Dependency injection
This page
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.
,
but also a bit about services in general, about General
and the
Singleton
. And since the TYPO3 core already had an object lifecycle management
solution with the extbase Object
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
General
and hand over
mandatory and optional __
arguments as additional arguments.
There are two quirks to that:
- First, a class instantiated through makeInstance() can implement
Singleton
. 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.Interface - Second,
make
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.Instance ()
Using make
worked very well for a long time. It however lacked a feature
that has been added to the PHP world after make
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 make
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 Object
.
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 make
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 make
and the Singleton
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.
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.
Attention
Errors in the DI cache may block frontend and backend!
The DI cache does not heal by itself but needs to be cleared manually!
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/
) 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/
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/
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
__
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.construct () - 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
get
method that parks the instance in a property. In TYPO3, this can also be achieved by implementing theInstance () Singleton
, whereInterface make
then stores the object internally. Within containers, this can be done by declaring the object as shared (Instance () 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 withnew
or() General
. Domain models or DTO are a typical example of data objects.Utility:: make Instance () 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.
When to use Dependency Injection in TYPO3
Class dependencies to services should be injected via constructor injection or
setter methods. Where possible, Symfony dependency injection should be used for
all cases where DI is required.
Non-service "data objects" like Extbase domain model instances or DTOs should
be instantiated via \TYPO3\
if they are non-final and support XCLASSing. For final classes without
dependencies plain instantiation via the new
keyword must be used.
In some APIs dependency injection cannot be used yet. This applies to classes that need specific data in their constructors or classes that are serialized and deserialized as, for example, scheduler tasks.
When dependency injection cannot be used directly yet, create a service class and make it public
in the Configuration/
. Create an instance of the service class via
General
you can then use dependency injection in the service class.
Constructor injection
Assume we're writing a controller that renders a list of users. Those users are
found using a custom User
, so the repository service is a direct dependency of the
controller service. A typical constructor dependency injection to resolve the
dependency by the framework looks like this:
<?php
declare(strict_types=1);
namespace MyVendor\MyExtension\Controller;
use MyVendor\MyExtension\Repository\UserRepository;
final class UserController
{
public function __construct(
private readonly UserRepository $userRepository,
) {}
}
Here the Symfony container sees a dependency to User
when scanning __
of the User
. Since autowiring is enabled by default (more on that below), an instance of the
User
is created and provided when the controller is created. The example uses constructor
property promotion
and sets the property readonly
, so it can not be written a second time.
Method injection
A second way to get services injected is by using inject*
methods:
<?php
declare(strict_types=1);
namespace MyVendor\MyExtension\Controller;
use MyVendor\MyExtension\Repository\UserRepository;
final class UserController
{
private ?UserRepository $userRepository = null;
public function injectUserRepository(UserRepository $userRepository)
{
$this->userRepository = $userRepository;
}
}
This ends up with the basically the same result as above: The controller instance has
an object of type User
in class property $user
.
The injection via methods was introduced by Extbase and TYPO3 core implemented it in addition to
the default Symfony constructor injection. Why did we do that, you may ask? Both
strategies have subtle differences: First, when using inject*
methods, the type
hinted class property needs to be nullable, otherwise PHP >= 7.4 throws a warning
since the instance is not set during __
. But that's just an
implementation detail. More important is an abstraction scenario. Consider this case:
<?php
declare(strict_types=1);
namespace MyVendor\MyExtension\Controller;
use MyVendor\MyExtension\Logger\Logger;
use MyVendor\MyExtension\Repository\UserRepository;
abstract class AbstractController
{
protected ?Logger $logger = null;
public function injectLogger(Logger $logger)
{
$this->logger = $logger;
}
}
final class UserController extends AbstractController
{
public function __construct(
private readonly UserRepository $userRepository,
) {}
}
We have an abstract controller service with a dependency plus a controller service that extends the abstract and has further dependencies.
Now assume the abstract class is provided by TYPO3 core and the consuming class
is provided by an extension. If the abstract class would use constructor injection,
the consuming class would need to know the dependencies of the abstract, add its
own dependency to the constructor, and then call parent::__
to
satisfy the dependency of the abstract. This would hardcode all dependencies
of the abstract into extending classes. If later the abstract is changed and
another dependency is added to the constructor, this would break consuming
classes since they did not know that.
Differently put: When core classes "pollute" __
with dependencies,
the core can not add dependencies without being breaking. This is the reason why
for example the extbase Abstract
uses inject*
methods for its
dependencies: Extending classes can then use constructor injection, do not need
to call parent::__
, and the core is free to change dependencies of
the abstract.
In general, when the core provides abstract classes that are expected to be
extended by extensions, the abstract class should use inject*
methods instead of
constructor injection. Extensions of course can follow this idea in similar
scenarios.
This construct has some further implications: Abstract classes should
think about making their dependency properties private
, so extending classes
can not rely on them. Furthermore, classes that should not be extended by extensions
are free to use constructor injection and should be marked final
, making
sure they can't be extended to allow internal changes.
As a last note on method injection, there is another way to do it: It is possible
to use a set
method if it has the annotation @required
.
This second way of method injection however is not used within the TYPO3
framework, should be avoided in general, and is just mentioned here for completeness.
Interface injection
Apart from constructor injection and inject*
method injection, there is another
useful dependency injection scenario. Look at this example:
<?php
declare(strict_types=1);
namespace MyVendor\MyExtension\Controller;
use MyVendor\MyExtension\Logger\LoggerInterface;
final class UserController extends AbstractController
{
public function __construct(
private readonly LoggerInterface $logger,
) {}
}
See the difference? We're requesting the injection of an interface and not a class! It works for both constructor and method injection. It forces the service container to look up which specific class is configured as implementation of the interface and inject an instance of it. This is the true heart of dependency injection: A consuming class no longer codes on a specific implementation, but on the signature of the interface. The framework makes sure something is injected that satisfies the interface, the consuming class does not care, it just knows about the interface methods. An instance administrator can decide to configure the framework to inject some different implementation than the default, and that's fully transparent for consuming classes.
Here's an example scenario that demonstrates how you can define the specific implementations that shall be used for an interface type hint:
<?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
Configure dependency injection in extensions
Extensions have to configure their classes to make use of the
dependency injection. This can be done in Configuration/
.
Alternatively, Configuration/
can also be used.
A basic Services.
file of an extension looks like the following.
Note
Whenever the service configuration or class dependencies change, the Core
cache must be flushed in the Admin Tools > Maintenance or via the CLI
command cache:
to rebuild the compiled Symfony container. Flushing
all caches from the Clear cache menu does not flush the compiled Symfony
container.
services:
_defaults:
autowire: true
autoconfigure: true
public: false
MyVendor\MyExtension\:
resource: '../Classes/*'
exclude: '../Classes/Domain/Model/*'
- autowire
-
autowire: true
instructs the dependency injection component to calculate the required dependencies from type declarations. This works for constructor injection andinject*
methods. The calculation generates a service initialization code which is cached in the TYPO3 Core cache.() Attention
An extension does not have to use autowiring, but can wire dependencies manually in the service configuration file.
- autoconfigure
- It is suggested to enable
autoconfigure: true
as this automatically adds Symfony service tags based on implemented interfaces or base classes. For example, autoconfiguration ensures that classes implementing\TYPO3\
are publicly available from the Symfony container and marked as shared (CMS\ Core\ Singleton Interface shared: true
). - Model exclusion
- The path exclusion
exclude: '../
excludes your models from the dependency injection container, which means you cannot inject them nor inject dependencies into them. Models are not services and therefore should not require dependency injection. Also, these objects are created by the Extbase persistence layer, which does not support the DI container.Classes/ Domain/ Model/*'
Arguments
In case you turned off autowire
or need special arguments, you can
configure those as well. This means that you can set autowire: false
for
an extension, but provide the required arguments via config specifically for
the desired classes. This can be done in chronological order or by naming.
services:
# Place here the default dependency injection configuration from above
MyVendor\MyExtension\UserFunction\ClassA:
arguments:
$argA: '@TYPO3\CMS\Core\Database\ConnectionPool'
MyVendor\MyExtension\UserFunction\ClassB:
arguments:
- '@TYPO3\CMS\Core\Database\ConnectionPool'
This allows you to inject concrete objects like the Connection:
services:
# Place here the default dependency injection configuration from above
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'
Now you can access the Connection
instance within Class
. This
allows you to execute your queries without further instantiation. For example,
this method of injecting objects also works with extension configurations and
with TypoScript settings.
Public
public: false
is a performance optimization and should therefore be set
in extensions. This settings controls which services are available through the
dependency injection container used internally by
General
. However, some classes that need to be
public are automatically marked as public due to autoconfigure: true
being set. These classes include singletons, as they must be shared with code
that uses General
and Extbase controllers.
What to make public
Every class that is instantiated using General
and requires dependency injection must be marked as public. The same goes
for instantiation via General
using constructor
arguments.
Any other class which requires dependency injection and is retrieved by dependency injection itself can be private.
Instances of \TYPO3\
and Extbase controllers
are automatically marked as public. This allows them to be retrieved using
General
as done by TYPO3 internally.
More examples of classes that must be marked as public:
- User functions
- Non-Extbase controllers
- Classes registered in hooks
- Authentication services
- Fluid data processors (only necessary if not tagged as data.processor).
For such classes, an extension can override the global configuration
public: false
in Configuration/
for each affected
class:
services:
_defaults:
autowire: true
autoconfigure: true
public: false
MyVendor\MyExtension\:
resource: '../Classes/*'
exclude: '../Classes/Domain/Model/*'
MyVendor\MyExtension\UserFunction\ClassA:
public: true
With this configuration, you can use dependency injection in
\My
when it is created, for example
in the context of a USER
TypoScript object, which would not be
possible if this class were private.
See also
Symfony: How to Create Service Aliases and Mark Services as Private
Errors resulting from wrong configuration
If objects that use dependency injection are not configured properly, one or
more of the following issues may result. In such a case, check whether the
class has to be configured as public: true
.
Argument
is raised on missing dependency injection for
Constructor injection:
(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
An Error
is thrown on missing dependency injection for
Method injection, once the dependency is used within the code:
(1/1) Error
Call to a member function methodName() on null
Installation-wide configuration
New in version 12.1
One can set up a global service configuration for a project that can be used in multiple project-specific extensions. For example, this way you can alias an interface with a concrete implementation that can be used in several extensions. It is also possible to register project-specific CLI commands without requiring a project-specific extension.
However, this only works - due to security restrictions - if TYPO3 is configured in a way that the project root is outside the document root, which is usually the case in Composer-based installations.
The global service configuration files services.
and
services.
are now read within the config/
path
of a TYPO3 project in Composer-based installations.
Example:
You want to use the PSR-20 clock interface as a type hint for dependency
injection in the service classes of your project's various extensions. Then the
concrete implementation may change without touching your code. In this example,
we use lcobucci/
for the concrete implementation.
<?php
declare(strict_types=1);
use Lcobucci\Clock\SystemClock;
use Psr\Clock\ClockInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
return static function (
ContainerConfigurator $containerConfigurator,
ContainerBuilder $containerBuilder,
): void {
$services = $containerConfigurator->services();
$services->set(ClockInterface::class)
->factory([SystemClock::class, 'fromUTC']);
};
The concrete clock implementation is now injected when a type hint to the interface is given:
User functions and their restrictions
It is possible to use dependency injection when calling custom user functions,
for example .userFunc within TypoScript or
in (legacy) hooks, usually via
\TYPO3\
.
This method call
internally uses the dependency-injection-aware
helper General
, which can recognize and inject
classes/services that are marked public.
Attention
The backend module Admin Tools > Extensions > Configuration is also
able to specify user functions for input options provided via an
ext_
(see Configuration).
However, this backend module is executed in a special low-level context that disables some functionality for failsafe-reasons. Specifically, this prevents dependency injection from being used in this scenario.
If you need to utilize services and other classes inside user functions
that are called there, you need to perform custom General
calls inside your own user function method to initialize those needed classes/services.
Dependency injection in a XCLASSed class
When extending an existing class (for example, an Extbase controller) using
XCLASS and injecting additional dependencies using constructor
injection, ensure that a reference to the extended class is added in the
Configuration/
file of the extending extension, as shown in
the example below:
TYPO3\CMS\Belog\Controller\BackendLogController: '@MyVendor\MyExtension\Controller\ExtendedBackendLogController'
Further information
- Symfony dependency injection component
- Symfony service container
- Dependency Injection in TYPO3 - Blog Article by Daniel Goerz
- Dependency Injection in "PHP The Right Way"
- What is dependency injection? by Fabien Potencier