layout: pattern

title: Registry

folder: registry

permalink: /patterns/registry/

categories: Creational

tags:

- Instantiation

Stores the objects of a single class and provide a global point of access to them.

Similar to Multiton pattern, only difference is that in a registry there is no restriction on the number of objects.

In Plain Words

**Programmatic Example**

Below is a `Customer` Class

public class Customer {

private final String id;

private final String name;

public Customer(String id, String name) {

this.id = id;

this.name = name;

}

public String getId() {

return id;

}

public String getName() {

return name;

}

}

This registry of the `Customer` objects is `CustomerRegistry`

public final class CustomerRegistry {

private static final CustomerRegistry instance = new CustomerRegistry();

public static CustomerRegistry getInstance() {

return instance;

}

private final Map<String, Customer> customerMap;

private CustomerRegistry() {

customerMap = new ConcurrentHashMap<>();

}

public Customer addCustomer(Customer customer) {

return customerMap.put(customer.getId(), customer);

}

public Customer getCustomer(String id) {

return customerMap.get(id);

}

}


Use Registry pattern when

* client wants reference of some object, so client can lookup for that object in the object's registry.

Large number of bulky objects added to registry would result in a lot of memory consumption as objects in the registry are not garbage collected.

* https://www.martinfowler.com/eaaCatalog/registry.html

* https://wiki.c2.com/?RegistryPattern

package com.iluwatar.registry {

class App {

- LOGGER : Logger {static}

+ App()

+ main(args : String[]) {static}

}

class Customer {

- id : String

- name : String

+ getId() : String

+ getName() : String

+ toString() : String

}

class CustomerRegistry {

+ addCustomer(customer : Customer)

+ getCustomer(id : String)

}

}

Customer --> "-addCustomer" CustomerRegistry

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

<modelVersion>4.0.0</modelVersion>

<parent>

<groupId>com.iluwatar</groupId>

<artifactId>java-design-patterns</artifactId>

<version>1.24.0-SNAPSHOT</version>

</parent>

<artifactId>registry</artifactId>

<dependencies>

<dependency>

<groupId>org.junit.jupiter</groupId>

<artifactId>junit-jupiter-engine</artifactId>

<scope>test</scope>

</dependency>

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

</dependency>

</dependencies>

<build>

<plugins>

<plugin>

<groupId>org.apache.maven.plugins</groupId>

<artifactId>maven-assembly-plugin</artifactId>

<executions>

<execution>

<configuration>

<archive>

<manifest>

<mainClass>com.iluwatar.registry.App</mainClass>

</manifest>

</archive>

</configuration>

</execution>

</executions>

</plugin>

</plugins>

</build>

</project>

package com.iluwatar.registry;

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

public class App {

private static final Logger LOGGER = LoggerFactory.getLogger(App.class);

/**

public static void main(String[] args) {

CustomerRegistry customerRegistry = CustomerRegistry.getInstance();

var john = new Customer("1", "John");

customerRegistry.addCustomer(john);

var julia = new Customer("2", "Julia");

customerRegistry.addCustomer(julia);

LOGGER.info("John {}", customerRegistry.getCustomer("1"));

LOGGER.info("Julia {}", customerRegistry.getCustomer("2"));

}

}

package com.iluwatar.registry;

public class Customer {

private final String id;

private final String name;

public Customer(String id, String name) {

this.id = id;

this.name = name;

}

public String getId() {

return id;

}

public String getName() {

return name;

}

@Override

public String toString() {

return "Customer{"

+ "id='" + id + '\''

+ ", name='" + name + '\''

+ '}';

}

}

package com.iluwatar.registry;

import java.util.Map;

import java.util.concurrent.ConcurrentHashMap;

public final class CustomerRegistry {

private static final CustomerRegistry instance = new CustomerRegistry();

public static CustomerRegistry getInstance() {

return instance;

}

private final Map<String, Customer> customerMap;

private CustomerRegistry() {

customerMap = new ConcurrentHashMap<>();

}

public Customer addCustomer(Customer customer) {

return customerMap.put(customer.getId(), customer);

}

public Customer getCustomer(String id) {

return customerMap.get(id);

}

}

package com.iluwatar.registry;

import org.junit.jupiter.api.BeforeAll;

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

import static org.junit.jupiter.api.Assertions.assertNotNull;

import static org.junit.jupiter.api.Assertions.assertNull;

public class CustomerRegistryTest {

private static CustomerRegistry customerRegistry;

@BeforeAll

public static void setUp() {

customerRegistry = CustomerRegistry.getInstance();

}

@Test

public void shouldBeAbleToAddAndQueryCustomerObjectFromRegistry() {

Customer john = new Customer("1", "john");

Customer julia = new Customer("2", "julia");

customerRegistry.addCustomer(john);

customerRegistry.addCustomer(julia);

Customer customerWithId1 = customerRegistry.getCustomer("1");

assertNotNull(customerWithId1);

assertEquals("1", customerWithId1.getId());

assertEquals("john", customerWithId1.getName());

Customer customerWithId2 = customerRegistry.getCustomer("2");

assertNotNull(customerWithId2);

assertEquals("2", customerWithId2.getId());

assertEquals("julia", customerWithId2.getName());

}

@Test

public void shouldReturnNullWhenQueriedCustomerIsNotInRegistry() {

Customer customerWithId5 = customerRegistry.getCustomer("5");

assertNull(customerWithId5);

}

}