Service Locators
Edit this pageWarning: You are browsing the documentation for Symfony 3.3, which is no longer maintained.
Read the updated version of this page for Symfony 6.3 (the current stable version).
Service Locators
Sometimes, a service needs access to several other services without being sure
that all of them will actually be used. In those cases, you may want the
instantiation of the services to be lazy. However, that's not possible using
the explicit dependency injection since services are not all meant to
be lazy (see Lazy Services).
A real-world example are applications that implement the Command pattern using a CommandBus to map command handlers by Command class names and use them to handle their respective command when it is asked for:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
// src/AppBundle/CommandBus.php
namespace AppBundle;
// ...
class CommandBus
{
/**
* @var CommandHandler[]
*/
private $handlerMap;
public function __construct(array $handlerMap)
{
$this->handlerMap = $handlerMap;
}
public function handle(Command $command)
{
$commandClass = get_class($command);
if (!isset($this->handlerMap[$commandClass])) {
return;
}
return $this->handlerMap[$commandClass]->handle($command);
}
}
// ...
$commandBus->handle(new FooCommand());Considering that only one command is handled at a time, instantiating all the other command handlers is unnecessary. A possible solution to lazy-load the handlers could be to inject the whole dependency injection container:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
// ...
use Symfony\Component\DependencyInjection\ContainerInterface;
class CommandBus
{
private $container;
public function __construct(ContainerInterface $container)
{
$this->container = $container;
}
public function handle(Command $command)
{
$commandClass = get_class($command);
if ($this->container->has($commandClass)) {
$handler = $this->container->get($commandClass);
return $handler->handle($command);
}
}
}However, injecting the entire container is discouraged because it gives too broad access to existing services and it hides the actual dependencies of the services.
Service Locators are intended to solve this problem by giving access to a set of predefined services while instantiating them only when actually needed.
Defining a Service Locator
First, define a new service for the service locator. Use its arguments
option to include as many services as needed to it and add the
container.service_locator tag to turn it into a service locator:
1 2 3 4 5 6 7 8 9
// app/config/services.yml
services:
app.command_handler_locator:
class: Symfony\Component\DependencyInjection\ServiceLocator
tags: ['container.service_locator']
arguments:
-
AppBundle\FooCommand: '@app.command_handler.foo'
AppBundle\BarCommand: '@app.command_handler.bar'Note
The services defined in the service locator argument must include keys, which later become their unique identifiers inside the locator.
Now you can use the service locator injecting it in any other service:
1 2 3 4
// app/config/services.yml
services:
AppBundle\CommandBus:
arguments: ['@app.command_handler_locator']Tip
If the service locator is not intended to be used by multiple services, it's better to create and inject it as an anonymous service.
Usage
Back to the previous CommandBus example, it looks like this when using the
service locator:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
// ...
use Psr\Container\ContainerInterface;
class CommandBus
{
/**
* @var ContainerInterface
*/
private $handlerLocator;
// ...
public function handle(Command $command)
{
$commandClass = get_class($command);
if (!$this->handlerLocator->has($commandClass)) {
return;
}
$handler = $this->handlerLocator->get($commandClass);
return $handler->handle($command);
}
}The injected service is an instance of ServiceLocator
which implements the PSR-11 ContainerInterface, but it is also a callable:
1 2 3 4 5
// ...
$locateHandler = $this->handlerLocator;
$handler = $locateHandler($commandClass);
return $handler->handle($command);
