Singleton class is a class which has only one instance in whole JVM.
Example of singleton class in jdk is java.lang.Runtime, getRuntime() method is used to create
instance.
There are following ways to create Singleton class
What is Early and Lazy loading of Singleton and how will you
implement it?
Early Loading: Singleton
instance is created when class is loaded into memory.
Lazy Loading: Singleton
instance is not created when class is loaded into memory. When we call its getInstance() method only
then its instance is created. Such kind
of phenomena is called lazy loading.
e.g Early Loading
public class Singleton {
public
static final Singleton INSTANCE = new Singleton ();
private
Singleton (){
}
}
Lazy Loading
public class Singleton {
private
static final Singleton INSTANCE = new Singleton ();
private
Singleton (){ }
public Singleton
getInstance(){
return
INSTANCE;
}
}
Double checked
locking in Singleton
public static Singleton getInstance(){
if(null ==
INSTANCE){
synchronized(Singleton.class){
//double
checked locking - because second check of Singleton instance with lock
if(null == INSTANCE){
INSTANCE = new Singleton();
}
}
}
return INSTANCE;
}
Double checked locking should only be used when you have
requirement for lazy initialization otherwise use Enum to implement singleton or simple static final variable.
In Java 1.5, there is an approach to implementing
singletons. Simply make an enum type with one element:
Singleton using Enum
in Java
// Enum singleton - This is the preferred approach
public enum Elvis {
INSTANCE;
}
This approach is functionally equivalent to the public field
approach, except that it is more concise, provides the serialization machinery
for free, and provides an ironclad guarantee against multiple instantiation,
even in the face of sophisticated serialization or reflection attacks. While
this approach has yet to be widely adopted, a single-element enum type is the
best way to implement a singleton.
