Multiton
Also known as
- Registry of Singletons
Intent
The Multiton pattern is a variation of the Singleton design pattern that manages a map of named instances as key-value pairs.
Explanation
Real-world example
The Nazgûl, also called ringwraiths or the Nine Riders, are Sauron's most terrible servants. By definition, there's always nine of them.
In plain words
Multiton pattern ensures there are a predefined amount of instances available globally.
Wikipedia says
In software engineering, the multiton pattern is a design pattern which generalizes the singleton pattern. Whereas the singleton allows only one instance of a class to be created, the multiton pattern allows for the controlled creation of multiple instances, which it manages through the use of a map.
Programmatic Example
Nazgul is the multiton class.
public enum NazgulName {
KHAMUL, MURAZOR, DWAR, JI_INDUR, AKHORAHIL, HOARMURATH, ADUNAPHEL, REN, UVATHA
}
public final class Nazgul {
private static final Map<NazgulName, Nazgul> nazguls;
@Getter
private final NazgulName name;
static {
nazguls = new ConcurrentHashMap<>();
nazguls.put(NazgulName.KHAMUL, new Nazgul(NazgulName.KHAMUL));
nazguls.put(NazgulName.MURAZOR, new Nazgul(NazgulName.MURAZOR));
nazguls.put(NazgulName.DWAR, new Nazgul(NazgulName.DWAR));
nazguls.put(NazgulName.JI_INDUR, new Nazgul(NazgulName.JI_INDUR));
nazguls.put(NazgulName.AKHORAHIL, new Nazgul(NazgulName.AKHORAHIL));
nazguls.put(NazgulName.HOARMURATH, new Nazgul(NazgulName.HOARMURATH));
nazguls.put(NazgulName.ADUNAPHEL, new Nazgul(NazgulName.ADUNAPHEL));
nazguls.put(NazgulName.REN, new Nazgul(NazgulName.REN));
nazguls.put(NazgulName.UVATHA, new Nazgul(NazgulName.UVATHA));
}
private Nazgul(NazgulName name) {
this.name = name;
}
public static Nazgul getInstance(NazgulName name) {
return nazguls.get(name);
}
}
And here's how we access the Nazgul instances.
LOGGER.info("Printing out eagerly initialized multiton contents");
LOGGER.info("KHAMUL={}", Nazgul.getInstance(NazgulName.KHAMUL));
LOGGER.info("MURAZOR={}", Nazgul.getInstance(NazgulName.MURAZOR));
LOGGER.info("DWAR={}", Nazgul.getInstance(NazgulName.DWAR));
LOGGER.info("JI_INDUR={}", Nazgul.getInstance(NazgulName.JI_INDUR));
LOGGER.info("AKHORAHIL={}", Nazgul.getInstance(NazgulName.AKHORAHIL));
LOGGER.info("HOARMURATH={}", Nazgul.getInstance(NazgulName.HOARMURATH));
LOGGER.info("ADUNAPHEL={}", Nazgul.getInstance(NazgulName.ADUNAPHEL));
LOGGER.info("REN={}", Nazgul.getInstance(NazgulName.REN));
LOGGER.info("UVATHA={}", Nazgul.getInstance(NazgulName.UVATHA));
Program output:
20:35:07.413 [main] INFO com.iluwatar.multiton.App - Printing out eagerly initialized multiton contents
20:35:07.417 [main] INFO com.iluwatar.multiton.App - KHAMUL=com.iluwatar.multiton.Nazgul@48cf768c
20:35:07.419 [main] INFO com.iluwatar.multiton.App - MURAZOR=com.iluwatar.multiton.Nazgul@7960847b
20:35:07.419 [main] INFO com.iluwatar.multiton.App - DWAR=com.iluwatar.multiton.Nazgul@6a6824be
20:35:07.419 [main] INFO com.iluwatar.multiton.App - JI_INDUR=com.iluwatar.multiton.Nazgul@5c8da962
20:35:07.419 [main] INFO com.iluwatar.multiton.App - AKHORAHIL=com.iluwatar.multiton.Nazgul@512ddf17
20:35:07.419 [main] INFO com.iluwatar.multiton.App - HOARMURATH=com.iluwatar.multiton.Nazgul@2c13da15
20:35:07.419 [main] INFO com.iluwatar.multiton.App - ADUNAPHEL=com.iluwatar.multiton.Nazgul@77556fd
20:35:07.419 [main] INFO com.iluwatar.multiton.App - REN=com.iluwatar.multiton.Nazgul@368239c8
20:35:07.420 [main] INFO com.iluwatar.multiton.App - UVATHA=com.iluwatar.multiton.Nazgul@9e89d68
Class diagram
Applicability
Use the Multiton pattern when
- A class must have named instances, but only one instance for each unique key.
- Global access to these instances is necessary without requiring global variables.
- You want to manage shared resources categorized by key.
Known Uses
- Managing database connections in different contexts.
- Configuration settings for different environments in an application.
Consequences
Benefits:
- Ensures controlled access to instances based on key.
- Reduces global state usage by encapsulating instance management within the pattern.
Trade-offs:
- Increased memory usage if not managed properly due to multiple instances.
- Potential issues with concurrency if not implemented with thread safety in mind.
Related Patterns
Singleton: Multiton can be seen as an extension of the Singleton pattern where Singleton allows only one instance of a class, Multiton allows one instance per key.
Factory Method: Multiton uses a method to create or retrieve instances, similar to how a Factory Method controls object creation.