1.7. Singleton
THIS IS CONSIDERED TO BE AN ANTI-PATTERN! FOR BETTER TESTABILITY AND MAINTAINABILITY USE DEPENDENCY INJECTION!
1.7.1. Purpose
To have only one instance of this object in the application that will handle all calls.
1.7.2. Examples
DB Connector
Logger
Config Manager
Threads Handling
Lock file for the application (there is only one in the filesystem …)
1.7.3. UML Diagram
1.7.4. Code
You can also find this code on GitHub
Singleton.php
1<?php
2
3declare(strict_types=1);
4
5namespace DesignPatterns\Creational\Singleton;
6
7use Exception;
8
9final class Singleton
10{
11 private static ?Singleton $instance = null;
12
13 /**
14 * gets the instance via lazy initialization (created on first usage)
15 */
16 public static function getInstance(): Singleton
17 {
18 if (self::$instance === null) {
19 self::$instance = new self();
20 }
21
22 return self::$instance;
23 }
24
25 /**
26 * is not allowed to call from outside to prevent from creating multiple instances,
27 * to use the singleton, you have to obtain the instance from Singleton::getInstance() instead
28 */
29 private function __construct()
30 {
31 }
32
33 /**
34 * prevent the instance from being cloned (which would create a second instance of it)
35 */
36 private function __clone()
37 {
38 }
39
40 /**
41 * prevent from being unserialized (which would create a second instance of it)
42 */
43 public function __wakeup()
44 {
45 throw new Exception("Cannot unserialize singleton");
46 }
47}
1.7.5. Test
Tests/SingletonTest.php
1<?php
2
3declare(strict_types=1);
4
5namespace DesignPatterns\Creational\Singleton\Tests;
6
7use DesignPatterns\Creational\Singleton\Singleton;
8use PHPUnit\Framework\TestCase;
9
10class SingletonTest extends TestCase
11{
12 public function testUniqueness()
13 {
14 $firstCall = Singleton::getInstance();
15 $secondCall = Singleton::getInstance();
16
17 $this->assertInstanceOf(Singleton::class, $firstCall);
18 $this->assertSame($firstCall, $secondCall);
19 }
20}