Marker interface pattern: Difference between revisions

Revision as of 08:21, 30 May 2013

Revision as of 15:21, 8 June 2013

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The '''marker interface pattern''' is a [[design pattern (computer science)|design pattern]] in [[computer science]], used with languages that provide run-time type information about objects. It provides a means to associate metadata with a class where the language does not have explicit support for such metadata.

To use this pattern, a [[Class (computer science)|class]] implements a '''marker interface''',<ref name="EffectiveJava">

{{cite book

| last = Bloch

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| year = 2008

| isbn = 978-0-321-35668-0

| publisher = Addison-Wesley}}</ref>, and methods that interact with instances of that class test for the existence of the interface. Whereas a typical [[interface (computer science)|interface]] specifies functionality (in the form of method declarations) that an implementing class must support, a marker interface need not do so. The mere presence of such an interface indicates specific behavior on the part of the implementing class. Hybrid interfaces, which both act as markers and specify required methods, are possible but may prove confusing if improperly used.

An example of the application of marker interfaces from the [[Java (programming language)|Java programming language]] is the {{Javadoc:SE|java/io|Serializable}} interface. A class implements this interface to indicate that its non-[[Transient (computer programming)|transient]] data members can be written to an {{Javadoc:SE|java/io|ObjectOutputStream}}. The <code>ObjectOutputStream</code> private method <code>writeObject()</code> contains a series of <code>instanceof</code> tests to determine writeability, one of which looks for the <code>Serializable</code> interface. If any of these tests fails, the method throws a <code>NotSerializableException</code>.

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Another solution is for the language to support [[metadata]] directly:

* Both the [[.NET Framework|.NET framework]] and [[Java (software platform)|Java]] (as of Java 5 (1.5)) provide support for such metadata. In .NET, they are called ''"custom attributes"'', in Java they are called ''"annotations"''. Despite the different name, they are conceptually the same thing. They can be defined on classes, member variables, methods, and method parameters and may be accessed using [[Reflection_Reflection (computer_sciencecomputer science)|reflection]].

* In [[Python (programming language)|Python]], the term "marker interface" is common in [[Zope]] and [[Plone (software)|Plone]]. Interfaces are declared as metadata and subclasses can use <code>implementsOnly</code> to declare they do not implement everything from their super classes.

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* [[Design marker]]s for an expansion of this pattern.

* Joshua Bloch, "Effective Java (Second edition)," Item 37: Use marker interfaces to define types, page 179.

[[Category:Software design patterns]]

[[Category:Java programming language]]

==References==

{{reflist|colwidth = 30em}}

[[Category:Software design patterns]]

[[Category:Java programming language]]