Implementing Singleton with an Enum in Java

Question

I have read that it is possible to implement Singleton in Java using an Enum such as   public enum MySingleton        INSTANCE         But  how does the above work  Specifically  an Object has to be instantiated  Here  how is MySingleton being instantiated  Who is doing new MySingleton

User · Answer

Like all enum instances  Java instantiates each object when the class is loaded  with some guarantee that it s instantiated exactly once per JVM  Think of the INSTANCE declaration as a public static final field  Java will instantiate the object the first time the class is referred to   The instances are created during static initialization  which is defined in the Java Language Specification  section 12 4   For what it s worth  Joshua Bloch describes this pattern in detail as item 3 of Effective Java Second Edition

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An enum type is a special type of class   Your enum will actually be compiled to something like  public final class MySingleton       public final static MySingleton INSTANCE   new MySingleton        private MySingleton          When your code first accesses INSTANCE  the class MySingleton will be loaded and initialized by the JVM  This process initializes the static field above once  lazily

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This   public enum MySingleton     INSTANCE         has an implicit empty constructor  Make it explicit instead   public enum MySingleton       INSTANCE      private MySingleton             System out println  Here              If you then added another class with a main   method like  public static void main String   args        System out println MySingleton INSTANCE       You would see  Here INSTANCE   enum fields are compile time constants  but they are instances of their enum type  And  they re constructed when the enum type is referenced for the first time

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In this Java best practices book by Joshua Bloch  you can find explained why you should enforce the Singleton property with a private constructor or an Enum type  The chapter is quite long  so keeping it summarized   Making a class a Singleton can make it difficult to test its clients  as it   s impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type  Recommended approach is implement Singletons by simply make an enum type with one element      Enum singleton - the preferred approach public enum Elvis   INSTANCE  public void leaveTheBuilding               This approach is functionally equivalent to the public field approach  except that it is more concise  provides the serialization machinery for free  and provides an ironclad guarantee against multiple instantiation  even in the face of sophisticated serialization or reflection attacks       While this approach has yet to be widely   adopted  a single-element enum type is the best way to implement a singleton

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Since Singleton Pattern is about having a private constructor and calling some method to control the instantiations  like some getInstance   in Enums we already have an implicit private constructor   I don t exactly know how the JVM or some container controls the instances of our Enums  but it seems it already use an implicit Singleton Pattern  the difference is we don t call a getInstance  we just call the Enum

User · Answer

As has  to some extent  been mentioned before  an enum is a java class with the special condition that its definition must start with at least one  enum constant    Apart from that  and that enums cant can t be extended or used to extend other classes  an enum is a class like any class and you use it by adding methods below the constant definitions   public enum MySingleton       INSTANCE       public void doSomething                public synchronized String getSomething      return something         private String something      You access the singleton s methods along these lines   MySingleton INSTANCE doSomething    String something   MySingleton INSTANCE getSomething      The use of an enum  instead of a class  is  as has been mentioned in other answers  mostly about a thread-safe instantiation of the singleton and a guarantee that it will always only be one copy   And  perhaps  most importantly  that this behavior is guaranteed by the JVM itself and the Java specification   Here s a section from the Java specification on how multiple instances of an enum instance is prevented      An enum type has no instances other than those defined by its enum constants  It is a compile-time error to attempt to explicitly instantiate an enum type  The final clone method in Enum ensures that enum constants can never be cloned  and the special treatment by the serialization mechanism ensures that duplicate instances are never created as a result of deserialization  Reflective instantiation of enum types is prohibited  Together  these four things ensure that no instances of an enum type exist beyond those defined by the enum constants    Worth noting is that after the instantiation any thread-safety concerns must be handled like in any other class with the synchronized keyword etc

[java] Implementing Singleton with an Enum (in Java)

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