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Ilkka Seppälä 2020-08-29 21:55:51 +03:00
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@ -9,16 +9,19 @@ tags:
--- ---
## Also known as ## Also known as
Registry Registry
## Intent ## Intent
Ensure a class only has limited number of instances and provide a global point of access to them. Ensure a class only has limited number of instances and provide a global point of access to them.
## Explanation ## Explanation
Real world example 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. > 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 In plain words
@ -26,11 +29,14 @@ In plain words
Wikipedia says 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. > 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** **Programmatic Example**
Nazgul is the multiton class. `Nazgul` is the multiton class.
```java ```java
public enum NazgulName { public enum NazgulName {
@ -71,7 +77,7 @@ public final class Nazgul {
} }
``` ```
And here's how we access the Nazgul instances. And here's how we access the `Nazgul` instances.
```java ```java
LOGGER.info("KHAMUL={}", Nazgul.getInstance(NazgulName.KHAMUL)); LOGGER.info("KHAMUL={}", Nazgul.getInstance(NazgulName.KHAMUL));
@ -83,22 +89,29 @@ And here's how we access the Nazgul instances.
LOGGER.info("ADUNAPHEL={}", Nazgul.getInstance(NazgulName.ADUNAPHEL)); LOGGER.info("ADUNAPHEL={}", Nazgul.getInstance(NazgulName.ADUNAPHEL));
LOGGER.info("REN={}", Nazgul.getInstance(NazgulName.REN)); LOGGER.info("REN={}", Nazgul.getInstance(NazgulName.REN));
LOGGER.info("UVATHA={}", Nazgul.getInstance(NazgulName.UVATHA)); LOGGER.info("UVATHA={}", Nazgul.getInstance(NazgulName.UVATHA));
```
// KHAMUL=com.iluwatar.multiton.Nazgul@2b214b94
// MURAZOR=com.iluwatar.multiton.Nazgul@17814b1c Program output:
// DWAR=com.iluwatar.multiton.Nazgul@7ac9af2a
// JI_INDUR=com.iluwatar.multiton.Nazgul@7bb004b8 ```
// AKHORAHIL=com.iluwatar.multiton.Nazgul@78e89bfe KHAMUL=com.iluwatar.multiton.Nazgul@2b214b94
// HOARMURATH=com.iluwatar.multiton.Nazgul@652ce654 MURAZOR=com.iluwatar.multiton.Nazgul@17814b1c
// ADUNAPHEL=com.iluwatar.multiton.Nazgul@522ba524 DWAR=com.iluwatar.multiton.Nazgul@7ac9af2a
// REN=com.iluwatar.multiton.Nazgul@29c5ee1d JI_INDUR=com.iluwatar.multiton.Nazgul@7bb004b8
// UVATHA=com.iluwatar.multiton.Nazgul@15cea7b0 AKHORAHIL=com.iluwatar.multiton.Nazgul@78e89bfe
HOARMURATH=com.iluwatar.multiton.Nazgul@652ce654
ADUNAPHEL=com.iluwatar.multiton.Nazgul@522ba524
REN=com.iluwatar.multiton.Nazgul@29c5ee1d
UVATHA=com.iluwatar.multiton.Nazgul@15cea7b0
``` ```
## Class diagram ## Class diagram
![alt text](./etc/multiton.png "Multiton") ![alt text](./etc/multiton.png "Multiton")
## Applicability ## Applicability
Use the Multiton pattern when Use the Multiton pattern when
* there must be specific number of instances of a class, and they must be accessible to clients from a well-known access point * There must be specific number of instances of a class, and they must be accessible to clients from
a well-known access point.