Why Enum Singleton are better in Java? Examples

Enum Singletons are new way to implement Singleton pattern in Java by using Enum with just one instance. Though Singleton pattern in Java exists from long time Enum Singletons are relatively new concept and in practice from Java 5 onwards after introduction of Enum as keyword and feature. This article is somewhat related to my earlier post on Singleton, 10 interview questions on Singleton pattern in Java where we have discussed common questions asked on interviews about Singleton pattern and 10 Java enum examples, where we have seen how versatile enum can be. This post is about why should we use Enum as Singleton in Java, What benefit it offers compared to conventional singleton methods etc.

Java Enum and Singleton Pattern

Following are some reasons which make sense to me for using Enum to implement Singleton pattern in Java. By the way, If you like articles on design patterns then you can also check my post on Builder design patterns and Decorator design pattern.

1) Enum Singletons are easy to write

This is by far the biggest advantage if you have been writing Singletons prior ot Java 5 than you know that even with double-checked locking you can have more than one instance. though that issue is fixed with Java memory model improvement and guarantee provided by volatile variables from Java 5 onwards it still tricky to write for many beginners. 

Compared to double-checked locking with synchronization Enum singletons are cake walk. If you don't believe then just compare the below code for conventional singleton with double-checked locking and Enum Singletons:

2. Singleton using Enum in Java

This is the way we generally declare Enum Singleton, it may contain instace variable and instance method but for sake of simplicity I haven’t used any, just beware that if you are using an instance method then you need to ensure thread-safety of that method if at all it affects the state of an object. 

By default creation of Enum instance is thread safe but any other method on Enum is programmers responsibility.

/**
* Singleton pattern example using Java Enumj
*/

public enum EasySingleton{
    INSTANCE;
}

You can acess it by EasySingleton.INSTANCE, much easier than calling getInstance() method on Singleton.

3. Singleton example with double-checked locking

Below code is an example of double-checked locking in Singleton pattern, here getInstance() method checks two times to see whether INSTANCE is null or not and that’s why it’s called double-checked locking pattern, remember that double-checked locking is broker before Java 5 but with the guaranteed of volatile variable in Java 5 memory model, it should work perfectly.

/**
* Singleton pattern example with Double-checked Locking
*/

public class DoubleCheckedLockingSingleton{
     private volatile DoubleCheckedLockingSingleton INSTANCE;
 
     private DoubleCheckedLockingSingleton(){}
 
     public DoubleCheckedLockingSingleton getInstance(){
         if(INSTANCE == null){
            synchronized(DoubleCheckedLockingSingleton.class){
                //double checking Singleton instance
                if(INSTANCE == null){
                    INSTANCE = new DoubleCheckedLockingSingleton();
                }
            }
         }
         return INSTANCE;
     }
}

You can call DoubleCheckedLockingSingleton.getInstance() to get access of this Singleton class.

Now Just look at amount of code needed to create a lazy loaded thread-safe Singleton. With Enum Singleton pattern you can have that in one line because creation of Enum instance is thread-safe and guranteed by JVM.

People may argue that there are better way to write Singleton instead of Double-checked locking approach but every approach has there own advantages and disadvantages like I mostly prefer static field Singleton initialized during classloading as shown in below example, but keep in mind that is not a lazy-loaded Singleton:

Singleton pattern with a static factory method

This is one of my favorite method to impelemnt Singleton pattern in Java, Since Singleton instance is static and final variable it initialized when class is first loaded into memeory so creation of instance is inherently thread-safe.

/**
* Singleton pattern example with static factory method
*/

public class Singleton{
    //initailzed during class loading
    private static final Singleton INSTANCE = new Singleton();
 
    //to prevent creating another instance of Singleton
    private Singleton(){}

    public static Singleton getSingleton(){
        return INSTANCE;
    }
}

You can call Singleton.getSingleton() to get access of this class.

2) Enum Singletons handled Serialization by themselves

Another problem with conventional Singletons are that once you implement serializable interface they are no longer remain Singleton because readObject() method always return a new instance just like a constructor in Java. you can avoid that by using readResolve() method and discarding newly created instance by replacing with Singleton as shwon in below example :

    //readResolve to prevent another instance of Singleton
    private Object readResolve(){
        return INSTANCE;
    }

This can become even more complex if your Singleton Class maintains state, as you need to make them transient, but with Enum Singleton, Serialization is guaranteed by JVM.

3) Creation of Enum instance is thread-safe

As stated in point 1 since creation of Enum instance is thread-safe by default you don't need to worry about double-checked locking.

In summary, given the Serialization and thread-safety guaranteed and with a couple of lines of code enum, the Singleton pattern is the best way to create Singleton in Java 5 world. you can still use other popular methods if you feel so but I still have to find a convincing reason not to use Enum as Singleton, let me know if you got any.

Other Java articles from Javarevisited blog:

Why multiple inheritance is not supported in Java

Why operator overloading is not supported in Java

How Generics works in Java

10 tips to write better comments while coding

10 Object oriented design principles every Java programmer should know

10 JVM options you should know.