Design Patterns : Singleton
Motivation
It is important for some classes to have exactly one instance. As an example although there can be many printers in a system, there should be only one printer spooler. How do we ensure that a class has only one instance and that the instance is easily accessible within the network?
Intent
Applicability
Use Singleton pattern when
- There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point.
- When the sole instance should be extensible by sub-classing, and clients should be able to use an extended instance without modifying their code.
Why is it a Responsibility Pattern?
Objects can usually act responsibly just by performing their own work. but some some objects take on further responsibilities such as;
- Representing real-world entities
- Coordinating work
- Modeling the state of a system
When they have that extra responsibility to carry they become responsible for self and other clients too.
How to instantiate a single instance and global access?
The mechanics of Singleton are more memorable than its intent.It is easier to explain how to ensure that a class has only one instance than it is to say why you might want this restriction.You might categorize Singleton as a “creational” pattern.
One way create this instance as a static field in the class.
- example: The Runtime class in java.lang includes the line:
–private static Runtime currentRuntime = new Runtime();
Lazy Initialization
The system might wait until the instance is first needed.
Example : A Factory class might make its single instance available with
public static Factory getFactory() {
if (factory == null) {
factory = new Factory();
// …
}
return factory;
}
Garbage Collection
Once you create a singleton instance it might get garbage-collected. If there is you might want to prevent this, specially if the objects has a significant startup cost or if it is gathering statistics.
Tip: This can be handled by having a reference to the object on the main process.
