Dependency Injection Smells
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This document discusses dependency injection smells in PHP code from frameworks like Zend, Symfony, and Doctrine. It identifies common anti-patterns like static dependencies, missing dependency auto-recovery, hidden dependencies, creation logic reduction, factory methods, programming against implementations, and prohibiting dependencies. The document provides examples of these smells and recommends refactoring approaches, emphasizing that dependencies should be explicit, only required minimum dependencies injected, and API design consider the needs of users.
![DoctrineDBAL
Dependency Injection Smell: Dependencies prohibited
abstractclassType
{
publicstaticfunctionaddType($name,$className)
{
self::$typesMap[$name]=$className;
}
publicstaticfunctiongetType($name)
{
if(!isset(self::$typeObjects[$name])){
self::$typeObjects[$name]=newself::$typesMap[$name]();
}
returnself::$typeObjects[$name];
}
}
Type::getType('datetime')->convertToPHPValue('2013-06-08');
PHP
36/44](https://image.slidesharecdn.com/dependency-injection-smells-dutch-php-conference-130607093431-phpapp01/85/Dependency-Injection-Smells-36-320.jpg)
Dependency Injection Smells
- 1. Dependency Injection Smells Featuring Zend,Symfonyand Doctrinecode Matthias Noback - PHP developer and consultant Dutch PHP Conference - 6/7/2013
- 2. Matthias Noback Dutch developer Consultancy, training, writing Clean code · · · 2/44
- 3. What is a dependency? A needs a B to do its job therefore B is a dependency of A 3/44
- 4. Dependency management To make sure that A will have a B, A can: 4/44
- 5. 1. Create a B 5/44
- 6. 2. Fetch a B 6/44
- 7. 3. Receive a B 7/44
- 8. Dependency management Strategy 1: Do it yourself 8/44
- 9. Why not... Instantiate classes with the newoperator? classSomeClass { publicfunctiondoSomething() { $logger=newLogger(); $logger->debug('Nothingtoseehere'); } } PHP 9/44
- 10. Why not... Instantiate variable classes? classSomeClass { publicfunction__construct($loggerClass) { $this->loggerClass=$loggerClass; } publicfunctiondoSomething() { $logger=new{$this->loggerClass}(); $logger->debug('Veryflexible,right?'); } } PHP 10/44
- 11. Why not... Instantiate in overridable methods? classSomeClass { publicfunctiondoSomething() { $logger=$this->createLogger(); $logger->debug('Matthiaswashere'); } protectedfunctioncreateLogger() { returnnewLogger(); } } PHP 11/44
- 12. Dependency management Strategy 2: Fetch your dependencies 12/44
- 13. Why not... Use a global/static variable? classSomeClass { publicfunctiondoSomething() { Logger::getInstance()->debug('Matthiaswashere'); } } PHP 13/44
- 14. Why not... Use a service container/manager/registry? classSomeClass { publicfunction__construct(ContainerInterface$container) { $this->container=$container; } publicfunctiondoSomething() { $this ->container ->get('logger') ->log('Wow,thatwasquiteadistance!') } } PHP 14/44
- 15. Dependency management Strategy 3: Be passive about your dependencies 15/44
- 16. Injecting dependencies by their interface classSomeClass { publicfunction__construct(LoggerInterface$logger) { $this->logger=$logger; } publicfunctiondoSomething() { $this->logger->info('Ah,muchbetter'); } } PHP 16/44
- 17. Strategy review 1. Create your own dependencies 2. Fetch your dependencies 17/44
- 18. Strategy review You are in control of your dependencies 18/44
- 19. Strategy review 3. Have your dependencies injected 19/44
- 20. Strategy review You are not in control of your dependencies Someone else This is called "inversion of control" (IoC) · · · 20/44
- 21. Dependency injection Dependency injection is a , used to 21/44
- 24. ZendCrypt Dependency Injection Smell: Static dependency Static dependencies are hard-to-control dependencies classBlockCipher { publicstaticfunctionfactory($adapter,$options=array()) { $plugins=static::getSymmetricPluginManager(); $adapter=$plugins->get($adapter,(array)$options); returnnewstatic($adapter); } } PHP 24/44
- 25. ZendCrypt Dependency Injection Smell: Missing dependency auto-recovery If you have a dependency, act like you are . publicstaticfunctiongetSymmetricPluginManager() { if(static::$symmetricPlugins===null){ static::setSymmetricPluginManager(newSymmetricPluginManager()); } returnstatic::$symmetricPlugins; } PHP 25/44
- 26. ZendCrypt Dependency Injection Smell: Hidden dependencies If you have a dependency, make it explicit. classSymmetricPluginManagerextendsAbstractPluginManager { protected$invokableClasses=array( 'mcrypt'=>'ZendCryptSymmetricMcrypt', ); } PHP 26/44
- 27. ZendCrypt Suggested refactoring useZendCryptSymmetricMcrypt; useZendCryptSymmetricBlockCipher; $cipher=newMcrypt(array('algo'=>'aes')); $blockCipher=newBlockCipher($cipher); PHP No SymmetricPluginManager No factory()method · · 27/44
- 28. ZendCrypt Dependency Injection Smell: Creation logic reduction Make way for complex creation logic (and don't think the newoperator will suffice). publicstaticfunctionsetSymmetricPluginManager($plugins) { if(is_string($plugins)){ $plugins=new$plugins(); } if(!$pluginsinstanceofSymmetricPluginManager){ thrownewExceptionInvalidArgumentException(); } static::$symmetricPlugins=$plugins; } PHP 28/44
- 31. SymfonyBundleFrameworkBundle Dependency Injection Smell: Factory methods Don't require developers to use inheritance for replacing dependencies. abstractclassHttpCacheextendsBaseHttpCache { protectedfunctioncreateEsi() { returnnewEsi(); } protectedfunctioncreateStore() { returnnewStore($this->cacheDir?:$this->kernel->getCacheDir().'/http_cache'); } } PHP 31/44
- 32. SymfonyBundleFrameworkBundle Dependency Injection Smell: Programming against an implementation HttpCachehas a dependency on HttpKernelInterface... abstractclassHttpCacheextendsBaseHttpCache { publicfunction__construct(HttpKernelInterface$kernel,$cacheDir=null) { ... } } PHP interfaceHttpKernelInterface { publicfunctionhandle(Request$request,$type=self::MASTER_REQUEST,$catch=true); } PHP 32/44
- 33. SymfonyBundleFrameworkBundle Dependency Injection Smell: Programming against an implementation ... but it uses of the interface methods array('debug'=>$kernel->isDebug()), PHP returnnewStore($this->cacheDir?:$this->kernel->getCacheDir().'/http_cache'); PHP $this->getKernel()->boot(); PHP 33/44
- 34. SymfonyBundleFrameworkBundle Suggested refactoring Program against an interface (strictly) Or: program against an implementation (and type-hint accordingly) Or: expand the interface to contain the methods you need · · · 34/44
- 36. DoctrineDBAL Dependency Injection Smell: Dependencies prohibited abstractclassType { publicstaticfunctionaddType($name,$className) { self::$typesMap[$name]=$className; } publicstaticfunctiongetType($name) { if(!isset(self::$typeObjects[$name])){ self::$typeObjects[$name]=newself::$typesMap[$name](); } returnself::$typeObjects[$name]; } } Type::getType('datetime')->convertToPHPValue('2013-06-08'); PHP 36/44
- 37. DoctrineDBAL Dependency Injection Smell: Dependencies prohibited classEncryptedStringTypeextendsType { publicfunction__construct(BlockCipher$blockCipher) { $this->blockCipher=$blockCipher; } publicfunctionconvertToDatabaseValue($value) { return$this->blockCipher->encrypt($value); } publicfunctionconvertToPHPValue($value) { return$this->blockCipher->decrypt($value); } } PHP 37/44
- 38. DoctrineDBAL Dependency Injection Smell: Dependencies prohibited We can not override Type::__construct()since it is final. Don't neglect other classes' needs (even if your class has everything it needs) Type::addType('encrypted_string','EncryptedStringType'); PHP abstractclassType { finalprivatefunction__construct(){} } PHP 38/44
- 39. DoctrineDBAL Dependency Injection Smell: Dependencies prohibited Work-around: static dependency classEncryptedStringType { publicstatic$blockCiper; } EncryptedStringType::$blockCipher=...; PHP 39/44
- 40. DoctrineDBAL Recommended refactoring 1. Split the Typeclass into an AbstractTypeand a TypeRegistry. 2. Set a type instance, instead of just a class. $blockCipher=...;//ha,weknowhowtomakeone! $typeRegistry=newTypeRegistry(); $encryptedStringType=newEncryptedStringType($blockCipher); $typeRegistry->setType('encrypted_string',$encryptedStringType); PHP 40/44
- 41. In conclusion Good API design
- 42. Dependency injection smells Static dependency Missing dependency auto-recovery Hidden dependencies Creation logic reduction Factory methods Programming against an implementation Dependencies prohibited · · · · · · · 42/44
- 43. Keep in mind... Be clear and open about what your dependencies are Require only a minimum amount of dependencies Develop with your users (other developers) in mind · · · 43/44
- 44. Thank you Please leave some feedback at joind.in/8447 twitter @matthiasnoback www php-and-symfony.matthiasnoback.nl github github.com/matthiasnoback leanpub leanpub.com/a-year-with-symfony