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Ilkka Seppälä
4108f86177
* Changed database implementation. Removed static objects. * Fix Logs * Fix 40 errors from checkstyle plugin run. 139 left)) * Fix CacheStore errors from checkstyle plugin 107 left * Fix last errors in checkstyle. * Fix sonar issues * Fix issues in VALIDATE phase * Fix Bug with mongo connection. Used "Try with resources" * Add test * Added docker-compose for mongo db. MongoDb db work fixed. * Provided missing tests * Comments to start Application with mongo. * Fix some broken links * Remove extra space * Update filename * Fix some links in localization folders * Fix link * Update frontmatters * Work on patterns index page * Work on index page * Fixes according PR comments. Mainly Readme edits. * fix frontmatter * add missing png * Update pattern index.md * Add index.md for Chinese translation * update image paths * update circuit breaker image paths * Update image paths for localizations * add generated puml * Add missing image * Update img file extensions * Update the rest of the EN and ZH patterns to conform with the new website Co-authored-by: Victor Zalevskii <zvictormail@gmail.com>
141 lines
5.1 KiB
Markdown
141 lines
5.1 KiB
Markdown
---
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title: Multiton
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category: Creational
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language: en
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tags:
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- Instantiation
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---
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## Also known as
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Registry
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## Intent
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Ensure a class only has a limited number of instances and provide a global point of access to them.
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## Explanation
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Real-world example
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> The Nazgûl, also called ringwraiths or the Nine Riders, are Sauron's most terrible servants. By
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> definition, there's always nine of them.
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In plain words
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> Multiton pattern ensures there are a predefined amount of instances available globally.
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Wikipedia says
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> In software engineering, the multiton pattern is a design pattern which generalizes the singleton
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> pattern. Whereas the singleton allows only one instance of a class to be created, the multiton
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> pattern allows for the controlled creation of multiple instances, which it manages through the use
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> of a map.
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**Programmatic Example**
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`Nazgul` is the multiton class.
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```java
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public enum NazgulName {
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KHAMUL, MURAZOR, DWAR, JI_INDUR, AKHORAHIL, HOARMURATH, ADUNAPHEL, REN, UVATHA
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}
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public final class Nazgul {
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private static final Map<NazgulName, Nazgul> nazguls;
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private final NazgulName name;
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static {
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nazguls = new ConcurrentHashMap<>();
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nazguls.put(NazgulName.KHAMUL, new Nazgul(NazgulName.KHAMUL));
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nazguls.put(NazgulName.MURAZOR, new Nazgul(NazgulName.MURAZOR));
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nazguls.put(NazgulName.DWAR, new Nazgul(NazgulName.DWAR));
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nazguls.put(NazgulName.JI_INDUR, new Nazgul(NazgulName.JI_INDUR));
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nazguls.put(NazgulName.AKHORAHIL, new Nazgul(NazgulName.AKHORAHIL));
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nazguls.put(NazgulName.HOARMURATH, new Nazgul(NazgulName.HOARMURATH));
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nazguls.put(NazgulName.ADUNAPHEL, new Nazgul(NazgulName.ADUNAPHEL));
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nazguls.put(NazgulName.REN, new Nazgul(NazgulName.REN));
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nazguls.put(NazgulName.UVATHA, new Nazgul(NazgulName.UVATHA));
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}
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private Nazgul(NazgulName name) {
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this.name = name;
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}
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public static Nazgul getInstance(NazgulName name) {
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return nazguls.get(name);
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}
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public NazgulName getName() {
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return name;
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}
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}
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```
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And here's how we access the `Nazgul` instances.
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```java
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// eagerly initialized multiton
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LOGGER.info("Printing out eagerly initialized multiton contents");
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LOGGER.info("KHAMUL={}", Nazgul.getInstance(NazgulName.KHAMUL));
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LOGGER.info("MURAZOR={}", Nazgul.getInstance(NazgulName.MURAZOR));
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LOGGER.info("DWAR={}", Nazgul.getInstance(NazgulName.DWAR));
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LOGGER.info("JI_INDUR={}", Nazgul.getInstance(NazgulName.JI_INDUR));
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LOGGER.info("AKHORAHIL={}", Nazgul.getInstance(NazgulName.AKHORAHIL));
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LOGGER.info("HOARMURATH={}", Nazgul.getInstance(NazgulName.HOARMURATH));
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LOGGER.info("ADUNAPHEL={}", Nazgul.getInstance(NazgulName.ADUNAPHEL));
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LOGGER.info("REN={}", Nazgul.getInstance(NazgulName.REN));
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LOGGER.info("UVATHA={}", Nazgul.getInstance(NazgulName.UVATHA));
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// enum multiton
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LOGGER.info("Printing out enum-based multiton contents");
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LOGGER.info("KHAMUL={}", NazgulEnum.KHAMUL);
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LOGGER.info("MURAZOR={}", NazgulEnum.MURAZOR);
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LOGGER.info("DWAR={}", NazgulEnum.DWAR);
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LOGGER.info("JI_INDUR={}", NazgulEnum.JI_INDUR);
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LOGGER.info("AKHORAHIL={}", NazgulEnum.AKHORAHIL);
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LOGGER.info("HOARMURATH={}", NazgulEnum.HOARMURATH);
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LOGGER.info("ADUNAPHEL={}", NazgulEnum.ADUNAPHEL);
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LOGGER.info("REN={}", NazgulEnum.REN);
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LOGGER.info("UVATHA={}", NazgulEnum.UVATHA);
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```
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Program output:
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```
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20:35:07.413 [main] INFO com.iluwatar.multiton.App - Printing out eagerly initialized multiton contents
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20:35:07.417 [main] INFO com.iluwatar.multiton.App - KHAMUL=com.iluwatar.multiton.Nazgul@48cf768c
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20:35:07.419 [main] INFO com.iluwatar.multiton.App - MURAZOR=com.iluwatar.multiton.Nazgul@7960847b
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20:35:07.419 [main] INFO com.iluwatar.multiton.App - DWAR=com.iluwatar.multiton.Nazgul@6a6824be
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20:35:07.419 [main] INFO com.iluwatar.multiton.App - JI_INDUR=com.iluwatar.multiton.Nazgul@5c8da962
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20:35:07.419 [main] INFO com.iluwatar.multiton.App - AKHORAHIL=com.iluwatar.multiton.Nazgul@512ddf17
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20:35:07.419 [main] INFO com.iluwatar.multiton.App - HOARMURATH=com.iluwatar.multiton.Nazgul@2c13da15
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20:35:07.419 [main] INFO com.iluwatar.multiton.App - ADUNAPHEL=com.iluwatar.multiton.Nazgul@77556fd
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20:35:07.419 [main] INFO com.iluwatar.multiton.App - REN=com.iluwatar.multiton.Nazgul@368239c8
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20:35:07.420 [main] INFO com.iluwatar.multiton.App - UVATHA=com.iluwatar.multiton.Nazgul@9e89d68
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20:35:07.420 [main] INFO com.iluwatar.multiton.App - Printing out enum-based multiton contents
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20:35:07.420 [main] INFO com.iluwatar.multiton.App - KHAMUL=KHAMUL
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20:35:07.420 [main] INFO com.iluwatar.multiton.App - MURAZOR=MURAZOR
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20:35:07.420 [main] INFO com.iluwatar.multiton.App - DWAR=DWAR
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20:35:07.420 [main] INFO com.iluwatar.multiton.App - JI_INDUR=JI_INDUR
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20:35:07.421 [main] INFO com.iluwatar.multiton.App - AKHORAHIL=AKHORAHIL
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20:35:07.421 [main] INFO com.iluwatar.multiton.App - HOARMURATH=HOARMURATH
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20:35:07.421 [main] INFO com.iluwatar.multiton.App - ADUNAPHEL=ADUNAPHEL
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20:35:07.421 [main] INFO com.iluwatar.multiton.App - REN=REN
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20:35:07.421 [main] INFO com.iluwatar.multiton.App - UVATHA=UVATHA
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```
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## Class diagram
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## Applicability
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Use the Multiton pattern when
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* There must be a specific number of instances of a class, and they must be accessible to clients from
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a well-known access point.
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