| Purpose |
Ensure that a class has only 1 instance, which is accessible from a
well-known access point.
Use Singleton for objects that are unique in the system (e.g., an operating
system has only 1 instance of a FileSystem).
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| Structure |
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| Consequences |
- The Singleton class can be subclassed. For example, in Jicos,
each JVM in the system is running exactly 1 of either a hosting service provider
(HSP), task server, or host. Each of these extend CellImpl, which therefore
should be a Singleton.
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| Implementation |
- Enforce the singleton property with a private constructor.
- If the singleton class is to be Serializable, override readResolve
to ensure that no other instances are constructed unintentionally. See sample
code below.
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| Sample Code |
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| Related Patterns |
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