Early and lazy loading

Singletons can be split into two categories, depending on when the instance object is created. If the singleton is created when the application is started, it is considered an early/eager instantiation. Otherwise, if the singleton constructor is invoked when the getInstance method is invoked for the first time, it is considered a lazy-loading singleton.

The lock-free thread-safe singleton presented in the previous example is considered an early-loading singleton in the first version of Java. However, in the latest version of Java, classes are loaded when they are needed, so that version is also a lazy-loading version. Furthermore, the moment that a class is loaded depends on the JVM implementation and may differ from one version to another. Making design decisions based on JVM implementation should be avoided.

Currently, there is no reliable option in Java for creating an early loading singleton. If we really need an early instantiation, we should enforce it at the start of the application, by simply invoking the getInstance() method, as shown in the following code:

Singleton.getInstance();