Marker interface in Java is interfaces with no field or methods or in simple word empty interface in java is called marker interface. Example of market interface is Serializable, Clonnable and Remote interface. Now if marker interface doesn't have any field or method or behavior they why would Java needs it? This was asked to my friend in one of his core java interview and then I thought to touch based on it. In this article we will see couple of reason on what marker interface do in Java and what is use of marker interface in Java. Marker interface are also called tag interface in Java.
What is Marker interfaces in Java and why required
Why Marker or Tag interface do in Java
1) Looking carefully on marker interface in Java e.g. Serializable, Clonnable and Remote it looks they are used to indicate something to compiler or JVM. So if JVM sees a Class is Serializable it done some special operation on it, similar way if JVM sees one Class is implement Clonnable it performs some operation to support cloning. Same is true for RMI and Remote interface. So in short Marker interface indicate, signal or a command to Compiler or JVM.
This is pretty standard answer of question about marker interface and once you give this answer most of the time interviewee definitely asked "Why this indication can not be done using a flag inside a class?” this make sense right? Yes this can be done by using a boolean flag or a String but doesn't marking a class like Serializable or Clonnable makes it more readable and it also allows to take advantage of Polymorphism in Java.Where Should I use Marker interface in Java
Apart from using built in marker interface for making a class Serializable or Clonnable. One can also develop his own marker interface. Marker interface is a good way to classify code. You can create marker interface to logically divide your code and if you have your own tool than you can perform some pre-processing operation on those classes. Particularly useful for developing API and framework like Spring or Struts.
After introduction of Annotation on Java5, Annotation is better choice than marker interface and JUnit is a perfect example of using Annotation e.g. @Test for specifying a Test Class. Same can also be achieved by using Test marker interface.
Another use of marker interface in Java
One more use of marker interface in Java can be commenting. a marker interface called Thread Safe can be used to communicate other developers that classes implementing this marker interface gives thread-safe guarantee and any modification should not violate that. Marker interface can also help code coverage or code review tool to find bugs based on specified behavior of marker interfaces.
Again Annotations are better choice @ThreadSafe looks lot better than implementing ThraedSafe marker interface.
In summary marker interface in Java is used to indicate something to compiler, JVM or any other tool but Annotation is better way of doing same thing.
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12 comments:
nice explanation
I agree Annotation is better than marker interface in Java. marker interface just contain one information like Serializable, Clonnable etc but with Annotation you can carry more information and it can also server purpose of marker interface in java.
very good explanation, thanks
One more difference: Annotations can carry more information to the JVM or other tools, but marker interfaces can be used to pass these objects as method arguments - just like with Serializable.
Good explanation of marker interface... Thanks..!!
thanks
This is the BEST site for interviews
Can we create our own marker interface? If we can please explain how to do it.
Javin gr8 article ..!! perfect Expnation..!!
@electronics for u ...yeah we can create our own marker interface...
For example, suppose I declare a new marker interface Foo:
public interface Foo {
}
... and then declare a class Bar that implements Foo:
public class Bar implements Foo {
private final int i;
public Bar(int i) { this.i = i; }
}
I am now able to refer to an instance of Bar through a reference of type Foo:
Foo foo = new Bar(5);
... and also check (at runtime) whether an object implements Foo:
if (o instanceof Foo) {
System.err.println("It's a Foo!");
}
This latter case is typically the driver behind using marker interfaces; the former case offers little benefit as there are no methods that can be called on Foo (without first attempting a downcast)
How JVM invoke this specific behavior
ObjectOutputStream and ObjectInputStream will check your class whether or not it implementes Serializable, Externalizable. If yes it will continue or else will thrown NonSerializableException.
How to write our own marker interface
Create an interface without any method and that is your marker interface.
Sample
public interface IMarkerEntity {
}
If any class which implement this interface will be taken as database entity by your application.
Sample Code:
public boolean save(Object object) throws InvalidEntityException {
if(!(object instanceof IMarkerEntity)) {
throw new InvalidEntityException("Invalid Entity Found, cannot proceed);
}
database.save(object);
}
Is this possible to have methods in marker interface?
The whole idea of Marker Interface Pattern is to provide a mean to say "yes I am something" and then system will proceed with the default process, like when you mark your class as Serialzable it just tells that this class can be converted to bytes.
@Saral, as always great comment, but I think marker interface is now given way for annotations. Annotations are better way to represent metadata e.g. in place of IMarkerEntity, an annotation @Enttiy, would be much better. Also external tools e.g IDE, compiler, source code checker are making good use of annotations.
Another example of marker interface from Java SDK is Remote interface from rmi package. Remote method has no methods, it just used to provide compiler information that a class will be used as remote proxy.
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