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I dont understand the concept of a singleton class

I know how to make a singleton class in java but what I dont understand is the concept of singleton. Like why would I need a singleton class and why would I use a singleton instead of a regular class?
"Singleton pattern restricts the instantiation of a class and ensures
that only one instance of the class exists in the java virtual
machine."
I just read that definition but I dont get it, what does it change if there is one or more instances of a class.
Why would I want to only have one instance of a class.

In some cases, we need to expose a shared resource throughout the application e.g. DB connection but we don't want to
create shared object up-front (before creation of client objects).
explicitly pass shared object to each client object.
then we can use Singleton design pattern.
Typical Singleton class looks like
public class MySingleton {
private MySingleton INSTANCE
private MySingleton() {
}
public static MySingleton getInstance() {
if (INSTANCE == null) {
syncronized (MySingleton.class) {
if (INSTANCE == null) {
INSTANCE = new MySingleton();
}
}
}
return INSTANCE;
}
// instance methods exposing business operation
}
But we can achieve the similar behaviour by making each and every instance methods which are exposing business operation as static. In this approach we don't even need to create single object.
Then why do we need Singleton?
Well, the answer is simple. To isolate actual implementation from the client. Basically we are applying abstraction OOP principle here.
This is helpful If the singleton class is part of library which is used by various clients and library wants to vary the implementation as per the client.
Sample for such singleton can be
public class MySingleton {
private MySingleton INSTANCE
private MySingleton() {
}
public static MySingleton getInstance() {
if (INSTANCE == null) {
syncronized (MySingleton.class) {
if (INSTANCE == null) {
INSTANCE = new MySingletonChild(); // here we are creating an object of subclass of MySingleton.
}
}
}
return INSTANCE;
}
// instance methods exposing business operation
}
Hope this help in understanding Singleton design pattern.

Singletons are used for when you want exactly one instance of a class, that the entire application shares.
Good examples for this principle are classes that are in charge of accessing external resources. For example, you'd want the entire application share the same database connection (or at least connection pool), not have every class that needs it open its own connection.

Singletons are classes with properties which can be shared with other classes in the same context. (Application, session, ...)
For example if you have to count the number of connected users in a web application. Every time a user connect, you increment a counter in a unique shared class.

When should I use lazy Singletons over normal Singletons?

So far I have seen two examples of Singletons.
Normal Singletons,
public class Singleton {
private static Singleton instance;
static {
instance = new Singleton();
}
private Singleton() {
// hidden constructor
}
public static Singleton getInstance() {
return instance;
}
}
and Lazy Singletons,
public class Singleton {
private Singleton() {
// hidden constructor
}
private static class Holder {
static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return Holder.INSTANCE;
}
}
Coding is from this thread and this user. I have just recently gotten into trying to learn Singletons as my previous methods have been
1.) Using static in order to create something like ...
static MyClass instance;
2.) I would attempt to pass an instance in a seemingly odd way,
MyClass instance;
#Override
public void onEnable() { instance = this; }
// Transition to different class - - -
public OtherClass(MyClass myClass) {
this.instance = myClass;
}
Lastly, what is my end goal? I am mainly using it in order to pass variables from my main class to other classes. I'm currently attempting to learn how to properly use Files and FileConfiguration, so I want to easily share them throughout my classes.
If I seem like a beginner, instead of going out of your way to tell me to learn Java, please provide a resource to help me with my problem first and foremost.

As to when, rather than how: I would use lazy instantiation of a singleton or of any other object when there is a fair chance of the object not being needed, and immediate instantiation when the likelihood of it being needed is high. In general, if instantiation were to fail, and the object is needed, it is better that it fail as early as possible.

This link explains it fairly well and even uses a similar example.
In software engineering, the initialization-on-demand holder (design pattern) idiom is a lazy-loaded singleton. In all versions of Java, the idiom enables a safe, highly concurrent lazy initialization with good performance.
Regarding why you should use this: if the creation of this instance is expensive, then this design pattern essentially delegates the expensive computation for when it is needed, rather than when the outer class, Singleton in your case, is first accessed.
Another reason is given by this other link. It states:
A singleton implementation may use lazy initialization, where the instance is created when the static method is first invoked. If the static method might be called from multiple threads simultaneously, measures may need to be taken to prevent race conditions that could result in the creation of multiple instances of the class.

Can objects have access modifiers?

I was going through a piece of code when I came across this:
public class ClassicA {
private static ClassicA instance = null;
}
I have never used such a thing and wanted to know what it means and how it is used. Also, what is the purpose of the access modifier for the object? Any examples/links are welcome.

It probably means that ClassicA is a Singleton. It is usually involved with declaring a private constructor, and a single public static getInstance() method.
Singletons are used when you want to make sure there is only one global instance of ClassicA in your entire application. Instead of instantiating it, you call getInstance(), which will check if it was instantiated once or not. If it was, it will instantiate it, and store the the resulting object in the private instance field. If it was already constructed, just return the instance field without re-instantiating.
Note that this is considered bad practice. See: https://softwareengineering.stackexchange.com/questions/40373/so-singletons-are-bad-then-what

Well, the class ClassicA has a private and static field instance which is null.
If there are no getters/setters the only way to access that field would be using reflection.
Since this looks like a singleton, I guess there's a getter as well that returns instance and if it is null first creates an instance and assigns it to the field.

It's a (static) member of the class, and yes, these can have access modifiers. (And as others have noted, it indeed looks like a portion of a Singleton.)

its a singleton
basically the author intended there to be only 1 instance of this class alive (the instance field)
the constructor is probably private and used only within the class.

This is used typically for a class that exhibits the Singleton design pattern.
The point is that for these types of objects you only want a single instance at most. What you do is create a private constructor for the class and then usually a public static method called, getInstance in which you check if the private instance variable has been set yet or not. If it has not yet set, you create a new instance of the class and assign it to instance, then you return that instance. If the object was already created you simply return it. Like this:
public class MySingleton {
private static MySingleton instance=null;
private MySingleton() {
//do stuff
}
public static MySingleton getInstance() {
if (instance == null) {
instance = new MySingleton();
}
return instance;
}
}
Then from throughout your program you can easily get the singleton object from anywhere.
Thus, a singleton is effectively just a glorified global variable in languages such as PHP. I would argue though that it is a lot cleaner as you can prevent others from reassigning the reference to the instance and other trickery that may be very bad from a design standpoint.
Typically people use it for classes that manage some type of data access, such as a DB object, for Factory classes and builder classes, see this for more info

This is used for singleton pattern. See here

private static means that the instance is available to all the instances and static methods of the class, but only to them. It's actually like having static private method.
Such tricks can be used for example for implementing a singleton: you keep internally a single instance of a class, and you can give it to the class's clients on demand. Or for any other case when you need something shared just between the static methods and instances of some class.

How do I find where an instance of a Java singleton is created?

In a large, complex program it may not be simple to discover where in the
code a Singleton has been instantiated. What is the best approach to keep track of created singleton instances in order to re-use them?
Regards,
RR

A Singleton usually has a private constructor, thus the Singleton class is the only class which can instantiate the one and only singleton instance.

It's the responsibilty of singleton class developer to make sure that the instance is being reused on multiple calls.
As a user, you shouldn't worry about it.
class Singelton
{
private static Singelton _singelton = null;
private Singelton()
{
}
// NOT usable for Multithreaded program
public static Singelton CreateMe()
{
if(_singelton == null)
_singelton = new Singelton();
return _singelton;
}
}
Now, from anywhere in your code, you can instantiate Singelton, how many times you like and each time assign it to different reference. but c'tor is called ONLY once.

I would use an enum
enum Singleton {
INSTANCE:
}
or something similar which cannot be instantiated more than once and globally accessible.

General practice for naming methods which create/return singletons is getInstance(). I don't understand the situation when you can't find the place in code where singletons created, but you can search for this method name.
If you want to catch the exact moment of singleton creation - you can use AOP. AspectJ is a good example in java. You will be able to execute your code before/after creation of class or calling getInstance() method.
If your question is about reusing of created Singletons, then search this site. For example

Java Singleton Design Pattern : Questions

I had an interview recently and he asked me about Singleton Design Patterns about how are they implemented and I told him that using static variables and static methods we can implement Singleton Design Patterns.
He seems to be half satisfied with the answer but I want to know
How many different ways we can
implement Singleton Design Pattern
in Java ?
What is the scope of Singleton Object and how does it actually work inside JVM ? I know we would always have one instance of Singleton Object but what is the actual scope of that object, is it in JVM or if there are multiple application running than it's scope is per context basis inside the JVM, I was really stumped at this and was unable to give satisfying explanation ?
Lastly he asked if it is possible to used Singleton Object with Clusters with explanation and is there any way to have Spring not implement Singleton Design Pattern when we make a call to Bean Factory to get the objects ?
Any inputs would be highly appreciated about Singleton and what are the main things to keep in mind while dealing with Singletons ?
Thanks.

There are a few ways to implement a Singleton pattern in Java:
// private constructor, public static instance
// usage: Blah.INSTANCE.someMethod();
public class Blah {
public static final Blah INSTANCE = new Blah();
private Blah() {
}
// public methods
}
// private constructor, public instance method
// usage: Woo.getInstance().someMethod();
public class Woo {
private static final Woo INSTANCE = new Woo();
private Woo() {
}
public static Woo getInstance() {
return INSTANCE;
}
// public methods
}
// Java5+ single element enumeration (preferred approach)
// usage: Zing.INSTANCE.someMethod();
public enum Zing {
INSTANCE;
// public methods
}
Given the examples above, you will have a single instance per classloader.
Regarding using a singleton in a cluster...I'm not sure what the definition of "using" is...is the interviewer implying that a single instance is created across the cluster? I'm not sure if that makes a whole lot of sense...?
Lastly, defining a non-singleton object in spring is done simply via the attribute singleton="false".

I disagree with #irreputable.
The scope of a Singleton is its node in the Classloader tree. Its containing classloader, and any child classloaders can see the Singleton.
It's important to understand this concept of scope, especially in the application servers which have intricate Classloader hierarchies.
For example, if you have a library in a jar file on the system classpath of an app server, and that library uses a Singleton, that Singleton is going to (likely) be the same for every "app" deployed in to the app server. That may or may not be a good thing (depends on the library).
Classloaders are, IMHO, one of the most important concepts in Java and the JVM, and Singletons play right in to that, so I think it is important for a Java programmer to "care".

I find it hard to believe that so many answers missed the best standard practice for singletons - using Enums - this will give you a singleton whose scope is the class loader which is good enough for most purposes.
public enum Singleton { ONE_AND_ONLY_ONE ; ... members and other junk ... }
As for singletons at higher levels - perhaps I am being silly - but my inclination would be to distribute the JVM itself (and restrict the class loaders). Then the enum would be adequate to the job .

Singleton is commonly implemented by having a static instance object (private SingletonType SingletonType.instance) that is lazily instantiated via a static SingletonType SingletonType.getInstance() method. There are many pitfalls to using singletons, so many, in fact, that many consider singleton to be a design anti-pattern. Given the questions about Spring, the interviewer probably was looking for an understanding not only of singletons but also their pitfalls as well as a workaround for these pitfalls known as dependency injection. You may find the video on the Google Guice page particularly helpful in understanding the pitfalls of singletons and how DI addresses this.

3: Lastly he asked if it is possible to used Singleton Object with Clusters with explanation and is there any way to have Spring not implement Singleton Design Pattern when we make a call to Bean Factory to get the objects ?
The first part of this question is hard to answer without a technological context. If the cluster platform includes the ability to make calls on remote objects as if they were local objects (e.g. as is possible with EJBs using RMI or IIOP under the hood) then yes it can be done. For example, the JVM resident singleton objects could be proxies for a cluster-wide singleton object, that was initially located / wired via JNDI or something. But cluster-wide singletons are a potential bottleneck because each call on one of the singleton proxies results in an (expensive) RPC to a single remote object.
The second part of the question is that Spring Bean Factories can be configured with different scopes. The default is for singletons (scoped at the webapp level), but they can also be session or request scoped, or an application can define its own scoping mechanism.

a static field can have multiple occurrences in one JVM - by using difference class loaders, the same class can be loaded and initialized multiple times, but each lives in isolation and JVM treat the result loaded classes as completely different classes.
I don't think a Java programmer should care, unless he's writing some frameworks. "One per VM" is a good enough answer. People often talk that way while strictly speaking they are saying "one per classloader".
Can we have one singleton per cluster? Well that's a game of concepts. I would not appreciate an interviewer word it that way.

There's the standard way, which you already covered. Also, most dependency-injection schemes have some way to mark a class as a singleton; this way, the class looks just like any other, but the framework makes sure that when you inject instances of that class, it's always the same instance.
That's where it gets hairy. For example, if the class is initialized inside a Tomcat application context, then the singleton instance's lifetime is bound to that context. But it can be hard to predict where your classes will be initialized; so it's best not to make any assumptions. If you want to absolutely make sure that there's exactly one instance per context, you should bind it as an attribute of the ServletContext. (Or let a dependency-injection framework take care of it.)
--
Not sure I understand the question - but if you're talking about having a singleton instance that's shared between several cluster nodes, then I think EJB makes this possible (by way of remote beans), though I've never tried it. No idea how Spring does it.

Singleton is a creational pattern and hence governs object instantiation. Creating singletons would mandate that you voluntarily or involuntarily give up control on creating the object and instead rely on some way of obtaining access to it.
This can be achieved using static methods or by dependency injection or using the factory pattern. The means is immaterial. In case of the normal protected constructor() approach, the consumer perforce needs to use the static method for accessing the singleton. In case of DI, the consumer voluntarily gives up control over the instantiation of the class and instead relies on a DI framework to inject the instance into itself.
As pointed out by other posters, the class loader in java would define the scope of the singleton. Singletons in clusters are usually "not single instances" but a collection of instances that exhibit similar behavior. These can be components in SOA.

The Following Code is from here
The Key point is you should Override the clone method...The Wikipedia example also is helpful.
public class SingletonObject
{
private SingletonObject()
{
// no code req'd
}
public static SingletonObject getSingletonObject()
{
if (ref == null)
// it's ok, we can call this constructor
ref = new SingletonObject();
return ref;
}
public Object clone()
throws CloneNotSupportedException
{
throw new CloneNotSupportedException();
// that'll teach 'em
}
private static SingletonObject ref;
}

Query 1:
Different ways of creating Singleton
Normal Singleton : static initialization
ENUM
Lazy Singleton : Double locking Singleton & : Initialization-on-demand_holder_idiom singleton
Have a look at below code:
public final class Singleton{
private static final Singleton instance = new Singleton();
public static Singleton getInstance(){
return instance;
}
public enum EnumSingleton {
INSTANCE;
}
public static void main(String args[]){
System.out.println("Singleton:"+Singleton.getInstance());
System.out.println("Enum.."+EnumSingleton.INSTANCE);
System.out.println("Lazy.."+LazySingleton.getInstance());
}
}
final class LazySingleton {
private LazySingleton() {}
public static LazySingleton getInstance() {
return LazyHolder.INSTANCE;
}
private static class LazyHolder {
private static final LazySingleton INSTANCE = new LazySingleton();
}
}
Related SE questions:
What is an efficient way to implement a singleton pattern in Java?
Query 2:
One Singleton instance is created per ClassLoader. If you want to avoid creation of Singleton object during Serializaiton, override below method and return same instance.
private Object readResolve() {
return instance;
}
Query 3:
To achieve a cluster level Singleton among multiple servers, store this Singleton object in a distributed caches like Terracotta, Coherence etc.

Singleton is a creational design pattern.
Intents of Singleton Design Pattern :
Ensure a class has only one instance, and provide a global point of
access to it.
Encapsulated "just-in-time initialization" or "initialization on
first use".
I'm showing three types of implementation here.
Just in time initialization (Allocates memory during the first run, even if you don't use it)
class Foo{
// Initialized in first run
private static Foo INSTANCE = new Foo();
/**
* Private constructor prevents instantiation from outside
*/
private Foo() {}
public static Foo getInstance(){
return INSTANCE;
}
}
Initialization on first use (or Lazy initialization)
class Bar{
private static Bar instance;
/**
* Private constructor prevents instantiation from outside
*/
private Bar() {}
public static Bar getInstance(){
if (instance == null){
// initialized in first call of getInstance()
instance = new Bar();
}
return instance;
}
}
This is another style of Lazy initialization but the advantage is, this solution is thread-safe without requiring special language constructs (i.e. volatile or synchronized). Read More at SourceMaking.com
class Blaa{
/**
* Private constructor prevents instantiation from outside
*/
private Blaa() {}
/**
* BlaaHolder is loaded on the first execution of Blaa.getInstance()
* or the first access to SingletonHolder.INSTANCE, not before.
*/
private static class BlaaHolder{
public static Blaa INSTANCE = new Blaa();
}
public static Blaa getInstance(){
return BlaaHolder.INSTANCE;
}
}