Handlers should be lazy loaded

src/Symfony/Bundle/FrameworkBundle/Resources/config/message.xml

@@ -7,20 +7,20 @@
     <services>
         <defaults public="false" />
 
-        <!-- Bus -->
+        <!-- Message Bus -->
         <service id="message_bus" class="Symfony\Component\Message\MessageBus" public="true">
             <argument type="collection" /> <!-- Middlewares -->
         </service>
 
         <service id="Symfony\Component\Message\MessageBusInterface" alias="message_bus" />
 
         <!-- Handlers -->
-        <service id="message.handler_resolver" class="Symfony\Component\Message\HandlerLocator">
-            <argument type="collection" /> <!-- Message to handler mapping -->
+        <service id="message.container_message_handler_resolver" class="Symfony\Component\Message\ContainerHandlerLocator">
+            <argument type="service" id="service_container"/>
         </service>
 
         <service id="message.middleware.call_message_handler" class="Symfony\Component\Message\Middleware\HandleMessageMiddleware">
-            <argument type="service" id="message.handler_resolver" />
+            <argument type="service" id="message.container_message_handler_resolver" />
 
             <tag name="message_middleware" priority="-10" />
         </service>

src/Symfony/Component/Message/ContainerHandlerLocator.php

@@ -0,0 +1,43 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Message;
+
+use Psr\Container\ContainerInterface;
+use Symfony\Component\Message\Exception\NoHandlerForMessageException;
+
+/**
+ * @author Miha Vrhovnik <miha.vrhovnik@gmail.com>
+ */
+class ContainerHandlerLocator implements HandlerLocatorInterface
+{
+    /**
+     * @var ContainerInterface
+     */
+    private $serviceLocator;
+
+    public function __construct(ContainerInterface $serviceLocator)
+    {
+        $this->serviceLocator = $serviceLocator;
+    }
+
+    public function resolve($message): callable
+    {
+        $messageKey = get_class($message);
+
+        if (!$this->serviceLocator->has($messageKey)) {
+            throw new NoHandlerForMessageException(sprintf('No handler for message "%s"', $messageKey));
+        }
+
+        return $this->serviceLocator->get($messageKey);
+    }
+
+}

src/Symfony/Component/Message/DependencyInjection/MessagePass.php

@@ -13,9 +13,12 @@
 
 use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
 use Symfony\Component\DependencyInjection\Compiler\PriorityTaggedServiceTrait;
+use Symfony\Component\DependencyInjection\Compiler\ServiceLocatorTagPass;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Definition;
 use Symfony\Component\DependencyInjection\Exception\RuntimeException;
 use Symfony\Component\DependencyInjection\Reference;
+use Symfony\Component\Message\Handler\MessageHandlerCollection;
 
 /**
  * @author Samuel Roze <samuel.roze@gmail.com>
@@ -29,7 +32,7 @@ class MessagePass implements CompilerPassInterface
     private $messageHandlerResolverService;
     private $handlerTag;
 
-    public function __construct(string $messageBusService = 'message_bus', string $middlewareTag = 'message_middleware', string $messageHandlerResolverService = 'message.handler_resolver', string $handlerTag = 'message_handler')
+    public function __construct(string $messageBusService = 'message_bus', string $middlewareTag = 'message_middleware', string $messageHandlerResolverService = 'message.container_message_handler_resolver', string $handlerTag = 'message_handler')
     {
         $this->messageBusService = $messageBusService;
         $this->middlewareTag = $middlewareTag;
@@ -53,11 +56,10 @@ public function process(ContainerBuilder $container)
         $busDefinition = $container->getDefinition($this->messageBusService);
         $busDefinition->replaceArgument(0, $middlewares);
 
-        $handlerResolver = $container->getDefinition($this->messageHandlerResolverService);
-        $handlerResolver->replaceArgument(0, $this->findHandlers($container));
+        $this->registerHandlers($container);
     }
 
-    private function findHandlers(ContainerBuilder $container): array
+    private function registerHandlers(ContainerBuilder $container)
     {
         $handlersByMessage = array();
 
@@ -79,7 +81,23 @@ private function findHandlers(ContainerBuilder $container): array
             $handlersByMessage[$message] = call_user_func_array('array_merge', $handlersByMessage[$message]);
         }
 
-        return $handlersByMessage;
+
+        $definitions = array();
+        foreach ($handlersByMessage as $message => $handlers) {
+            if (1 === count($handlers)) {
+                $handlersByMessage[$message] = current($handlers);
+            } else {
+                $d = new Definition(MessageHandlerCollection::class, array($handlers));
+                $d->setPrivate(true);
+                $serviceId = hash('sha1', $message);
+                $definitions[$serviceId] = $d;
+                $handlersByMessage[$message] = new Reference($serviceId);
+            }
+        }
+        $container->addDefinitions($definitions);
+
+        $handlerResolver = $container->getDefinition($this->messageHandlerResolverService);
+        $handlerResolver->replaceArgument(0, ServiceLocatorTagPass::register($container, $handlersByMessage));
     }
 
     private function guessHandledClass(ContainerBuilder $container, string $serviceId): string