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。

目标

让应用只存在一个对象的实例，处理所有的调用。

例子

• 数据库连接器
• 日志记录器
• 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）

UML 图

代码

Singleton.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Creational\Singleton;

use Exception;

final class Singleton
{
    private static ?Singleton $instance = null;

    /**
     * gets the instance via lazy initialization (created on first usage)
     */
    public static function getInstance(): Singleton
    {
        if (static::$instance === null) {
            static::$instance = new static();
        }

        return static::$instance;
    }

    /**
     * is not allowed to call from outside to prevent from creating multiple instances,
     * to use the singleton, you have to obtain the instance from Singleton::getInstance() instead
     */
    private function __construct()
    {
    }

    /**
     * prevent the instance from being cloned (which would create a second instance of it)
     */
    private function __clone()
    {
    }

    /**
     * prevent from being unserialized (which would create a second instance of it)
     */
    public function __wakeup()
    {
        throw new Exception("Cannot unserialize singleton");
    }
}

测试

Tests/SingletonTest.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Creational\Singleton\Tests;

use DesignPatterns\Creational\Singleton\Singleton;
use PHPUnit\Framework\TestCase;

class SingletonTest extends TestCase
{
    public function testUniqueness()
    {
        $firstCall = Singleton::getInstance();
        $secondCall = Singleton::getInstance();

        $this->assertInstanceOf(Singleton::class, $firstCall);
        $this->assertSame($firstCall, $secondCall);
    }
}