Singleton:- only one instance of a class is created in JVM - lazy/early loading mechanism.
used to provide global point of access to the object.
used in logging, caches, thread pools, configuration settings etc.
In the multi-threading environment to prevent each thread to create another instance of singleton object and thus creating concurrency issue we will need to use locking mechanism.
This can be achieved by synchronized keyword. By using this synchronized keyword we prevent Thread2 or Thread3 to access the singleton instance while Thread1 inside the method getInstance().
Enum is thread safe and implementation of Singleton through Enum ensures that your singleton will have only one instance even in a multi-threaded environment.
public class SingletonClass {
// static member
private static SingletonClass singletonInstance;
// private constructor
private SingletonClass() {
}
// global point of access
public static synchronized SingletonClass getInstance(){
if(singletonInstance == null){
singletonInstance=new SingletonClass();
System.out.println("Created");
}
return singletonInstance;
}
}
public enum SingletonEnum {
INSTANCE;
public void doPrint(){
System.out.println("Singleton using Enum");
}
}
public class SingletonTest {
public static void main(String[] args) {
System.out.println(SingletonClass.getInstance());
System.out.println(SingletonClass.getInstance());
SingletonEnum.INSTANCE.doPrint();
}
}
How to prevent Singleton Pattern from Reflection, Serialization and Cloning?
There are mainly 3 concepts which can break singleton property of a singleton class.
Reflection:- Reflection allows instantiation of new objects, invocation of methods, and get/set operations on class variables dynamically at run time without having prior knowledge of its implementation.
// Singleton class
class Singleton
{
// public instance initialized when loading the class
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());
}
}
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.
Overcome reflection issue: To overcome 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.
//Java program for Enum type singleton
public enum GFG
{
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.
Serialization:- 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.
class Singleton implements Serializable
{
// public instance initialized when loading the class
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();
}
}
}
hashCode() of both instances is different, hence there are 2 objects of a singleton class. Thus, the class is no more singleton.
Overcome serialization issue:- To overcome this issue, we have to implement method readResolve() method.
class Singleton implements Serializable
{
// public instance initialized when loading the class
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
// implement readResolve method
protected Object readResolve()
{
return instance;
}
}
The readResolve method is called when ObjectInputStream has read an object from the stream and is preparing to return it to the caller. ObjectInputStream checks whether the class of the object defines the readResolve method. If the method is defined, the readResolve method is called to allow the object in the stream to designate the object to be returned. The object returned should be of a type that is compatible with all uses. If it is not compatible, a ClassCastException will be thrown when the type mismatch is discovered.
Cloning:- It 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();
}
}
// Singleton class
class Singleton extends SuperClass
{
// public instance initialized when loading the class
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
}
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());
}
}
Two different hashCode means there are 2 different objects of singleton class.
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.
// Singleton class
class Singleton extends SuperClass
{
// public instance initialized when loading the class
public static Singleton instance = new Singleton();
private Singleton()
{
// private constructor
}
@Override
protected Object clone() throws CloneNotSupportedException
{
throw new CloneNotSupportedException();
}
}
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.
@Override
protected Object clone() throws CloneNotSupportedException
{
return instance;
}
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