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
A real-world example of the Multiton pattern is a printer management system in a large office. In this scenario, the office has several printers, each serving a different department. Instead of creating a new printer object every time a printing request is made, the system uses the Multiton pattern to ensure that each department has exactly one printer instance. When a printing request comes from a specific department, the system checks the registry of printer instances and retrieves the existing printer for that department. If no printer exists for that department, it creates one, registers it, and then returns it. This ensures efficient management of printer resources and prevents unnecessary creation of multiple printer instances for the same department.
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
The Nazgûl, also called ringwraiths or the Nine Riders, are Sauron's most terrible servants. By definition, there's always nine of them.
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.
public static void main(String[] args) {
// eagerly initialized multiton
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));
// enum multiton
LOGGER.info("Printing out enum-based multiton contents");
LOGGER.info("KHAMUL={}", NazgulEnum.KHAMUL);
LOGGER.info("MURAZOR={}", NazgulEnum.MURAZOR);
LOGGER.info("DWAR={}", NazgulEnum.DWAR);
LOGGER.info("JI_INDUR={}", NazgulEnum.JI_INDUR);
LOGGER.info("AKHORAHIL={}", NazgulEnum.AKHORAHIL);
LOGGER.info("HOARMURATH={}", NazgulEnum.HOARMURATH);
LOGGER.info("ADUNAPHEL={}", NazgulEnum.ADUNAPHEL);
LOGGER.info("REN={}", NazgulEnum.REN);
LOGGER.info("UVATHA={}", NazgulEnum.UVATHA);
}
Program output:
15:16:10.597 [main] INFO com.iluwatar.multiton.App -- Printing out eagerly initialized multiton contents
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- KHAMUL=com.iluwatar.multiton.Nazgul@4141d797
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- MURAZOR=com.iluwatar.multiton.Nazgul@38cccef
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- DWAR=com.iluwatar.multiton.Nazgul@5679c6c6
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- JI_INDUR=com.iluwatar.multiton.Nazgul@27ddd392
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- AKHORAHIL=com.iluwatar.multiton.Nazgul@19e1023e
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- HOARMURATH=com.iluwatar.multiton.Nazgul@7cef4e59
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- ADUNAPHEL=com.iluwatar.multiton.Nazgul@64b8f8f4
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- REN=com.iluwatar.multiton.Nazgul@2db0f6b2
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- UVATHA=com.iluwatar.multiton.Nazgul@3cd1f1c8
15:16:10.600 [main] INFO com.iluwatar.multiton.App -- Printing out enum-based multiton contents
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- KHAMUL=KHAMUL
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- MURAZOR=MURAZOR
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- DWAR=DWAR
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- JI_INDUR=JI_INDUR
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- AKHORAHIL=AKHORAHIL
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- HOARMURATH=HOARMURATH
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- ADUNAPHEL=ADUNAPHEL
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- REN=REN
15:16:10.601 [main] INFO com.iluwatar.multiton.App -- UVATHA=UVATHA
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.