Marker interface pattern

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The marker interface pattern is a 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 implements a marker interface, and methods that interact with instances of that class test for the existence of the interface. Whereas a typical 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 is the Serializable interface. A class implements this interface to indicate that its non-transient data members can be written to an ObjectOutputStream. The ObjectOutputStream private method writeObject() contains a series of instanceof tests to determine writeability, one of which looks for the Serializable interface. If none of these tests pass, the method throws a NotSerializableException.

[edit] Critique

A major problem with marker interfaces is that an interface defines a contract for implementing classes, and that contract is inherited by all subclasses. This means that you cannot "unimplement" a marker. In the example given, if you create a subclass that you do not want to serialize (perhaps because it depends on transient state), you must resort to explicitly throwing NotSerializableException (per ObjectOutputStream docs).

A better solution is for the language to support metadata directly:

The .NET framework supports attributes that can be used to associate any type of data with a class or with its members.
Java, as of Java 5 (1.5), also provides annotations support on classes, member variables, methods, and method parameters that may be accessed from an instance using reflection.
In Python, the use and term marker interfaces is common in Zope 3 and Plone. Interfaces are declared as metadata and subclasses can use implementsOnly to declare they do not implement everything from their super classes.