The marker interface pattern is a design pattern

Design pattern (computer science)

In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...

 in computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, 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

Class (computer science)

In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...

 implements a marker interface, and methods that interact with instances of that class test for the existence of the interface. Whereas a typical interface

Interface (computer science)

In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...

 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)

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

 is the interface. A class implements this interface to indicate that its non-transient

Transient (computer programming)

-Java:In the Java programming language, transient is a keyword used as a field modifier. When a field is declared transient, it would not be serialized even if the class to which it belongs is serialized...

 data members can be written to an . The ObjectOutputStream private method writeObject 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.

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).

Another solution is for the language to support metadata

Metadata

The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...

 directly:

• Both the .NET framework
  .NET Framework
  The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
  
   and 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 science)
  In computer science, reflection is the process by which a computer program can observe and modify its own structure and behavior at runtime....
  
  .
• In Python
  Python (programming language)
  Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
  
  , the term "marker interface" is common in Zope
  Zope
  Zope is a free and open-source, object-oriented Web application server written in the Python programming language. Zope stands for "Z Object Publishing Environment", and was the first system using the now common object publishing methodology for the Web...
  
   and Plone. Interfaces are declared as metadata and subclasses can use implementsOnly to declare they do not implement everything from their super classes.

See also

• Design marker
  Design marker
  In software engineering, a design marker is a technique of documenting design choices in source code using the Marker Interface pattern. Marker interfaces have traditionally been limited to those interfaces intended for explicit, runtime verification . A design marker is a marker interface used to...
  
  s for an expansion of this pattern.
• Joshua Bloch, "Effective Java (Second edition)," Item 37: Use marker interfaces to define types, page 179.