Java: Marker Interfaces

A marker interface, or tag interface, is an interface without any methods or fields. It’s typically used to flag that instances of the implementing classes have some internal capability and behaves in a certain way when interacting with them.

It may also be the case that they have no semantical purpose other than to group related types together. It then serves as slightly more typesafe alternative to Object and can be seen as a poor man’s substitute for discriminated union types.

Examples from the Java API

RandomAccess is a marker interface that should be implemented by List classes that provide efficient non-sequential access. Specifically the Javadoc says:

As a rule of thumb, a List implementation should implement this interface if, for typical instances of the class, this loop:
for (int i = 0, n = list.size(); i < n; i++)
     list.get(i);
runs faster than this loop:
for (Iterator i = list.iterator(); i.hasNext();)
     i.next();

This allows clients to choose the most efficient code for traversing a given list. ArrayList implements this interface, while LinkedList does not.

Note: Since RandomAccess is intended specifically for lists, an alternative approach would have been to simply add a boolean providesRandomAccess() method to the List interface. However, at the time RandomAccess was introduced this was not an option, since List was already around, and adding methods to an interface is not a backward compatible change. Had this feature been introduced today on the other hand, a default method could have been a good alternative.

Other examples from the standard API:

Interface Purpose
Serializable Says that the object state may be serialized
Cloneable Says that Object.clone may perform a field-by-field copy of the object
Remote Says that methods may be invoked from a non-local virtual machine
EventListener Interface that all AWT event listeners must implement
CopyOption Union type for LinkOption and StandardCopyOption

A custom marker interface

Suppose we have a LogUtil.logAsynchronously(Object o) method that writes o.toString() to a log file. To return as quickly as possible from logAsynchronously we would like to call toString in the background thread. This is however only safe if other threads don’t change the state of o. For this reason we add an ImmutableObject marker interface to allow for clients to opt in for this feature.

interface ImmutableObject {}

The logAsynchronously method could then use instanceof as follows:

class LogUtil {
    public void logAsynchronously(Object o) {
        if (o instanceof ImmutableObject) {
            runInBackground(() -> log(o.toString()));
        } else {
            String msg = o.toString();
            runInBackground(() -> log(msg));
        }
    }
    …
}

Use as discriminated union type

Other languages, such as F# and TypeScript support discriminated union types which means you can write things like:

function print(value: string | number) {
    // value can be a string or a number
}

Java does not provide support for this. If you want to express “an instance of either A or B you can create an empty interface AorB and let both classes implement this interface.

CopyOption is an example of this. It is an empty interface implemented by LinkOption and StandardCopyOption. This allows for methods such as Files.copy to have a signature like this:

public static Path copy(Path source,
                        Path target,
                        CopyOption... options) {
    …
}

This approach doesn’t let you create union types of classes out of your control, like String and Number as in the TypeScript example above. On the upside, you can easily extend the union type with new classes by letting more classes implement the empty interface.

Restricting Use

By letting the marker interface, M, extend another interface, I, you make sure that M can only be implemented by classes that also implement I.

For example, RandomAccess is intendend to be used by List implementations. If the authors wanted to enforce this, they could have implemented RandomAccess like this:

interface RandomAccess extends List {}

Criticism

The use of marker interfaces have long been criticized and nowadays most of the use cases are better solved with annotations.

Violates the principle of least astonishment
The primary purpose of an interface is to establish a contract of communication. Since an empty interface is a contract that says nothing, it's at first glance quite useless. Some argue that marker interfaces aren't actual interfaces at all.
Forces a non-object oriented programming style
The use of instanceof almost always begs the question: Why isn't this solved with polymorphism / virtual methods? Where it would be natural to do
obj.doSomething()
marker interfaces requires you to do
if (obj instanceof Marker) {
    doSomething(obj);
}
I.e. the logic is pushed outside the class itself. If this affects many places in the code, maintenance becomes a nightmare.
Requires casting
If you have say, a Cloneable variable, you'd expect to be able to clone it. But no; You're forced to cast it to something else first. This is because the contract of a marker interface such as Cloneable is useless. All you get are the implicitly declared methods from Object, which is rarely what you're looking for. This ties back to the first item: Marker interfaces aren't proper interfaces at all.
Requires out of band knowledge
If objects implementing marker interfaces requires special treatment, the programmer must be aware of this. Even if the programmer reads up on the documentation of the marker interface, it may be unclear where to be cautious. While an Animal doesn't implement the marker interface, a Cat might. Or some future—not yet known—subclass might. Or an undocumented anonymous subclass might.

Marker Interfaces vs Annotations

Marker interfaces and annotations serve the same purpose: Convey metadata about the class to its consumers. The fundamental difference however, is that annotations are meant to be used for this purpose, while interfaces are not.

Parameters
A marker interface is like a boolean flag, while an annotation can carry an actual value. In other words, annotations let you express things like @Author("John").
Inheritance
Whether or not an annotation is inherited can be controlled through the @Inherited annotation. Marker interfaces are always inherited and there's no way for subclasses to opt out. For example, if you extend a Serializable class, your class will automatically also be serializable.
Retention
Sometimes the metadata is only relevant during compilation. Sometimes it should be available also in runtime. @Retention allows you to control this aspect for annotations. Marker interfaces are always available in runtime.
Applicability
Marker interfaces can only be "applied" to classes and interfaces. Annotations can also be applied to methods, fields, local variables, parameters, packages and other annotations.
Repetition
A marker interface can be applied at most once. Annotations can be applied multiple times. You can for example have multiple @Author annotations on the same class.

When to use marker interfaces?

As shown above, annotations are superior in many ways. Marker interfaces however, give you something that annotations don’t: an actual type. If (and only if) you need to express something like “This method accepts an immutable value as argument” then you should use an ImmutableValue marker interface rather than an @ImmutableValue annotation. (You can actually still use an annotation for this, but you would need to use a compiler plugin like the Checker Framework.)

Rule of thumb: Use marker interfaces if you want to define a type, otherwise use an annotation.

The EventListener interface is an example where an annotation would not have been sufficient. Most annotations (even non-repeating, inherited, without arguments) would not have been suitable as a marker interface: @NotNull, @JsonIgnore, @Interned to mention a few.

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