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{{Short description|Software design pattern}}
[[Image:Specification_UML_v2.png|right|thumb|300px|Specification Pattern in [[Unified Modeling Language|UML]] ]]
[[File:Specification UML.png|right|thumb|300px|Specification Pattern in [[Unified Modeling Language|UML]] ]]
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. The pattern is frequently used in the context of [[___domain-driven design]].
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 algebra|boolean logic]]. The pattern is frequently used in the context of [[___domain-driven design]].
 
A specification pattern outlines a business rule that is combinable with other business rules. In this pattern, a unit of business logic inherits its functionality from the abstract aggregate Composite Specification class. The Composite Specification class has one function called IsSatisfiedBy that returns a boolean value. After instantiation, the specification is "chained" with other specifications, making new specifications easily maintainable, yet highly customizable business logic. Furthermore, upon instantiation the business logic may, through method invocation or [[inversion of control]], have its state altered in order to become a delegate of other classes such as a persistence repository.
 
As a consequence of performing runtime composition of high-level business/___domain logic, the Specification pattern is a convenient tool for converting ad-hoc user search criteria into low level logic to be processed by repositories.
 
Since a specification is an encapsulation of logic in a reusable form it is very simple to thoroughly unit test, and when used in this context is also an implementation of the humble object pattern.
 
== Code examples ==
=== [[C Sharp (programming language)|C#]] ===
 
=== C# ===
<source lang="csharp">
{{Further|C Sharp (programming language)}}
public interface ISpecification
<syntaxhighlight lang="csharp">
public interface ISpecification
{
bool IsSatisfiedBy(object candidate);
ISpecification And(ISpecification other);
ISpecification AndNot(ISpecification other);
ISpecification Or(ISpecification other);
ISpecification OrNot(ISpecification other);
ISpecification Not();
}
 
public abstract class CompositeSpecification : ISpecification
{
public abstract bool IsSatisfiedBy(object candidate);
 
public ISpecification And(ISpecification other)
{
boolreturn IsSatisfiedBynew AndSpecification(objectthis, candidateother);
ISpecification And(ISpecification other);
ISpecification Or(ISpecification other);
ISpecification Not();
}
 
public ISpecification AndNot(ISpecification other)
public abstract class CompositeSpecification : ISpecification
{
publicreturn abstractnew bool IsSatisfiedByAndNotSpecification(objectthis, candidateother);
}
 
public ISpecification AndOr(ISpecification other)
{
return new AndSpecificationOrSpecification(this, other);
}
 
public ISpecification OrOrNot(ISpecification other)
{
return new OrSpecificationOrNotSpecification(this, other);
}
 
public ISpecification Not()
{
return new NotSpecification(this);
}
}
 
public ISpecification Not()
public class AndSpecification : CompositeSpecification
{
return privatenew ISpecification OneNotSpecification(this);
}
private ISpecification Other;
}
 
public publicclass AndSpecification(ISpecification x, ISpecification: y)CompositeSpecification
{
{
private ISpecification _leftCondition;
One = x;
private ISpecification _rightCondition;
Other = y;
}
 
public AndSpecification(ISpecification left, ISpecification right)
public override bool IsSatisfiedBy(object candidate)
{
_leftCondition = left;
return One.IsSatisfiedBy(candidate) && Other.IsSatisfiedBy(candidate);
}_rightCondition = right;
}
 
public override bool IsSatisfiedBy(object candidate)
public class OrSpecification : CompositeSpecification
{
return _leftCondition.IsSatisfiedBy(candidate) && _rightCondition.IsSatisfiedBy(candidate);
private ISpecification One;
}
private ISpecification Other;
}
 
public class AndNotSpecification : CompositeSpecification
public OrSpecification(ISpecification x, ISpecification y)
{
{
private ISpecification _leftCondition;
One = x;
private ISpecification _rightCondition;
Other = y;
}
 
public AndNotSpecification(ISpecification left, ISpecification right)
public override bool IsSatisfiedBy(object candidate)
{
_leftCondition = left;
return One.IsSatisfiedBy(candidate) || Other.IsSatisfiedBy(candidate);
}_rightCondition = right;
}
 
public override bool IsSatisfiedBy(object candidate)
public class NotSpecification : CompositeSpecification
{
return _leftCondition.IsSatisfiedBy(candidate) && _rightCondition.IsSatisfiedBy(candidate) != true;
private ISpecification Wrapped;
}
}
 
public class OrSpecification : CompositeSpecification
public NotSpecification(ISpecification x)
{
{
private ISpecification _leftCondition;
Wrapped = x;
private ISpecification _rightCondition;
}
 
public OrSpecification(ISpecification left, ISpecification right)
public override bool IsSatisfiedBy(object candidate)
{
_leftCondition = left;
return !Wrapped.IsSatisfiedBy(candidate);
}_rightCondition = right;
}
 
public override bool IsSatisfiedBy(object candidate)
</source>
{
return _leftCondition.IsSatisfiedBy(candidate) || _rightCondition.IsSatisfiedBy(candidate);
}
}
 
public class OrNotSpecification : CompositeSpecification
=== [[C Sharp (programming language)|C# 3.0]], simplified with generics and extension methods ===
{
private ISpecification _leftCondition;
private ISpecification _rightCondition;
 
public OrNotSpecification(ISpecification left, ISpecification right)
<source lang="csharp">
public interface ISpecification<T>
{
bool_leftCondition IsSatisfiedBy(T= entity)left;
_rightCondition = right;
ISpecification<T> And(ISpecification<T> other);
ISpecification<T> AndNot(ISpecification<T> other);
ISpecification<T> Or(ISpecification<T> other);
ISpecification<T> Not();
}
 
public override bool IsSatisfiedBy(object candidate)
public abstract class LinqSpecification<T> : CompositeSpecification<T>
{
return _leftCondition.IsSatisfiedBy(candidate) || _rightCondition.IsSatisfiedBy(candidate) != true;
public abstract Expression<Func<T, bool>> AsExpression();
}
}
 
public class NotSpecification : CompositeSpecification
public override bool IsSatisfiedBy(T entity)
{
{
private ISpecification _wrapped;
Func<T, bool> predicate = AsExpression().Compile();
 
return predicate(entity);
public NotSpecification(ISpecification x)
}
{
_wrapped = x;
}
 
public override bool IsSatisfiedBy(object candidate)
public abstract class CompositeSpecification<T> : ISpecification<T>
{
public abstract boolreturn !_wrapped.IsSatisfiedBy(T entitycandidate);
}
}
</syntaxhighlight>
 
=== C# 6.0 with generics ===
public ISpecification<T> And(ISpecification<T> other)
{{Further|C Sharp (programming language)}}
{
<syntaxhighlight lang="csharp">
return new AndSpecification<T>(this, other);
public interface ISpecification<T>
}
{
bool IsSatisfiedBy(T candidate);
ISpecification<T> And(ISpecification<T> other);
ISpecification<T> AndNot(ISpecification<T> other);
ISpecification<T> Or(ISpecification<T> other);
ISpecification<T> OrNot(ISpecification<T> other);
ISpecification<T> Not();
}
 
public abstract class LinqSpecification<T> : CompositeSpecification<T>
public ISpecification<T> AndNot(ISpecification<T> other)
{
{
public abstract Expression<Func<T, bool>> AsExpression();
return new AndNotSpecification<T>(this, other);
public override bool IsSatisfiedBy(T candidate) => AsExpression().Compile()(candidate);
}
}
 
public abstract class public ISpecificationCompositeSpecification<T> Or(: ISpecification<T> other)
{
{
public abstract bool IsSatisfiedBy(T candidate);
return new OrSpecification<T>(this, other);
public ISpecification<T> And(ISpecification<T> other) => new AndSpecification<T>(this, other);
}
public ISpecification<T> AndNot(ISpecification<T> other) => new AndNotSpecification<T>(this, other);
public ISpecification<T> Or(ISpecification<T> other) => new OrSpecification<T>(this, other);
public ISpecification<T> OrNot(ISpecification<T> other) => new OrNotSpecification<T>(this, other);
public ISpecification<T> Not() => new NotSpecification<T>(this);
}
 
public class AndSpecification<T> : CompositeSpecification<T>
public ISpecification<T> Not()
{
{
private ISpecification<T> _left;
return new NotSpecification<T>(this);
private ISpecification<T> _right;
}
 
public AndSpecification(ISpecification<T> left, ISpecification<T> right)
{
_left = left;
_right = right;
}
 
public override bool IsSatisfiedBy(T candidate) => _left.IsSatisfiedBy(candidate) && _right.IsSatisfiedBy(candidate);
public class AndSpecification<T> : CompositeSpecification<T>
}
{
private readonly ISpecification<T> left;
private readonly ISpecification<T> right;
 
public class AndNotSpecification<T> : CompositeSpecification<T>
public AndSpecification(ISpecification<T> left, ISpecification<T> right)
{
{
private ISpecification<T> _left;
this.left = left;
private ISpecification<T> _right;
this.right = right;
}
 
public AndNotSpecification(ISpecification<T> left, ISpecification<T> right)
public override bool IsSatisfiedBy(T candidate)
{
_left = left;
return left.IsSatisfiedBy(candidate) && right.IsSatisfiedBy(candidate);
}_right = right;
}
 
public override bool IsSatisfiedBy(T candidate) => _left.IsSatisfiedBy(candidate) && !_right.IsSatisfiedBy(candidate);
public class AndNotSpecification<T> : CompositeSpecification<T>
}
{
private readonly ISpecification<T> left;
private readonly ISpecification<T> right;
 
public class OrSpecification<T> : CompositeSpecification<T>
public AndNotSpecification(ISpecification<T> left, ISpecification<T> right)
{
{
private ISpecification<T> _left;
this.left = left;
private ISpecification<T> _right;
this.right = right;
}
 
public OrSpecification(ISpecification<T> left, ISpecification<T> right)
public override bool IsSatisfiedBy(T candidate)
{
_left = left;
return left.IsSatisfiedBy(candidate) && right.IsSatisfiedBy(candidate) != true;
}_right = right;
}
 
public override bool IsSatisfiedBy(T candidate) => _left.IsSatisfiedBy(candidate) || _right.IsSatisfiedBy(candidate);
public class OrSpecification<T> : CompositeSpecification<T>
}
public class OrNotSpecification<T> : CompositeSpecification<T>
{
private ISpecification<T> _left;
private ISpecification<T> _right;
 
public OrNotSpecification(ISpecification<T> left, ISpecification<T> right)
{
private_left readonly ISpecification<T>= left;
private_right readonly ISpecification<T>= right;
}
 
public override bool IsSatisfiedBy(T candidate) => _left.IsSatisfiedBy(candidate) || !_right.IsSatisfiedBy(candidate);
public OrSpecification(ISpecification<T> left, ISpecification<T> right)
}
{
this.left = left;
this.right = right;
}
 
public class NotSpecification<T> : CompositeSpecification<T>
public override bool IsSatisfiedBy(T candidate)
{
{
ISpecification<T> other;
return left.IsSatisfiedBy(candidate) || right.IsSatisfiedBy(candidate);
public NotSpecification(ISpecification<T> other) => this.other = other;
}
public override bool IsSatisfiedBy(T candidate) => !other.IsSatisfiedBy(candidate);
}
}
</syntaxhighlight>
 
=== Python ===
public class NotSpecification<T> : CompositeSpecification<T>
{{Further|Python (programming language)}}
{
<syntaxhighlight lang="python">
private readonly ISpecification<T> other;
from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Any
 
class BaseSpecification(ABC):
public NotSpecification(ISpecification<T> other)
{@abstractmethod
def is_satisfied_by(self, candidate: Any) -> bool:
this.other = other;
}raise NotImplementedError()
 
def __call__(self, candidate: Any) -> bool:
public override bool IsSatisfiedBy(T candidate)
return self.is_satisfied_by(candidate)
{
return !other.IsSatisfiedBy(candidate);
}
}
</source>
 
def __and__(self, other: "BaseSpecification") -> "AndSpecification":
==Example of use==
return AndSpecification(self, other)
 
def __or__(self, other: "BaseSpecification") -> "OrSpecification":
In the following example, we are retrieving invoices and sending them to a collection agency if
return OrSpecification(self, other)
 
def __neg__(self) -> "NotSpecification":
# they are overdue,
return NotSpecification(self)
# notices have been sent, and
# they are not already with the collection agency.
 
@dataclass(frozen=True)
This example is meant to show the end result of how the logic is 'chained' together.
class AndSpecification(BaseSpecification):
first: BaseSpecification
second: BaseSpecification
 
def is_satisfied_by(self, candidate: Any) -> bool:
This usage example assumes a previously defined OverdueSpecification class that is satisfied when an invoice's due date is 30 days or older, a NoticeSentSpecification class that is satisfied when three notices have been sent to the customer, and an InCollectionSpecification class that is satisfied when an invoice has already been sent to the collection agency. The implementation of these classes isn't important here.
return self.first.is_satisfied_by(candidate) and self.second.is_satisfied_by(candidate)
 
@dataclass(frozen=True)
Using these three specifications, we created a new specification called SendToCollection which will be satisfied when an invoice is overdue, when notices have been sent to the customer, and are not already with the collection agency.
class OrSpecification(BaseSpecification):
first: BaseSpecification
second: BaseSpecification
 
def is_satisfied_by(self, candidate: Any) -> bool:
<source lang="csharp">
return self.first.is_satisfied_by(candidate) or self.second.is_satisfied_by(candidate)
OverDueSpecification OverDue = new OverDueSpecification();
NoticeSentSpecification NoticeSent = new NoticeSentSpecification();
InCollectionSpecification InCollection = new InCollectionSpecification();
 
@dataclass(frozen=True)
// example of specification pattern logic chaining
class NotSpecification(BaseSpecification):
ISpecification<Invoice> SendToCollection = OverDue.And(NoticeSent).And(InCollection.Not());
subject: BaseSpecification
 
def is_satisfied_by(self, candidate: Any) -> bool:
InvoiceCollection = Service.GetInvoices();
return not self.subject.is_satisfied_by(candidate)
 
</syntaxhighlight>
foreach (Invoice currentInvoice in InvoiceCollection) {
 
if (SendToCollection.IsSatisfiedBy(currentInvoice)) {
=== C++ ===
currentInvoice.SendToCollection();
{{Further|C++}}
}
<syntaxhighlight lang="cpp">
template <class T>
class ISpecification
{
public:
virtual ~ISpecification() = default;
virtual bool IsSatisfiedBy(T Candidate) const = 0;
virtual ISpecification<T>* And(const ISpecification<T>& Other) const = 0;
virtual ISpecification<T>* AndNot(const ISpecification<T>& Other) const = 0;
virtual ISpecification<T>* Or(const ISpecification<T>& Other) const = 0;
virtual ISpecification<T>* OrNot(const ISpecification<T>& Other) const = 0;
virtual ISpecification<T>* Not() const = 0;
};
 
template <class T>
class CompositeSpecification : public ISpecification<T>
{
public:
virtual bool IsSatisfiedBy(T Candidate) const override = 0;
 
virtual ISpecification<T>* And(const ISpecification<T>& Other) const override;
virtual ISpecification<T>* AndNot(const ISpecification<T>& Other) const override;
virtual ISpecification<T>* Or(const ISpecification<T>& Other) const override;
virtual ISpecification<T>* OrNot(const ISpecification<T>& Other) const override;
virtual ISpecification<T>* Not() const override;
};
 
template <class T>
class AndSpecification final : public CompositeSpecification<T>
{
public:
const ISpecification<T>& Left;
const ISpecification<T>& Right;
 
AndSpecification(const ISpecification<T>& InLeft, const ISpecification<T>& InRight)
: Left(InLeft),
Right(InRight) { }
 
virtual bool IsSatisfiedBy(T Candidate) const override
{
return Left.IsSatisfiedBy(Candidate) && Right.IsSatisfiedBy(Candidate);
}
};
 
template <class T>
ISpecification<T>* CompositeSpecification<T>::And(const ISpecification<T>& Other) const
{
return new AndSpecification<T>(*this, Other);
}
</source>
 
template <class T>
Contrast without the Specification Pattern:
class AndNotSpecification final : public CompositeSpecification<T>
{
public:
const ISpecification<T>& Left;
const ISpecification<T>& Right;
 
AndNotSpecification(const ISpecification<T>& InLeft, const ISpecification<T>& InRight)
<source lang="csharp">
: Left(InLeft),
InvoiceCollection = Service.GetInvoices();
Right(InRight) { }
foreach (Invoice currentInvoice in InvoiceCollection) {
 
currentInvoice.SendToCollectionIfNecessary();
virtual bool IsSatisfiedBy(T Candidate) const override
{
return Left.IsSatisfiedBy(Candidate) && !Right.IsSatisfiedBy(Candidate);
}
};
 
template <class T>
class OrSpecification final : public CompositeSpecification<T>
{
public:
const ISpecification<T>& Left;
const ISpecification<T>& Right;
 
OrSpecification(const ISpecification<T>& InLeft, const ISpecification<T>& InRight)
: Left(InLeft),
Right(InRight) { }
 
virtual bool IsSatisfiedBy(T Candidate) const override
{
return Left.IsSatisfiedBy(Candidate) || Right.IsSatisfiedBy(Candidate);
}
};
 
template <class T>
class OrNotSpecification final : public CompositeSpecification<T>
{
public:
const ISpecification<T>& Left;
const ISpecification<T>& Right;
 
OrNotSpecification(const ISpecification<T>& InLeft, const ISpecification<T>& InRight)
: Left(InLeft),
Right(InRight) { }
 
virtual bool IsSatisfiedBy(T Candidate) const override
{
return Left.IsSatisfiedBy(Candidate) || !Right.IsSatisfiedBy(Candidate);
}
};
 
template <class T>
class NotSpecification final : public CompositeSpecification<T>
{
public:
const ISpecification<T>& Other;
 
NotSpecification(const ISpecification<T>& InOther)
: Other(InOther) { }
 
virtual bool IsSatisfiedBy(T Candidate) const override
{
return !Other.IsSatisfiedBy(Candidate);
}
};
 
template <class T>
ISpecification<T>* CompositeSpecification<T>::AndNot(const ISpecification<T>& Other) const
{
return new AndNotSpecification<T>(*this, Other);
}
 
template <class T>
//.. in the Invoice partial class:
ISpecification<T>* CompositeSpecification<T>::Or(const ISpecification<T>& Other) const
{
return new OrSpecification<T>(*this, Other);
}
 
template <class T>
public bool ShouldSendToCollection { get { return currentInvoice.OverDue && currentInvoice.NoticeSent == false && currentInvoice.InCollection == false; }}
ISpecification<T>* CompositeSpecification<T>::OrNot(const ISpecification<T>& Other) const
{
return new OrNotSpecification<T>(*this, Other);
}
 
template <class T>
public void SendToCollectionIfNecessary()
ISpecification<T>* CompositeSpecification<T>::Not() const
{
return new NotSpecification<T>(*this);
//Guard condition - with each of those new properties
if (!ShouldSendToCollection) return;
this.SendToCollection();
}
 
</source>
</syntaxhighlight>
 
=== TypeScript ===
{{Further|TypeScript}}
<syntaxhighlight lang="typescript">
export interface ISpecification {
isSatisfiedBy(candidate: unknown): boolean;
and(other: ISpecification): ISpecification;
andNot(other: ISpecification): ISpecification;
or(other: ISpecification): ISpecification;
orNot(other: ISpecification): ISpecification;
not(): ISpecification;
}
 
export abstract class CompositeSpecification implements ISpecification {
abstract isSatisfiedBy(candidate: unknown): boolean;
 
and(other: ISpecification): ISpecification {
return new AndSpecification(this, other);
}
 
andNot(other: ISpecification): ISpecification {
return new AndNotSpecification(this, other);
}
 
or(other: ISpecification): ISpecification {
return new OrSpecification(this, other);
}
 
orNot(other: ISpecification): ISpecification {
return new OrNotSpecification(this, other);
}
 
not(): ISpecification {
return new NotSpecification(this);
}
}
 
export class AndSpecification extends CompositeSpecification {
constructor(private leftCondition: ISpecification, private rightCondition: ISpecification) {
super();
}
 
isSatisfiedBy(candidate: unknown): boolean {
return this.leftCondition.isSatisfiedBy(candidate) && this.rightCondition.isSatisfiedBy(candidate);
}
}
 
export class AndNotSpecification extends CompositeSpecification {
constructor(private leftCondition: ISpecification, private rightCondition: ISpecification) {
super();
}
 
isSatisfiedBy(candidate: unknown): boolean {
return this.leftCondition.isSatisfiedBy(candidate) && this.rightCondition.isSatisfiedBy(candidate) !== true;
}
}
 
export class OrSpecification extends CompositeSpecification {
constructor(private leftCondition: ISpecification, private rightCondition: ISpecification) {
super();
}
 
isSatisfiedBy(candidate: unknown): boolean {
return this.leftCondition.isSatisfiedBy(candidate) || this.rightCondition.isSatisfiedBy(candidate);
}
}
 
export class OrNotSpecification extends CompositeSpecification {
constructor(private leftCondition: ISpecification, private rightCondition: ISpecification) {
super();
}
 
isSatisfiedBy(candidate: unknown): boolean {
return this.leftCondition.isSatisfiedBy(candidate) || this.rightCondition.isSatisfiedBy(candidate) !== true;
}
}
 
export class NotSpecification extends CompositeSpecification {
constructor(private wrapped: ISpecification) {
super();
}
 
isSatisfiedBy(candidate: unknown): boolean {
return !this.wrapped.isSatisfiedBy(candidate);
}
}
</syntaxhighlight>
 
==Example of use==
 
In the next example, invoices are retrieved and sent to a collection agency if:
 
# they are overdue,
# notices have been sent, and
# they are not already with the collection agency.
 
This example is meant to show the result of how the logic is 'chained' together.
 
This usage example assumes a previously defined <code>OverdueSpecification</code> class that is satisfied when an invoice's due date is 30 days or older, a <code>NoticeSentSpecification</code> class that is satisfied when three notices have been sent to the customer, and an <code>InCollectionSpecification</code> class that is satisfied when an invoice has already been sent to the collection agency. The implementation of these classes isn't important here.
 
Using these three specifications, we created a new specification called <code>SendToCollection</code> which will be satisfied when an invoice is overdue, when notices have been sent to the customer, and are not already with the collection agency.
 
<syntaxhighlight lang="csharp">
var overdue = new OverdueSpecification();
var noticeSent = new NoticeSentSpecification();
var inCollection = new InCollectionSpecification();
 
// Example of specification pattern logic chaining
var sendToCollection = overdue.And(noticeSent).And(inCollection.Not());
 
var invoices = InvoiceService.GetInvoices();
 
foreach (var invoice in invoices)
{
if (sendToCollection.IsSatisfiedBy(invoice))
{
invoice.SendToCollection();
}
}
</syntaxhighlight>
 
==References==
Line 264 ⟶ 553:
* [https://github.com/Happyr/Doctrine-Specification Happyr Doctrine Specification in PHP] by Happyr
* [https://github.com/neoneye/SpecificationPattern The Specification Pattern in Swift] by Simon Strandgaard
* [https://github.com/thiagodp/spec-pattern The Specification Pattern in TypeScript and JavaScript] by Thiago Delgado Pinto
* [https://web.archive.org/20110724151447/http://www.dpdk.nl/opensource/specification-pattern-for-selection-on-lists specification pattern in flash actionscript 3] by Rolf Vreijdenberger
* [https://web.archive.org/web/20110724151447/http://www.dpdk.nl/opensource/specification-pattern-for-selection-on-lists specification pattern in flash actionscript 3] by Rolf Vreijdenberger
 
 
{{Design Patterns Patternspatterns}}
 
[[Category:Architectural pattern (computer science)]]
[[Category:Software design patterns]]
[[Category:Programming language comparisons]]
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