Java Singleton Design Pattern : Interview Question

Question

Hi,

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

1. How many different ways we can implement Singleton Design Pattern in Java ?
2. 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 ?
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 ?

Any inputs would be highly appreciated about Singleton and what are the main things to keep in mind while dealing with Singletons ?

Thanks.

Answer 1

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.

Answer 2

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.

Answer 3

1. 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.

2. 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.)

3. --

4. 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.

Answer 4

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".

Answer 5

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".

Answer 6

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.

Answer 7

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 .

Answer 8

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.

Answer 9

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;
}