Singletons inSingletons in Why they are bad and how you can eliminate

them from your applications

Gordon OheimGordon Oheim@go_oh

a.k.a.The GoF Book

Singleton

Creational Pattern„control when and how objects are

created“

“Ensure a class has only one instance, and provide a global

access point to it.”

Singleton- instance : Singleton+ getInstance() : Singleton- __construct() : void- __clone() : void- __wakeup() : void

Singleton- instance : Singleton+ getInstance() : Singleton- __construct() : void- __clone() : void- __wakeup() : void

class Singleton… public static function getInstance() { if (!isset(self::$instance)) { self::$instance = new self; } return self::$instance; }}

abstract class Singleton… public static function getInstance() { return isset(static::$instance) ? static::$instance : static::$instance = new static; } final private function __construct() { static::init(); } protected function init() {}}class A extends Singleton { protected static $instance;}

trait Singleton… protected static $instance; final public static function getInstance() { return isset(static::$instance) ? static::$instance : static::$instance = new static; } final private function __construct() { static::init(); } protected function init() {}}class A { use Singleton;}

„Nobody should need a mechanism to make it as easy as pie to clutter the code base

with singletons“

- Sebastian Bergmann

Testing Singletons

„In my stilted view of the universe anything that impedes testing is something to be avoided. There are those who don't agree with this view, but I'll

pit my defect rates against theirs any time.“

- Robert „Uncle Bob“ C. Martin

class TableDataGateway… public function __construct() { $this->db = Db::getInstance(); } public insert(array $data) { $this->db->sql('INSERT …', $data); }}

class TableDataGatewayTest extends PHPUnit… /**

* @dataProvider provideInsertData */

public function testInsertCallsDbWithData($data) { $gateway = new TableDataGateway; $gateway->insert($data); $this->assertSame( $data, $gateway->findById($data['id']) ); }}

throws LogicException: No key found for 'pdo' in Registry

class Db extends Singleton… protected function init() { $this->logger = Logger::getInstance(); $this->pdo = new PDO(Registry::get('pdo')); } public sql($query, $data) { $this->logger->log(printf( 'Executed %s with %s', $query, implode(',', $data) )); $sth = $this->pdo->prepare($query); return $sth->execute($data); } }

Singletons are pathological liars- Miško Hevery

class TableDataGatewayTest extends PHPUnit… /**

* @dataProvider provideInsertData */

public function testInsertCallsDbWithData($data) { Registry::set('pdo', array('…')); Registry::set('log', '/dev/null'); $gateway = new TableDataGateway; $gateway->insert($data); $this->assertSame( $data, $gateway->findById($data['id']) ); }}

Your tests are not isolated.You still need a real database.

No easy way to mock the Db Singleton.

„The real problem with Singletons is that they give you such a good excuse not to think carefully

about the appropriate visibility of an object.“

- Kent Beck

Hard to change code manifests itself in a cascade of

subsequent changes in dependent code

Fragile code breaks in many places when you

change just one place

Non-reusable code is code that you cannot reuse in

another project because it contains too much extra baggage

S.O.L.I.D.

Single Responsibility Principle

A class should have one, and only one, reason to change.

Open Closed Principle

You should be able to extend a classes behavior, without modifying it.

Liskov Substitution Principle

Derived classes must be substitutable for their base classes.

Interface Segregation Principle

Make fine grained interfaces that are client specific.

Dependency Inversion PrincipleDepend on abstractions, not on concretions.

Solid Singleton?

SRP Violation

Creation Logic + Business Logic= 2 reasons to change

Mitigated by using Abstract Singletons

But responsibilities are still strongly coupled through inheritance

OCP Violation

In PHP < 5.2 Singletons are closed for extending

Boilerplate code has to be removed when no longer needed

DIP Violation

Global access breaks encapsulation

Hides dependencies

Adds concrete dependencies into Clients

Signature lies

Singletons in the Wild

Database

FrontController

Logger

Cache

Registry

Request

ResponseConfig

Bootstrap

Application

Acl

FooController

BarController

ThisService

ThatService

Database

FrontController

Logger

Cache

Registry

Request

ResponseConfig

Bootstrap

Application

Acl

FooController

BarController

ThisService

ThatService

Recap:“Ensure a class has only one

instance, and provide a global access point to it.”

„So much of the Singleton pattern is about coaxing language protection mechanisms into

protecting this one aspect: singularity. I guess it is important, but it seems to have grown out of

proportion.“

- Ward Cunningham

You do not need to ensure singularity when you are going to instantiate the

object only once anyway.

You do not need to provide a Global Access Point when you can inject the

object.

class TableDataGateway… public function __construct() { $this->db = Db::getInstance(); }}class Db extends Singleton… protected function init() { $this->logger = Logger::getInstance(); $this->pdo = new PDO(Registry::get('pdo')); } }

class TableDataGateway… public function __construct(Db $db) { $this->db = $db; }}class Db… public function __construct(PDO $pdo, Log $logger) { $this->logger = $logger; $this->pdo = $pdo; } }

But then I have to push dependencies all the way through my object graph?!

Recap: Creational Pattern

„control when and how objects are created“

Use Builders and Factories to create Collaborator Graphs

class UserControllerBuilder { public function build($config) { $log = new Log($config['log']); $pdo = new PDO($config['…']); $db = new Db($pdo, $log); $tdg = new TableDataGateway($db); return new UserController($tdg); }}

class UserControllerBuilder { public function build($config) { $log = new Log($config['log']); $pdo = new PDO($config['…']); $db = new Db($pdo, $log); $tdg = new TableDataGateway($db); return new UserController($tdg); }}

???

class UserControllerBuilder { public function build($config) { $db = new LogDecorator( new PDO($config['…']); new Log($config['log']); ); $tdg = new TableDataGateway($db); return new UserController($tdg); }}

// index.phpinclude '/path/to/autoload.php';$config = new Config('/path/to/config.ini');$router = new Router( array( '/user/{id}' => function() use ($config) { $builder = new UserControllerBuilder; return $builder->build($config); } ));$router->route( new Request($_GET, $_POST, $_SERVER), new Response);

„Singletons aren't necessary when you can design or redesign

to avoid using them.“

- Joshua Kerievsky

„I'm in favor of dropping Singleton. Its use is almost always a design

smell“

- Erich Gamma