Marker interface pattern

<h2 id="example">Example</h2><p>
An example of the application of marker interfaces from the <a href="/facts/Java_(programming_language)/9ScgFyAL">Java programming language</a> is the <a href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/io/Serializable.html">Serializable</a> interface:</p>package java.io;

public interface Serializable {
}
<p>A class implements this interface to indicate that its non-<a href="/facts/Transient_(computer_programming)/NKsNJgX1">transient</a> data members can be written to an <a href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/io/ObjectOutputStream.html">ObjectOutputStream</a>. The ObjectOutputStream private method writeObject0(Object,boolean) contains a series of instanceof tests to determine writeability, one of which looks for the Serializable interface. If any of these tests fails, the method throws a NotSerializableException.
</p><h2 id="critique">Critique</h2>
<p>One problem with marker interfaces is that, since an interface defines a contract for implementing classes, and that contract is inherited by all subclasses, a marker cannot be "unimplemented". In the example given, any subclass not intended for serialization (perhaps it depends on transient state), must explicitly throw NotSerializableException exceptions (per ObjectOutputStream docs).
</p><p>Another solution is for the language to support <a href="/facts/Metadata/qt35z79C">metadata</a> directly:
</p>
<ul><li>Both the <a href="/facts/.NET_Framework/J6QDAUhP">.NET Framework</a> and <a href="/facts/Java_(software_platform)/fpn6x8WL">Java</a> (as of Java 5 (1.5)) provide support for such metadata. In .NET, they are called <i>"custom attributes"</i>, in Java they are called <i>"<a href="/facts/Java_annotation/bEff4u2b">annotations</a>"</i>. 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 <a href="/facts/Reflection_(computer_science)/Nj8stFfp">reflection</a>.</li>
<li>In <a href="/facts/Python_(programming_language)/YbuGqofa">Python</a>, the term "marker interface" is common in <a href="/facts/Zope/QUkJsd03">Zope</a> and <a href="/facts/Plone_(software)/GVuT4KTR">Plone</a>. Interfaces are declared as metadata and subclasses can use implementsOnly to declare they do not implement everything from their super classes.</li></ul>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Design_marker/WHSdqA2K">Design markers</a> for an expansion of this pattern.</li></ul>

<h2 id="further-reading">Further reading</h2>
<p><i>Effective Java</i><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> by <a href="/facts/Joshua_Bloch/bPgktoww">Joshua Bloch</a>.
</p>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Bloch, Joshua (2008). "Item 37: Use marker interfaces to define types". Effective Java (Second ed.). Addison-Wesley. p. 179. ISBN 978-0-321-35668-0. <a href="978-0-321-35668-0" target="_blank">978-0-321-35668-0</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"Marker interface in Java". GeeksforGeeks. 2017-03-06. Retrieved 2022-05-01. <a href="https://www.geeksforgeeks.org/marker-interface-java/" target="_blank">https://www.geeksforgeeks.org/marker-interface-java/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Bloch, Joshua (2018). Effective Java (Third ed.). Boston. ISBN 978-0-13-468599-1. OCLC 1018432176.{{cite book}}:  CS1 maint: location missing publisher (link) <a href="978-0-13-468599-1" target="_blank">978-0-13-468599-1</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
</ol>

Marker interface pattern open-in-new

Marker interface pattern