We Keep Coding

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.

When to use proper version of singleton thread-safe implementation?

I have a stateless helper-like class which I want to make as a singleton. This class will be shared across different threads.
Am I correct that in this case (instance does not require huge memory allocation size and thus can be loaded several times without resources and performance impact) there is no need in implementing such a singleton with proper multi-threading lazy initialization strategy (Double Checked Locking & volatile, On Demand Holder idiom, Enum Singleton, Synchronized Accessor)?
Is it right to implement such a singleton with a simple non-multi-threading lazy initialization version strategy (like in the code below) in order to have less amount of boilerplate code?
public class Singleton {
private static Singleton INSTANCE;
private Singleton() {
}
public static Singleton getInstance() {
if (INSTANCE == null) {
INSTANCE = new Singleton();
}
return INSTANCE;
}
}
And only in case when state of the singleton class shared across different threads it is required to add proper multi-threading version of a singleton initialization?

If your class is entirely stateless - make it a util class with only static functions.
If you have state and want a semi-singleton I would say that what you have done is misleading since there is no way for the reader to know if you were aware of the fact that you can get multiple instances or not. If you decide to stick with your posted code - rename it multiton or document the behaviour carefully. But don't do it just to reduce boilerplate though - you are in fact creating more problems for the reader than you are removing.
In my opinion the "Initialization on Demand Holder" idiom is the nicest singleton pattern. But I would recommend against Singletons in general. You would be better off passing a reference to a shared instance to your thread when you start it.

To answer your question... No, it's not correct.
You say it's OK for it to be loaded several times, but in that case then it's not a singleton. The defining characteristic of a singleton is that there can only ever be one instance.
If it's OK for there to be several, then why make it a singleton at all?
If it's just stateless util methods then why not just make the static?

Initialize-On-Demand idiom vs simple static initializer in Singleton implementation

Is the Initialize-On-Demand idiom really necessary when implementing a thread safe singleton using static initialization, or would a simple static declaration of the instance suffice?
Simple declaration of instance as static field:
class Singleton
{
private static Singleton instance=new Singleton();
private Singleton () {..}
public static Singleton getInstance()
{
return instance;
}
}
vs
class Singleton {
static class SingletonHolder {
static final Singleton INSTANCE = new Singleton();
}
private Singleton () {..}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
I ask this because Brian Goetz recommends the 1st approach in this article:
http://www.ibm.com/developerworks/java/library/j-dcl/index.html
while he suggests the latter in this article
http://www.ibm.com/developerworks/library/j-jtp03304/
Does the latter approach provide any benefits that the former doesn't?

Well what i can say These articles are 7-9 years old.
Now we have > Java 1.5 where we have power of enumeration enum. According to 'Josh Block' The best way to write a singleton is to write a Single Element enum
public enum MySingleton{
Singleton;
// rest of the implementation.
// ....
}
But for your question I guess there is no issue in using either of the implementations. I personaly prefer the first option because its straightforward, simple to understand.
But watch out for the loop holes that we can be able to create more objects of these class in the same JVM at the same time by serializing and deserializing the object or by making the clone of the object.
Also make the class final, because we can violate the singleton by extending the class.

In first approach your singleton will get created once you load Singleton class. In the other, it will get created once you call getInstance() method. Singleton class may have many reasons to get loaded before you call getInstance. So you will most likely initialize it much earlier when you actually use it and that defeats the purpose of lazy initialization. Whether you need lazy initialization is a separate story.

The simple declaration pattern constructs the singleton when when the class Singleton is loaded. The initialize-on-demand idiom constructs the singleton when Singeton.getInstance() is called -- i.e., when class SingetonHolder is loaded.
So these are the same except for time; the second option allows you delay initialization. When to choose one or the other depends on (among other things) how much work you are doing in Singleton's constructor. If it's a lot, you may see improved application startup time with initialization-on-demand.
That said, my advice is to try not to do too much there so that the simplest pattern works for you.
-dg

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;
}
}

static method and thread safety

Is the following code threadsafe ?
public static Entity getInstance(){
//the constructor below is a default one.
return new Entity();
}

Assuming the constructor itself is thread-safe, that's fine.
It would be very unusual for a constructor not to be thread-safe, but possible... even if it's calling the default auto-generated constructor for Entity, the base constructor may not be thread-safe. I'm not saying it's likely, just possible :)
Basically there's no magic thread-safety applied to static methods or instance methods or constructors. They can all be called on multiple threads concurrently unless synchronization is applied. If they don't fetch or change any shared data, they will generally be safe - if they do access shared data, you need to be more careful. (If the shared data is immutable or only read, that's generally okay - but if one of the threads will be mutating it, you need to be really careful.)
Only static initializers (initialization expressions for static variables and static { ... } blocks directly within a class) have special treatment - the VM makes sure they're executed once and only once, blocking other threads which are waiting for the type to be initialized.

It depends on the details of the Entity constructor. If the Entity constructor modifies shared data, then it is not.

It's probably thread safe, but what's the point? If you're just using a factory method to redirect to the default constructor then why not use the constructor in the first place? So the question is: what are you trying to achieve? The name getInstance() suggests a singleton (at least that's common practice), but you clearly don't have a singleton there. If you do want a singleton, use a static inner holder class like this:
public class Singleton {
private Singleton() {
}
public static Singleton getInstance() {
return InstanceHolder.INSTANCE;
}
private static final class InstanceHolder {
public static final Singleton INSTANCE = new Singleton();
}
}
but if you don't, why bother with such a factory method, as you're not adding any value (method name semantics, object pooling, synchronization etc) through it

Thread safety is about access to shared data between different threads. The code in your example doesn't access shared data by itself, but whether it's thread-safe depends on whether the constructor accesses data that could be shared between different threads.
There are a lot of subtle and hard issues to deal with with regard to concurrent programming. If you want to learn about thread safety and concurrent programming in Java, then I highly recommend the book Java Concurrency in Practice by Brian Goetz.

Multiple threads could call this method and each one will get an unique instance of 'Entity'. So this method 'per se' is thread safe. But if there is code in the constructor or in one of the super constructors that is not thread safe you might have a safety problem anyhow.