Singleton pattern in Java

Simplest way to create a singleton instance is to use - static member/members, final class and private constructor.



class final Singleton{

private Singleton(){

}

static public doSomething(){
//do stuff here
}

}

class Client{

public static void main(String[] a){

Singleton.doSomething();

}

}



So wats wrong here?

a. Since Singleton class is final - cant subclass
b. Since Singleton class can effectively have only static members- it cannot implement any interface -very bad
c. all clients should access the Singleton with classname- hard binding
d. modifying existing pojo as singleton is nightmare of changing all over the places

Wat to do then?

Decouple the singleton functionality from singleton class with wrapper class.



class final SingletonWrapper{

static private Singleton instance_;

static synchronized public Singleton getInstance(){

if (instance_==null){
instance_=new Singleton();

}

return instance_;
}

}



Good article, why static singleton can't be a complete solution


http://radio.weblogs.com/0122027/stories/2003/10/20/implementingTheSingletonPatternInJava.html

Some more better models


/*
* Simple Traditional Way An instance is available whether used or not thread
* safe
*/
class AClass {

private final static AClass instance_ = new AClass();

private AClass() {
}

static AClass getInstance() {
return instance_;
}

void doStuff() {

}
}

/*
* New Java 5 way - use of volatile Instance is created with thread safe way
* when the fist getInstance is called
*/

class BClass {

private static volatile BClass instance_;

private BClass() {

}

static BClass getInstance() {

if (instance_ == null) {

synchronized (BClass.class) {

if (instance_ == null) {
instance_ = new BClass();
}
}
}

return instance_;
}

}

/*
* Better way to make use of Wrapper/Holder Lazy loaded when getInstance is
* called Thread safe
*/

class CClass {

private CClass() {

}

private static class CClassHolder {
private static final CClass instance = new CClass();
}

static CClass getInstance() {
return CClassHolder.instance;
}
}