single-element enum type singletone with lazy loading capability

Question

I read many forums and posts about different style to implement single-tone pattern in java and seems \"Enum are the best way to implement singletone pattern in java\"!! I w

Answer 1

The first time a class is used, it gets loaded by the JVM and all of its static initialization is done. the enum members are static , so they're all going to be initialized.

Actually, classloader loads classes (sounds funny) only after you are accessing this classes first time. And only one reason to access enum-singleton class is to get it's instance.

That is why single-element enum type singletone in Java are called lazy - it's value is not initialized before you first time access it.

Similar questions:

• Java: Lazy Initializing Singleton
• Lazy-loaded singleton: Double-checked locking vs Initialization on demand holder idiom

Answer 2

The reason the source you read said it's the easiest way to do lazy singletons is because it should just work. Try this:

public class LazyEnumTest {
  public static void main(String[] args) throws InterruptedException {
    System.out.println("Sleeping for 5 seconds...");
    Thread.sleep(5000);
    System.out.println("Accessing enum...");
    LazySingleton lazy = LazySingleton.INSTANCE;
    System.out.println("Done.");
  }
}

enum LazySingleton {
  INSTANCE;
  static { System.out.println("Static Initializer"); }
}

Here's the output I get in the console:

$ java LazyEnumTest
Sleeping for 5 seconds...
Accessing enum...
Static Initializer
Done.