[#specification-pattern]
= Specification pattern

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The interfaces and classes listed in this chapter provide support for the  `Specification` pattern, as described in Eric Evans' book _Domain Driven Design_, p224.

Apache Causeway will automatically apply such specifications as validation rules on properties (as per xref:refguide:applib:index/annotation/Property.adoc#mustSatisfy[@Property#mustSatisfy()]) and on action parameters (as per xref:refguide:applib:index/annotation/Parameter.adoc#mustSatisfy[@Parameter#mustSatisfy()]).

[#specification]
== `Specification`

The heart of the support for this pattern is the `Specification` interface:

[source,java]
----
public interface Specification {
    String satisfies(Object obj);  // <1>
}
----
<1> if returns `null`, then the constraint is satisfies; otherwise returns the reason why the constraint has not been satisfied.

For example:

[source,java]
----
public class StartWithCapitalLetterSpecification implements Specification {
    public String satisfies(Object proposedObj) {
        String proposed = (String)proposedObj;               // <1>
        return "".equals(proposed)
            ? "Empty string"
            : !Character.isUpperCase(proposed.charAt(0))
                ? "Does not start with a capital letter"
                : null;
    }
}
public class Customer {
    @Property(mustSatisfy=StartWithCapitalLetterSpecification.class)
    public String getFirstName() { /* ... */ }
    ...
}
----
<1> this ugly cast can be avoided using some of the other classes available; see below.

== `Specification2`

The `Specification2` interface extends the `Specification` API to add support for i18n.
This is done by defining an additional method that returns a xref:refguide:applib:index/services/i18n/TranslatableString.adoc[translatable string]:

[source,java]
----
public interface Specification2 extends Specification {
    public TranslatableString satisfiesTranslatable(Object obj);  // <1>
}
----
<1> if returns `null`, then the constraint is satisfies; otherwise returns the reason why the constraint has not been satisfied.

Note that if implementing `Specification2` then there is no need to also provide an implementation of the inherited `satisfies(Object)` method; this will never be called by the framework for `Specification2` instances.

== Adapter classes

The `AbstractSpecification` and `AbstractSpecification2` adapter classes provide a partial implementation of the respective interfaces, providing type-safety.
(Their design is modelled on the `TypesafeMatcher` class within link:http://hamcrest.org/JavaHamcrest/[Hamcrest]).

For example:

[source,java]
----
public class StartWithCapitalLetterSpecification extends AbstractSpecification<String> {
    public String satisfiesSafely(String proposed) {
        return "".equals(proposed)
            ? "Empty string"
            : !Character.isUpperCase(proposed.charAt(0))
                ? "Does not start with a capital letter"
                : null;
    }
}
public class Customer {
    @Property(mustSatisfy=StartWithCapitalLetterSpecification.class)
    public String getFirstName() { /* ... */ }
    ...
}
----

The `AbstractSpecification2` class is almost identical; its type-safe method is `satisfiesTranslatableSafely(T)` instead.

== Combining specifications

There are also adapter classes that can be inherited from to combine specifications:

* `SpecificationAnd` - all provided specifications' constraints must be met
* `SpecificationOr` - at least one provided specifications' constraints must be met
* `SpecificationNot` - its constraints are met if-and-only-if the provided specification's constraint was _not_ met.

Note that these adapter classes inherit `Specification` but do not inherit `Specification2`; in other words they do not support i18n.
