Dependency injection

Abstract

This chapter explains "Dependency Injection (DI)" as used in TYPO3. Readers interested in the general concepts and principles may want to look at, for example, Dependency Injection in "PHP The Right Way" or What is dependency injection? by Fabien Potencier. Whenever a class has a service dependency to another class the technique of dependency injection should be used to satisfy that need. TYPO3 uses a Symfony component for dependency injection. The component is PSR-11 compliant, and it is used throughout Core and extensions to standardize object initialization. By default all API services shipped with the TYPO3 Core system extensions offer dependency injection. The recommended usage is constructor injection. Available as well are method injection and interface injection. To activate the Symfony component for dependency injection a few lines of configuration are necessary.

Introduction

The title of this chapter is "dependency injection" (DI), but the scope is a bit broader: In general, this chapter is about TYPO3 object lifecycle management and how to obtain objects, with one sub-part of it being dependency injection.

The underlying interfaces are based on the PHP-FIG (PHP Framework Interop Group) standard PSR-11 Container interface, and the implementation is based on Symfony service container and Symfony dependency injection components, plus some TYPO3 specific sugar.

This chapter not only talks about Symfony DI and its configuration via Services.yaml, but also a bit about services in general, about GeneralUtility::makeInstance() and the SingletonInterface. And since the TYPO3 core already had an object lifecycle management solution with the extbase ObjectManager before Symfony services were implemented, we'll also talk about how to transition away from it towards the core-wide Symfony solution.

Background and history

Obtaining object instances in TYPO3 has always been straightforward: Call GeneralUtility::makeInstance(\MyVendor\MyExtension\Some\Class::class) and hand over mandatory and optional __construct() arguments as additional arguments.

There are two quirks to that:

• First, a class instantiated through makeInstance() can implement SingletonInterface. This empty interface definition tells makeInstance() to instantiate the object exactly once for this request, and if another makeInstance() call asks for it, the same object instance is returned - otherwise makeInstance() always creates a new object instance and returns it.
• Second, makeInstance() allows "XCLASSing". This is a - rather dirty - way to substitute a class with a custom implementation. XCLASS'ing in general is brittle and seen as a last resort hack if no better solution is available. In connection with Symfony containers, XCLASSing services should be avoided in general and service overrides should be used instead. Replacing the XCLASS functionality in TYPO3 is still work in progress. In contrast, XCLASSing is still useful for data objects, and there is no good other solution yet.

Using makeInstance() worked very well for a long time. It however lacked a feature that has been added to the PHP world after makeInstance() had been invented: Dependency injection. There are lots of articles about dependency injection on the net, so we won't go too deep here but rather explain the main idea: The general issue appears when classes follow the separation of concerns principle.

One of the standard examples is logging. Let's say a class's responsibility is the creation of users - it checks everything and finally writes a row to database. Now, since this is an important operation, the class wants to log an information like "I just created user 'foo'". And this is where dependency injection enters the game: Logging is a huge topic, there are various levels of error, information can be written to various destinations and so on. The little class does not want to deal with all those details, but just wants to tell the framework: "Please give me some logger I can use and that takes care of all details, I don't want to know about details". This separation is the heart of single responsibility and separation of concerns.

Dependency injection does two things for us here: First, it allows separating concerns, and second, it hands the task of finding an appropriate implementation of a dependency over to the framework, so the framework decides - based on configuration - which specific instance is given to the consumer. Note in our example, the logging instance itself may have dependencies again - the process of object creation and preparation may be further nested.

In more abstract software engineering terms: Dependency injection is a pattern used to delegate the task of resolving class dependencies away from a consumer towards the underlying framework.

Back to history: After makeInstance() has been around for quite a while and lacked an implementation of dependency injection, Extbase appeared in 2009. Extbase brought a first container and dependency injection solution, it's main interface being the Extbase ObjectManager. The Extbase object manager has been widely used for a long time, but suffered from some issues younger approaches don't face. One of the main drawbacks of Extbase object manager is the fact that it's based on runtime reflection: Whenever an object is to be instantiated, the object manager scans the class for needed injections and prepares dependencies to be injected. This process is quite slow though mitigated by various caches. And these also come with costs. In the end, these issues have been the main reason the object manager was never established as a main core concept but only lived in Extbase scope.

The object lifecycle and dependency injection solution based on Symfony DI has been added in TYPO3v10 and is a general core concept: Next to the native dependency injection, it is also wired into makeInstance() as a long living backwards compatibility solution, and it fully substitutes the Extbase object manager. In contrast to the Extbase solution, Symfony based object management does not have the overhead of expensive runtime calculations. Instead it is an instance wide build-time solution: When TYPO3 bootstraps, all object creation details of all classes are read from a single cache file just once, and afterwards no expensive calculation is required for actual creation.

Symfony based DI was implemented in TYPO3 v10 and usage of the Extbase ObjectManager was discouraged. With TYPO3 v11 the core doesn't use the ObjectManager any more. It is actively deprecated in v11 and thus leads to 'deprecation' level log entries.

With TYPO3 v12 the Extbase ObjectManager is actually gone. Making use of Symfony DI integration still continues. There are still various places in the core to be improved. Further streamlining will be done over time. For instance, the final fate of makeInstance() and the SingletonInterface has not fully been decided on yet. Various tasks remain to be solved in younger TYPO3 developments to further improve the object lifecycle management provided by the core.

Build-time caches

To get a basic understanding of the core's lifecycle management it is helpful to get a rough insight on the main construct. As already mentioned, object lifecycle management is conceptualized as steps to take place at build-time. It is done very early and only once during the TYPO3 bootstrap process. All calculated information is written to a special cache that can not be reconfigured and is available early. On subsequent requests the cache file is loaded. Extensions can not mess with the construct if they adhere to the core API.

Besides being created early, the state of the container is independent and exactly the same in frontend, backend and CLI scope. The same container instance may even be used for multiple requests. This is becoming more and more important nowadays with the core being able to execute sub requests. The only exception to this is the Install Tool: It uses a more basic container that "cannot fail". This difference is not important for extension developers however since they can't hook into the Install Tool at those places anyways.

The Symfony container implementation is usually configured to actively scan the extension classes for needed injections. All it takes are just a couple of lines within the Services.yaml file. This should be done within all extensions that contain PHP classes and it is the fundamental setup we will outline in the following sections.

For developers, it is important to understand that dealing with Symfony DI is an early core bootstrap thing. The system will fail upon misconfiguration, so frontend and backend may be unreachable.

The container cache entry (at the time of this writing) is not deleted when a backend admin user clicks "Clear all cache" in the backend top toolbar. The only way to force a DI recalculation is using the "Admin tools" -> "Maintenance" -> "Flush Caches" button of the backend embedded Install Tool or the standalone Install Tool (/typo3/install.php) itself. This means: Whenever core or an extension fiddles with DI (or more general "Container") configuration, this cache has to be manually emptied for a running instance by clicking this button. The backend Extension Manager however does empty the cache automatically when loading or unloading extensions. Another way to quickly drop this cache during development is to remove all var/cache/code/di/* files, which reside in typo3temp/ in Legacy Mode instances or elsewhere in Composer Mode instances (see Environment). TYPO3 will then recalculate the cache upon the next access, no matter if it's a frontend, a backend or a CLI request.

The main takeaway is: When a developer fiddles with container configuration, the cache needs to be manually cleared. And if some configuration issue slipped in, which made the container or DI calculation fail, the system does not heal itself and needs both a fix of the configuration plus probably a cache removal. The standalone Install Tool however should always work, even if the backend breaks down, so the "Flush caches" button is always reachable. Note that if the container calculation fails, the var/log/typo3_* files contain the exception with backtrace!

Important terms

We will continue to use a couple of technical terms in this chapter, so let's quickly define them to align. Some of them are not precisely used in our world, for instance some Java devs may stumble upon "our" understanding of a prototype.

Prototype

The broad understanding of a prototype within the TYPO3 community is that it's simply an object that is created anew every time. Basically the direct opposite of a singleton. In fact, the prototype pattern describes a base object that is created once, so __construct() is called to set it up, after that it is cloned each time one wants to have a new instance of it. The community isn't well aware of that, and the core provides no "correct" prototype API, so the word prototype is often misused for an object that is always created anew when the framework is asked to create one.

Singleton

A singleton is an object that is instantiated exactly once within one request. If an instance is requested and the object has been created once already, the same instance is returned. Codewise, this is sometimes done by implementing a static getInstance() method that parks the instance in a property. In TYPO3, this can also be achieved by implementing the SingletonInterface, where makeInstance() then stores the object internally. Within containers, this can be done by declaring the object as shared (shared: true), which is the default. We'll come back to details later. Singletons must not have state - they must act the same way each time they're used, no matter where, how often or when they've been used before. Otherwise the behavior of a singleton object is undefined and will lead to obscure errors.

Service

This is another "not by the book" definition. We use the understanding "What is a service?" from Symfony: In Symfony, everything that is instantiated through the service container (both directly via $container->get() and indirectly via DI) is a service. These are many things - for instance controllers are services, as well as - non static - utilities, repositories and obvious classes like mailers and similar. To emphasize: Not only classes named with a *Service suffix are services but basically anything. It does not matter much if those services are stateless or not. Controllers, for instance, are usually not stateless. (This is just a configuration detail from this point of view.) Note: The TYPO3 Core does not strictly follow this behavior in all cases yet, but it strives to get this more clean over time.

Data object

Data objects are the opposite of services. They are not available through service containers. Here calling $container->has() returns false and they can not be injected. They are instantiated either with new() or GeneralUtility::makeInstance(). Domain models or DTO are a typical example of data objects.

Note: Data objects are not "service container aware" and do not support DI. Although the TYPO3 core does not strictly follow this rule in all cases until now the ambition is to get this done over time.

Using DI

Now that we have a general understanding of what a service and what a data object is, let's turn to usages of services. We will mostly use examples for this.

The general rule is: Whenever your class has a service dependency to another class, one of the following solutions should be used.

namespace MyVendor\MyExtension\Controller;

use MyVendor\MyExtension\Repository\UserRepository;

final class UserController
{
     private UserRepository $userRepository;

     public function __construct(UserRepository $userRepository)
     {
          $this->userRepository = $userRepository;
     }
}

namespace MyVendor\MyExtension\Controller;

use MyVendor\MyExtension\Repository\UserRepository;

final class UserController
{
     private ?UserRepository $userRepository = null;

     public function injectUserRepository(UserRepository $userRepository)
     {
         $this->userRepository = $userRepository;
     }
}

namespace MyVendor\MyExtension\Controller;

use MyVendor\MyExtension\Repository\UserRepository;
use MyVendor\MyExtension\Logger\Logger;

abstract class AbstractController
{
     protected ?Logger $logger = null;

     public function injectLogger(Logger $logger)
     {
         $this->logger = $logger;
     }
}

final class UserController extends AbstractController
{
     private UserRepository $userRepository;

     public function __construct(UserRepository $userRepository)
     {
         $this->userRepository = $userRepository;
     }
}

namespace MyVendor\MyExtension\Controller;

use MyVendor\MyExtension\Logger\LoggerInterface;

final class UserController extends AbstractController
{
     protected LoggerInterface $logger;

     public function __construct(LoggerInterface $logger)
     {
         $this->logger = $logger;
     }
}

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Controller;

use MyVendor\MyExtension\Service\MyServiceInterface;

class MyController
{
    public function __construct(
        private readonly MyServiceInterface $myService
    ) {}
}

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Controller;

class MySecondController extends MyController {}

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Controller;

use MyVendor\MyExtension\Service\MyServiceInterface;

class MyThirdController
{
    private MyServiceInterface $myService;

    public function injectMyService(MyServiceInterface $myService)
    {
        $this->myService = $myService;
    }
}

services:
  _defaults:
    autowire: true
    autoconfigure: true
    public: false

  # Define the default implementation of an interface
  MyVendor\MyExtension\Service\MyServiceInterface: '@MyVendor\MyExtension\Service\MyDefaultService'

  # Within MySecond- and MyThirdController different implementations for said
  # interface shall be used instead.

  # Version 1: when working with constructor injection
  MyVendor\MyExtension\Controller\MySecondController:
    arguments:
      $service: '@MyVendor\MyExtension\Service\MySecondService'

  # Version 2: when working with method injection
  MyVendor\MyExtension\Controller\MyThirdController:
    calls:
      - method: 'injectMyService'
        arguments:
          $service: '@MyVendor\MyExtension\Service\MyThirdService'

Using container->get()

[WIP] Service containers provide two methods to obtain objects, first via $container->get(), and via DI. This is only available for services itself: Classes that are registered as a service via configuration can use injection or $container->get(). DI is supported in two ways: As constructor injection, and as inject*() method injection. They lead to the same result, but have subtle differences. More on that later.

In general, services should use DI (constructor or method injection) to obtain dependencies. This is what you'll most often find when looking at core implementations. However, it is also possible to get the container injected and then use $container->get() to instantiate services. This is useful for factory-like services where the exact name of classes is determined at runtime.

Configuration

services:
  _defaults:
    autowire: true
    autoconfigure: true
    public: false

  MyVendor\MyExtension\:
    resource: '../Classes/*'
    exclude: '../Classes/Domain/Model/*'

MyVendor\MyExtension\UserFunction\ClassA:
  arguments:
    $argA: '@TYPO3\CMS\Core\Database\ConnectionPool'

MyVendor\MyExtension\UserFunction\ClassB:
  arguments:
    - '@TYPO3\CMS\Core\Database\ConnectionPool'

connection.pages:
  class: 'TYPO3\CMS\Core\Database\Connection'
  factory:
    - '@TYPO3\CMS\Core\Database\ConnectionPool'
    - 'getConnectionForTable'
  arguments:
    - 'pages'

MyVendor\MyExtension\UserFunction\ClassA:
  public: true
  arguments:
    - '@connection.pages'

services:
  _defaults:
    autowire: true
    autoconfigure: true
    public: false

  MyVendor\MyExtension\:
    resource: '../Classes/*'
    exclude: '../Classes/Domain/Model/*'

  MyVendor\MyExtension\UserFunction\ClassA:
    public: true

(1/1) ArgumentCountError

Too few arguments to function MyVendor\MyExtension\Namespace\Class::__construct(),
0 passed in typo3/sysext/core/Classes/Utility/GeneralUtility.php on line 3461 and exactly 1 expected

(1/1) Error

Call to a member function methodName() on null

TYPO3\CMS\Belog\Controller\BackendLogController: '@MyVendor\MyExtension\Controller\ExtendedBackendLogController'