Autowire not working in Symfony's Dependency Injection component - php

Question

I am using Symfony's Dependency Injection component version 3.4 in my custom PHP project. My project is running on PHP 5.6
"symfony/dependency-injection": "^3.4"
I have defined my services.yaml file to contain following service definitions
logger:
class: Monolog\Logger
arguments: ["application"]
autowire: true
public: true
Monolog\Logger: '#logger'
plugin_context:
class: MyProject\PluginContext
autowire: true
public: true
I can confirm that the autoloading is working and the instance of both classes are present in the definition, but the Logger class is not autowired in PluginContext constructor. the class is defined in the following code
use Monolog\Logger;
class PluginContext
{
private $logger;
function __construct(Logger $logger) {
$this->logger = $logger;
}
}
When the following code is run, PHP throws an exception
$container->get("plugin_context");
Catchable fatal error: Argument 1 passed to MyProject\PluginContext::__construct() must be an instance of Monolog\Logger, none given

Answer 1

Change your FQCN $logger and use this one use Psr\Log\LoggerInterface instead Monolog\Logger
Another thing, thanks to autowiring you don't need to specify anything in service.yaml except this (the default configuration):
_defaults:
autowire: true # Automatically injects dependencies in your services.
autoconfigure: true # Automatically registers your services as commands, event subscribers, etc.
public: false # Allows optimizing the container by removing unused services; this also means
# fetching services directly from the container via $container->get() won't work.
# The best practice is to be explicit about your dependencies anyway.
# makes classes in src/ available to be used as services
# this creates a service per class whose id is the fully-qualified class name
App\:
resource: '../src/*'
exclude: '../src/{Entity,Migrations,Tests,Kernel.php}'
The Doc said : « Aliases are used by the core bundles to allow services to be autowired. For example, MonologBundle creates a service whose id is logger. But it also adds an alias: Psr\Log\LoggerInterface that points to the logger service. This is why arguments type-hinted with Psr\Log\LoggerInterface can be autowired » so in your case the Psr\Log\LoggerInterface is an alias for Monolog https://symfony.com/doc/current/service_container/autowiring.html#using-aliases-to-enable-autowiring

Answer 2

It seems that the either the contents of services.yaml are not full.
Your services file should be like this
services:
logger:
class: Monolog\Logger
arguments: ["application"]
autowire: true
public: true
Monolog\Logger: '#logger'
plugin_context:
class: MyProject\PluginContext
autowire: true
public: true