.submit(new Feind(creatures.get(i + 1), sword));
      service.submit(new Feind(creatures.get(i + 2), sword));
    }
    // Wait for program to terminate.
    try {
      if (!service.awaitTermination(WAIT_TIME, TimeUnit.SECONDS)) {
        LOGGER.info("The master of the sword is now {}.", sword.getLocker().getName());
      }
    } catch (InterruptedException e) {
      LOGGER.error(e.getMessage());
      Thread.currentThread().interrupt();
    } finally {
      service.shutdown();
    }
  }
}

Applicability

The Lockable Object pattern is ideal for non distributed applications, that needs to be thread-safe and keeping their domain models in memory(in contrast to persisted models such as databases).

Class diagram

Credits

Lockable Object - Chapter 10.3, J2EE Design Patterns, O'Reilly