1.7. 单例模式

注:单例模式可能被认为是一种“反模式”。为了获得更好的可测试性和可维护性,建议使用依赖注入。

1.7.1. 目标

让应用只存在一个对象的实例,处理所有的调用。

1.7.2. 例子

  • 数据库连接器

  • 日志记录器

  • Config Manager

  • Threads Handling

  • 应用程序的锁定文件(Lock file,理论上整个应用应该只有一个锁文件)

1.7.3. UML 图

Alt Singleton UML Diagram

1.7.4. 代码

你可以在 GitHub 上找到这些代码

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. 测试

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}