The Specification Pattern is a design pattern that allows you to encapsulate validation criteria in separate objects, making the code more modular and flexible. Instead of writing complex conditional logic directly in the main code, this responsibility is delegated to specification objects. This makes the code easier to understand, maintain, and extend.

What Is the Specification Pattern?

The Specification Pattern is a way to encapsulate business rules into reusable, composable objects.

Instead of cluttering your domain objects or services with multiple conditions, you define clear and testable specifications that determine whether an object meets certain criteria.

In simple terms:
🔹 It separates business logic from domain models.
🔹 It allows flexible and reusable rule composition (AND, OR, NOT conditions).
🔹 It makes code more readable, testable, and maintainable.

When to Use the Specification Pattern

  • Your business logic is complex and frequently changes.
  • You need to reuse rules across multiple parts of the application.
  • You want to keep domain models lightweight and focused.
  • You are following Domain-Driven Design (DDD) principle

Applying the Specification Pattern

Let’s consider an example: a weightlifting competition system where athletes qualify for specific events based on their performance in lifts like the Snatch and Clean & Jerk.

A bad approach would be stuffing complex if-else conditions directly into domain models. This makes the code harder to read and reduces flexibility when business rules change.

The Specification Pattern offers a cleaner solution: it extracts these conditions into self-contained objects that can be combined dynamically.

Example: Weightlifting Competition

Specification objects contain individual validation rules. In the given code, there are classes such as:

Specification Objects

public class CleanAndJerkOlympicsSpecification extends CompositeSpecification<Double> {

    private static final double MINIMUM_WEIGHT = 200.0;

    @Override
    public BiPredicate<Double, Double> toPredicate() {
        return (snatch, cleanAndJerk) -> cleanAndJerk > MINIMUM_WEIGHT;
    }
}

A similar specification would be created for the Snatch.

Composing Specifications

Specifications can be combined using logical operators like AND and OR. For example:

private void setEligibleForOlympics(double snatchWeight, double cleanAndJerkWeight) {
    this.eligibleForOlympics = new CleanAndJerkOlympicsSpecification()
            .and(new SnatchOlympicsSpecification())
            .toPredicate()
            .test(snatchWeight, cleanAndJerkWeight);
}

Each specification is transformed into a Predicate, which is a condition that can be tested. The toPredicate() method converts each specification into a boolean condition that can be tested using the athlete’s snatch and clean & jerk weights.

Additional Example: Email Validation

Specifications can also validate other types of data, such as email addresses:

public class EmailSpecification implements Specification<String> {

    @Override
    public boolean isSatisfiedBy(String email) {
        return Pattern.compile("^(.+)@(\\S+)$")
                .matcher(email)
                .matches();
    }
}

The code below shows how to use EmailSpecification to validate an athlete’s email.

private void setAthleteEmail(String athleteEmail) {
    boolean isValidEmail = new EmailSpecification().isSatisfiedBy(athleteEmail);

    if (!isValidEmail) {
        throw new IllegalArgumentException("Invalid email.");
    }

    this.athleteEmail = athleteEmail;
}

Conclusion

The Specification Pattern allows you to encapsulate business rules into separate, composable components, making the code modular, reusable, and easier to maintain. It is particularly useful when there are multiple complex conditions that need to be evaluated against objects, such as checking an athlete’s eligibility for competitions or validating input like email addresses.

Additional Resources

For a step-by-step video walkthrough of this example and further explanation of the pattern in action, watch the full tutorial:

🟥▶️https://www.youtube.com/watch?v=ByERVyGQN_A

Remember, real speed doesn’t come from rushing. It comes from doing things right. As Robert C. Martin said, “The only way to go fast is to do things well.”

References