| title | shortTitle | description | category | language | tag | |||||
|---|---|---|---|---|---|---|---|---|---|---|
Specification Pattern in Java: Enhancing Business Rules with Decoupled Logic |
Specification |
Dive deep into the Specification design pattern in Java, a strategic solution for encapsulating business rules. Learn how to implement, combine, and apply this pattern effectively in your software development projects. |
Behavioral |
en |
|
- Filter
- Criteria
Encapsulate business rules and criteria that an object must satisfy to enable checking these rules in various parts of the application.
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 in Java enables the efficient encapsulation and reuse of business rules, offering a flexible and dynamic way to combine criteria for robust software development
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.
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]
Adopting the Specification pattern significantly enhances the flexibility and reusability of business rules within Java applications, contributing to more maintainable code.
Apply the Java Specification pattern when
- You need to filter objects based on different criteria.
- The filtering criteria can change dynamically.
- Ideal for use cases involving complex business rules that must be reused across different parts of an application.
- Validating user inputs in enterprise applications.
- Filtering search results in e-commerce applications.
- Business rule validation in domain-driven design (DDD).
Benefits:
- Enhances the flexibility and reusability of business rules.
- Promotes single responsibility principle 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.
- Composite: Often used together with Specification to combine multiple specifications.
- Decorator: Can be used to add additional criteria to a specification dynamically.
- Strategy: Both patterns involve encapsulating a family of algorithms. Strategy encapsulates different strategies or algorithms, while Specification encapsulates business rules.