How to write a Singleton in proper manner?

Question

Today in my interview one interviewer asked me to write a Singleton class. And i gave my answer as

public class Singleton {

    private static Singleton re         


        

Answer 1

Latest Standard Solutions:

• Core java with Managed Beans / CDI

  @ApplicationScoped
  public class MySingleton { ... }
  

• EJB Lite (JEE6)

  @Singleton
  public class MySingleton { ... }
  

Prior Recommendation (from 'Effective Java 2'):

• Use an enum, with a single bogus enumeration constant e.g. INSTANCE. Data fields and methods can be either static or non-static (instance) - the two behave equivalently, since there's only one instance

• Advantages:

  • Yep, it's a singleton. Impossible to create more than one instance by any mechanism (within any given class loader on any JVM).
  • Singleton initialization is thread safe.

• Disadvantages (compared with above Standard Solutions):

  • The definition is a little obtuse - the 'enum' outer shell could misdirect inexperienced code maintainers. It's a small hack, not the original intention of enum.
  • The implementation leaks through the abstraction. It violates the Uniform Access Principle
    • It doesn't work via injection - the client must know it is a singleton & code against the precise enum instance
    • The client must use Singleton.INSTANCE.someMethod()
    • The client can't (simply) code against an interface
    • The client's impacted by design changes between singleton to/from multi-instance objects
  • Extends an ancestor class (Enum), preventing inheritance from other classes
  • Serialization & deserialization just transfer the enum constant name without state - good for technically ensuring there's only one instance, but a null operation with regards to data. In the (rare) event one wanted to share/synchronise singleton state across JVMs/class loaders, replication via serialization couldn't be used.