com.iparelan.util.annotations.patterns
Annotation Type Singleton

@Documented
@Target(value=TYPE)
@Retention(value=SOURCE)
@Copyright(value="Copyright © 2008, Iparelan Solutions, LLC. All rights reserved.")
public @interface Singleton

Marks a class as implementing the Singleton pattern.

There are several ways to implement the singleton pattern, but all of them strive to prohibit multiple instantiations of the singleton type by implementing an instantiation constraint.

Usually this constraint is implemented by creating only private constructors. However, this constraint cannot be enforced at compile-time in all situations. Some runtime constraint must be employed when a compile-time constraint cannot be specified. Examples are such situations are:

• The singleton is used as a plugin in some context that requires plugins to have a public no-arg constructor. This is often the case when using ServiceLoader
• The singlton is loaded lazily from within its own class and the lazy loading implementation desires to avoid generating a "hidden" accessor as described in the PMD rule "AccessorClassGeneration." Such accessors usually have class names with suffixes like access$000.

Version:

July 2008

Author:

Greg Mattes

See Also:

"Design Patterns," Erich Gamma, et. al., "Singleton"