There may be situations that we need
exactly one instance of a class to perform some functionalities.
Singleton design pattern ensures that only one instance of a class is
created and provides a global point of access to the object.
Public class MySingleton(){
private static MySingleton
singletonInstance;
private MySingleton(){
}
public static MySingleton
getSigleton(){
if(singletonInstance = null)
singletonInstance = new MySingleton()
return singletonInstance;
}
}
If we make the constructor of
MySingleton class private, The outside classes can't create instances
of MySingleton class. We can create instances only within MySingleton
class. Therefore, public static getter method is created to give the
instance to outside.
In the situations of using multi
threading, getter method has to be put as synchronized because if two
threads may access the method at the same time and may create two
instances of MySingleton class. But using synchronized keyword may
reduce the performance of the program.
Public class MySingleton(){
private static MySingleton
singletonInstance;
private MySingleton(){
}
public static synchronized MySingleton
getSigleton(){
if(singletonInstance = null)
singletonInstance = new MySingletn()
return singletonInstance;
}
}
The other option to create a Singleton
class is to create the instance when class is loaded. Multi
threading problems may not occur because before any of threads access
the instance is already created when the class is loaded.
Public class MySingleton(){
private static final MySingleton
singletonInstance = new MySingleton()
private MySingleton(){
}
public static MySingleton
getMySingletn (){
return singletonInstance;
}
}
According to the performance of your
program and needs, you can select which method would be better to
create the Singleton class. If you are using several class loaders,
it may create more than one instance. Singleton class cannot be
subclassed because it has a private constructor.
Best way to create a
sigleton class is to use enum as described in Effective Java Reloaded by
Joshua Bloch. Enum guarantees only one instance and thread safety. As the below code, we can declare enum Singleton easily.
public enum MySigleton{
INSTANCE;
public void execute(){
//Other functionalities
}
}
We can get the singleton instance by using MySingleton.INSTANCE.
We can get the singleton instance by using MySingleton.INSTANCE.
