Singletons in PHP - Why they are bad and how you can eliminate them from your applications

Singletons inWhy they are bad and how you can eliminate them from your applications

Gordon Oheim @go_oh

a.k.a.The GoF Book

Singleton

Creational Pattern„control when and how objects are created“

“Ensure a class has only oneinstance, and provide a global access point to it.”

Singleton Singleton- instance : Singleton- instance : Singleton++ getInstance() getInstance() : : Singleton Singleton-- __construct() __construct() : : void void-- __clone() __clone() : : void void-- __wakeup() __wakeup() : : void 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 thatimpedes testing is something to be avoided. There are those who dont agree with this view, but Ill 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 theygive 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 ofsubsequent changes in dependent code

Fragile codebreaks in many places when you change just one place

Non-reusable code is code that you cannot reuse inanother project because it contains too much extra baggage

S.O.L.I.D.

Single Responsibility PrincipleA class should have one, and only one, reason to change.

Open Closed PrincipleYou should be able to extend a classes behavior, without modifying it.

Liskov Substitution PrincipleDerived classes must be substitutable for their base classes.

Interface Segregation PrincipleMake 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 changeMitigated by using Abstract Singletons But responsibilities are still strongly coupled through inheritance

OCP ViolationIn 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 dependenciesAdds concrete dependencies into Clients Signature lies

Singletons in the Wild

Config FooController Response FrontControllerThatService Request Cache Database Application BarController Registry Logger ThisService Bootstrap Acl

Recap: “Ensure a class has only oneinstance, and provide a global access point to it.”

„So much of the Singleton pattern is about coaxing language protection mechanisms intoprotecting this one aspect: singularity. I guess itis important, but it seems to have grown out of proportion.“ - Ward Cunningham

You do not need to ensure singularitywhen you are going to instantiate the object only once anyway.

You do not need to provide a GlobalAccess 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 arent necessarywhen you can design or redesign to avoid using them.“ - Joshua Kerievsky

„Im in favor of dropping Singleton. Its use is almost always a design smell“ - Erich Gamma

Singletons in PHP - Why they are bad and how you can eliminate them from your applications

by go_oh on Aug 20, 2011

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