Specification

About 4 min

Intent

Encapsulate business rules and criteria that an object must satisfy to enable checking these rules in various parts of the application.

Explanation

Real-world example

Imagine you are organizing a conference and need to filter attendees based on specific criteria such as registration status, payment completion, and session interests.
Using the Specification design pattern, you would create separate specifications for each criterion (e.g., "IsRegistered", "HasPaid", "IsInterestedInSessionX"). These specifications can be combined dynamically to filter attendees who meet all the required criteria, such as those who are registered, have completed their payment, and are interested in a particular session. This approach allows for flexible and reusable business rules, ensuring that the filtering logic can be easily adjusted as needed without changing the underlying attendee objects.

In plain words

The Specification design pattern allows for the encapsulation and reuse of business rules and criteria in a flexible, combinable manner.

Wikipedia says

In computer programming, the specification pattern is a particular software design pattern, whereby business rules can be recombined by chaining the business rules together using boolean logic.

Programmatic Example

Let's consider a creature pool example. We have a collection of creatures with specific properties. These properties might belong to a predefined, limited set (represented by enums like Size, Movement, and Color) or they might be continuous values (e.g., the mass of a Creature). In cases with continuous values, it's better to use a "parameterized specification," where the property value is provided as an argument when the Creature is instantiated, allowing for greater flexibility. Additionally, predefined and/or parameterized properties can be combined using boolean logic, offering almost limitless selection possibilities (this is known as a "composite specification," explained further below). The advantages and disadvantages of each approach are detailed in the table at the end of this document.

First, here is interface Creature.

public interface Creature {
    String getName();

    Size getSize();

    Movement getMovement();

    Color getColor();

    Mass getMass();
}

And Dragon implementation looks like this.

public class Dragon extends AbstractCreature {

  public Dragon() {
    super("Dragon", Size.LARGE, Movement.FLYING, Color.RED, new Mass(39300.0));
  }
}

Now that we want to select some subset of them, we use selectors. To select creatures that fly, we should use MovementSelector. The snippet also shows the base class AbstractSelector.

public abstract class AbstractSelector<T> implements Predicate<T> {

    public AbstractSelector<T> and(AbstractSelector<T> other) {
        return new ConjunctionSelector<>(this, other);
    }

    public AbstractSelector<T> or(AbstractSelector<T> other) {
        return new DisjunctionSelector<>(this, other);
    }

    public AbstractSelector<T> not() {
        return new NegationSelector<>(this);
    }
}

public class MovementSelector extends AbstractSelector<Creature> {

  private final Movement movement;

  public MovementSelector(Movement m) {
    this.movement = m;
  }

  @Override
  public boolean test(Creature t) {
    return t.getMovement().equals(movement);
  }
}

On the other hand, when selecting creatures heavier than a chosen amount, we use MassGreaterThanSelector.

public class MassGreaterThanSelector extends AbstractSelector<Creature> {

  private final Mass mass;

  public MassGreaterThanSelector(double mass) {
    this.mass = new Mass(mass);
  }

  @Override
  public boolean test(Creature t) {
    return t.getMass().greaterThan(mass);
  }
}

With these building blocks in place, we can perform some searches.

@Slf4j
public class App {

  public static void main(String[] args) {
    // initialize creatures list
    var creatures = List.of(
        new Goblin(),
        new Octopus(),
        new Dragon(),
        new Shark(),
        new Troll(),
        new KillerBee()
    );
    // so-called "hard-coded" specification
    LOGGER.info("Demonstrating hard-coded specification :");
    // find all walking creatures
    LOGGER.info("Find all walking creatures");
    print(creatures, new MovementSelector(Movement.WALKING));
    // find all dark creatures
    LOGGER.info("Find all dark creatures");
    print(creatures, new ColorSelector(Color.DARK));
    LOGGER.info("\n");
    // so-called "parameterized" specification
    LOGGER.info("Demonstrating parameterized specification :");
    // find all creatures heavier than 500kg
    LOGGER.info("Find all creatures heavier than 600kg");
    print(creatures, new MassGreaterThanSelector(600.0));
    // find all creatures heavier than 500kg
    LOGGER.info("Find all creatures lighter than or weighing exactly 500kg");
    print(creatures, new MassSmallerThanOrEqSelector(500.0));
    LOGGER.info("\n");
    // so-called "composite" specification
    LOGGER.info("Demonstrating composite specification :");
    // find all red and flying creatures
    LOGGER.info("Find all red and flying creatures");
    var redAndFlying = new ColorSelector(Color.RED).and(new MovementSelector(Movement.FLYING));
    print(creatures, redAndFlying);
    // find all creatures dark or red, non-swimming, and heavier than or equal to 400kg
    LOGGER.info("Find all scary creatures");
    var scaryCreaturesSelector = new ColorSelector(Color.DARK)
        .or(new ColorSelector(Color.RED)).and(new MovementSelector(Movement.SWIMMING).not())
        .and(new MassGreaterThanSelector(400.0).or(new MassEqualSelector(400.0)));
    print(creatures, scaryCreaturesSelector);
  }

  private static void print(List<? extends Creature> creatures, Predicate<Creature> selector) {
    creatures.stream().filter(selector).map(Objects::toString).forEach(LOGGER::info);
  }
}

Console output:

12:49:24.808 [main] INFO com.iluwatar.specification.app.App -- Demonstrating hard-coded specification :
12:49:24.810 [main] INFO com.iluwatar.specification.app.App -- Find all walking creatures
12:49:24.812 [main] INFO com.iluwatar.specification.app.App -- Goblin [size=small, movement=walking, color=green, mass=30.0kg]
12:49:24.812 [main] INFO com.iluwatar.specification.app.App -- Troll [size=large, movement=walking, color=dark, mass=4000.0kg]
12:49:24.812 [main] INFO com.iluwatar.specification.app.App -- Find all dark creatures
12:49:24.815 [main] INFO com.iluwatar.specification.app.App -- Octopus [size=normal, movement=swimming, color=dark, mass=12.0kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Troll [size=large, movement=walking, color=dark, mass=4000.0kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- 

12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Demonstrating parameterized specification :
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Find all creatures heavier than 600kg
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Dragon [size=large, movement=flying, color=red, mass=39300.0kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Troll [size=large, movement=walking, color=dark, mass=4000.0kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Find all creatures lighter than or weighing exactly 500kg
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Goblin [size=small, movement=walking, color=green, mass=30.0kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Octopus [size=normal, movement=swimming, color=dark, mass=12.0kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Shark [size=normal, movement=swimming, color=light, mass=500.0kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- KillerBee [size=small, movement=flying, color=light, mass=6.7kg]
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- 

12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Demonstrating composite specification :
12:49:24.816 [main] INFO com.iluwatar.specification.app.App -- Find all red and flying creatures
12:49:24.817 [main] INFO com.iluwatar.specification.app.App -- Dragon [size=large, movement=flying, color=red, mass=39300.0kg]
12:49:24.817 [main] INFO com.iluwatar.specification.app.App -- Find all scary creatures
12:49:24.818 [main] INFO com.iluwatar.specification.app.App -- Dragon [size=large, movement=flying, color=red, mass=39300.0kg]
12:49:24.818 [main] INFO com.iluwatar.specification.app.App -- Troll [size=large, movement=walking, color=dark, mass=4000.0kg]

Applicability

Use when you need to filter objects based on different criteria.
Use when the filtering criteria can change dynamically.
Ideal for use cases involving complex business rules that must be reused across different parts of an application.

Known Uses

Validating user inputs in enterprise applications.
Filtering search results in e-commerce applications.
Business rule validation in domain-driven design (DDD).

Consequences

Benefits:

Enhances the flexibility and reusability of business rules.
Promotes single responsibility principleopen in new window by separating business rules from the entities.
Facilitates unit testing of business rules.

Trade-offs:

Can lead to a proliferation of small classes, increasing complexity.
Might introduce performance overhead due to the dynamic checking of specifications.

Compositeopen in new window: Often used together with Specification to combine multiple specifications.
Decoratoropen in new window: Can be used to add additional criteria to a specification dynamically.
Strategyopen in new window: Both patterns involve encapsulating a family of algorithms. Strategy encapsulates different strategies or algorithms, while Specification encapsulates business rules.