How to stop Singleton Pattern from Reflection, Serialization and Cloning?
1)Reflection: Reflection can be a caused to destroy singleton property of singleton class.but what is Reflection? in a short answer is: Reflection is an API which is used to examine or modify the behavior of methods, classes, interfaces at runtime.Let see the code.
import java.lang.reflect.Constructor;
class Singleton
{
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
}
public class Test
{
public static void main(String[] args)
{
Singleton instance1 = Singleton.instance;
Singleton instance2 = null;
try
{
Constructor[] constructors = Singleton.class.getDeclaredConstructors();
for (Constructor constructor : constructors)
{
// Below code will destroy the singleton pattern
constructor.setAccessible(true);
instance2 = (Singleton) constructor.newInstance();
break;
}
}
catch (Exception e)
{
e.printStackTrace();
}
System.out.println("instance1.hashCode():- "+ instance1.hashCode());
System.out.println("instance2.hashCode():- "+ instance2.hashCode());
}
}
Output:
instance1.hashCode():- 366712642
instance2.hashCode():- 1829164700
After running this class, you will see that hashCodes are different that means, 2 objects of same class are created and singleton pattern has been destroyed.
So how to overcome this issue:
To overcome this issue raised by reflection, enums are used because java ensures internally that enum value is instantiated only once. Since java Enums are globally accessible, they can be used for singletons. Its only drawback is that it is not flexible i.e it does not allow lazy initialization.
public enum Test
{
INSTANCE;
}
As enums don’t have any constructor so it is not possible for Reflection to utilize it. Enums have their by-default constructor, we can’t invoke them by ourself. JVM handles the creation and invocation of enum constructors internally. As enums don’t give their constructor definition to the program, it is not possible for us to access them by Reflection also. Hence, reflection can’t break singleton property in case of enums.
2)Serialization:
Serialization can also cause breakage of singleton property of singleton classes. Serialization is used to convert an object of byte stream and save in a file or send over a network. Suppose you serialize an object of a singleton class. Then if you de-serialize that object it will create a new instance and hence break the singleton pattern.
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
import java.io.Serializable;
class Singleton implements Serializable
{
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
}
public class Test
{
public static void main(String[] args)
{
try
{
Singleton instance1 = Singleton.instance;
ObjectOutput out= new ObjectOutputStream(new FileOutputStream("file.text"));
out.writeObject(instance1);
out.close();
// deserailize from file to object
ObjectInput in = new ObjectInputStream(new FileInputStream("file.text"));
Singleton instance2 = (Singleton) in.readObject();
in.close();
System.out.println("instance1 hashCode:- "+ instance1.hashCode());
System.out.println("instance2 hashCode:- "+ instance2.hashCode());
}
catch (Exception e)
{
e.printStackTrace();
}
}
}
Output:
instance1 hashCode:- 1550089733
instance2 hashCode:- 865113938
So how to over this issue .
Overcome serialization issue:- To overcome this issue, we have to implement method readResolve() method.
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
import java.io.Serializable;
class Singleton implements Serializable
{
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
// implement readResolve method
protected Object readResolve()
{
return instance;
}
}
public class Test
{
public static void main(String[] args)
{
try
{
Singleton instance1 = Singleton.instance;
ObjectOutput out = new ObjectOutputStream(new FileOutputStream("file.text"));
out.writeObject(instance1);
out.close();
// deserailize from file to object
ObjectInput in = new ObjectInputStream(new FileInputStream("file.text"));
Singleton instance2 = (Singleton) in.readObject();
in.close();
System.out.println("instance1 hashCode:- "+ instance1.hashCode());
System.out.println("instance2 hashCode:- "+ instance2.hashCode());
}
catch (Exception e)
{
e.printStackTrace();
}
}
}
Output:
instance1 hashCode:- 1550089733
instance2 hashCode:- 1550089733
3)Cloning:
Cloning is a concept to create duplicate objects. Using clone we can create copy of object. Suppose, we create clone of a singleton object, then it will create a copy that is there are two instances of a singleton class, hence the class is no more singleton.
class SuperClass implements Cloneable
{
int i = 10;
@Override
protected Object clone() throws CloneNotSupportedException
{
return super.clone();
}
}
class Singleton extends SuperClass
{
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
}
public class GFG
{
public static void main(String[] args) throws CloneNotSupportedException
{
Singleton instance1 = Singleton.instance;
Singleton instance2 = (Singleton) instance1.clone();
System.out.println("instance1 hashCode:- "+ instance1.hashCode());
System.out.println("instance2 hashCode:- "+ instance2.hashCode());
}
}
Output :
instance1 hashCode:- 366712642
instance2 hashCode:- 1829164700
So now as you can see two different hashcodes that means two different objects.How to
overcome this issue.
Overcome Cloning issue:- To overcome this issue, override clone() method and throw an exception from clone method that is CloneNotSupportedException. Now whenever user will try to create clone of singleton object, it will throw exception and hence our class remains singleton.
class SuperClass implements Cloneable
{
int i = 10;
@Override
protected Object clone() throws CloneNotSupportedException
{
return super.clone();
}
}
class Singleton extends SuperClass
{
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
@Override
protected Object clone() throws CloneNotSupportedException
{
throw new CloneNotSupportedException();
}
}
public class Test
{
public static void main(String[] args) throws CloneNotSupportedException
{
Singleton instance1 = Singleton.instance;
Singleton instance2 = (Singleton) instance1.clone();
System.out.println("instance1 hashCode:- "+ instance1.hashCode());
System.out.println("instance2 hashCode:- "+ instance2.hashCode());
}
}
Output:
Exception in thread "main" java.lang.CloneNotSupportedException
at Test.Singleton.clone(Test.java:29)
at Test.Test.main(Test.java:38)
Now we have stopped user to create clone of singleton class. If you don't want to throw exception you can also return the same instance from clone method.
class SuperClass implements Cloneable
{
int i = 10;
@Override
protected Object clone() throws CloneNotSupportedException
{
return super.clone();
}
}
class Singleton extends SuperClass
{
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
@Override
protected Object clone() throws CloneNotSupportedException
{
return instance;
}
}
public class GFG
{
public static void main(String[] args) throws CloneNotSupportedException
{
Singleton instance1 = Singleton.instance;
Singleton instance2 = (Singleton) instance1.clone();
System.out.println("instance1 hashCode:- "+ instance1.hashCode());
System.out.println("instance2 hashCode:- "+ instance2.hashCode());
}
}
Output:
instance1 hashCode: 366712642
instance2 hashCode: 366712642
Now, as hashcode of both the instances is same that means they represent a single instance.