This post is not in the continuation of the last post. But this week I was searching about the marker interfaces. So I thought of documenting my learning about Marker Interfaces.
As it is known, Java interface defines contract between real world and objects, which implies when a class in implementing a java interface it has to implement all the methods defined in the interface.
What is a marker interface?
Marker interface is an interface which does not have any field and method associated with it.
/** * This interface is a marker interface. * For JVM it is just another interface without any method and field.*/public interface MarkerInterface {
}Here as in the example above there is an interface which do not define any method and field. But this interface will compile without any issues.
/**
* MarkedClass is implementing MarkerInterface, * which means class MarkedClass is associated with MarkerInterface.* This class will compile without any issues.* */public class MarkedClass implements MarkerInterface{
}To compile this example you can copy the interface’s code to MarkerInterface.java file and copy class’s code to MarkedClass.java file.
What is the actual use of marker interfaces? Why will I prefer marker interface instead of an interface which has some methods?
Purpose of marker interfaces is to create a special type of Object, in such a way to restrict the classes that can be cast to it without placing ANY restrictions on the exported methods that must be provided. In another way these are used to provide metadata information to the classes.To explain it further we can use an example…
There are quite a few IT organizations which primarily concentrate on product whereas there services are limited to their products and beside these organizations there are organizations which concentrate on services of the others products. These are classified as Product Organization and service organization.
A product company may sell their products either to Customer directly or to Service Company. On the basis of the type of customer a product company wants to provide different set of contracts with the company. In general a product company has millions of customers and few service companies associated with it. So to distinguish between these two types of organization we mark Service companies with the marker interface.
This organization interface defines the contract.
/** * This interface will have common utility methods that will be used across all the * customer as well as service organizations. */
public interface Organization {
String[] getProduct(); void setProduct(String[] products);
//Many other such methods.
}This ServiceOrganization interface is a marker interface which will state the organization as a service organization.
This interface is to mark an Organization as a Service Organization.
**/public interface ServiceOrganization extends Organization{
}Here you can see customer is implementing Organization, i.e. it is stating this Customer is not a special customer to me.
/**
* Customer is implementing Normal organization.*/ public class Customer implements Organization {
@Overridepublic String[] getProduct() {
return null;
}@Overridepublic void setProduct(String[] products) {
}}/**
* Service Customer has been tagged as service Organization but it does not need * to implement something other than method which will be defined by a* common customer. */public class ServiceCustomer implements ServiceOrganization {
@Overridepublic String[] getProduct() {
return null;
}@Overridepublic void setProduct(String[] products) {
}}As both types of customers have been separated with the use of marking, now it is not difficult to judge the customers.
Does Java have by default marker interface? If Yes, what all marker interfaces are there and what are their uses?
Yes, Java has few marker interfaces. Those are as follows:
java.util.RandomAccess
java.io.Serializable
java.rmi.Remote
java.util.EventListner
java.lang.Clonable
java.io.Serializable
When a class is implementing this interface this internally suggests to JVM that object of this class may be serialized for persistence purposes.
Source code for the interface is as mentioned below.
public interface Serializable {
}
So it is clear, writeObject and readObject methods are not part of the interface, but rather part of the serialization process, and help determine how to serialize an object.
java.lang.Clonable
Clonable interface is necessary to use clone method associated with each object. If clone is used without implementing clonable interface the code will throw a runtime exception.
java.util.EventListner
This interface also does not have any specific method but it is necessary for a class to implement the interface to achieve the functionality of listening the events.
java.util.RandomAccess
Implementing this interface suggests data accessing in the list is fast. So it is just marking list as fast random access.
java.rmi.Remote
The remote interface serves to identify interfaces whose methods may be invoked from a non-local virtual machine. Any object that is a remote object must directly or indirectly implement this interface. Only those methods specified in a "remote interface", an interface that extends java.rmi.Remote are available remotely.
Here again marker interface has made sure if something has to access remotely then it should be marked with remote interface.
List of marker interfaces is long but the interfaces mentioned above are the widely known interfaces.
Like me You must be wondering, this all is fine but still there is one important question.
How does JVM deal with specific functionality of each marker interfaces?
I could not find something very specific to this question. But as most of the experts suggests Java does not do anything specific to identify the marker interface. instanceOf operator is used to identify whether the functionality is supported for the object or not.
If you look at the code of ObjectOutputStream.java, it is quite clear when serialization is supported by java.
//Piece of code of method - writeObject0(Object obj, boolean unshared){
.............
} else if (cl.isArray()) {
writeArray(obj, desc, unshared);} else if (obj instanceof Serializable) {
writeOrdinaryObject(obj, desc, unshared);} else {
throw new NotSerializableException(cl.getName());
}.................Now it is clear what are marker interfaces and at what instances do we use them. Java 5 release sun has introduced annotations, a way which will replace marker interface in future designs. Annotations are more descriptive and powerful mechanism to tag a class. I will talk about annotations in coming blogs..
Disclaimer: All the material here is the result of research and understanding of other material. If there is some content which is either incorrect or inappropriate to the context of the article I am always open for discussion.
