Laravel构造函数自动依赖注入的例子

在Laravel的构造函数中可以实现自动依赖注入，而不需要实例化之前先实例化需要的类，如代码所示：

namespace LioHttpControllersForum;
 
use LioForumRepliesReplyRepository;
use LioForumThreadsThreadCreator;
use LioForumThreadsThreadCreatorListener;
use LioForumThreadsThreadDeleterListener;
use LioForumThreadsThreadForm;
use LioForumThreadsThreadRepository;
use LioForumThreadsThreadUpdaterListener;
use LioHttpControllersController;
use LioTagsTagRepository;
 
 
class ForumThreadsController extends Controller implements ThreadCreatorListener, ThreadUpdaterListener, ThreadDeleterListener
{
    protected $threads;
    protected $tags;
    protected $currentSection;
    protected $threadCreator;
 
    public function __construct(
        ThreadRepository $threads,
        ReplyRepository $replies,
        TagRepository $tags,
        ThreadCreator $threadCreator
    ) {
        $this->threads = $threads;
        $this->tags = $tags;
        $this->threadCreator = $threadCreator;
        $this->replies = $replies;
    }
}

注意构造函数中的几个类型约束，其实并没有地方实例化这个Controller并把这几个类型的参数传进去，Laravel会自动检测类的构造函数中的类型约束参数，并自动识别是否初始化并传入。

源码vendor/illuminate/container/Container.php中的build方法：

$constructor = $reflector->getConstructor();
dump($constructor);
这里会解析类的构造函数，在这里打印看：

reflection_construct

它会找出构造函数的参数，再看完整的build方法进行的操作：

public function build($concrete, array $parameters = [])
{
    // If the concrete type is actually a Closure, we will just execute it and
    // hand back the results of the functions, which allows functions to be
    // used as resolvers for more fine-tuned resolution of these objects.
    if ($concrete instanceof Closure) {
        return $concrete($this, $parameters);
    }
 
    $reflector = new ReflectionClass($concrete);
 
    // If the type is not instantiable, the developer is attempting to resolve
    // an abstract type such as an Interface of Abstract Class and there is
    // no binding registered for the abstractions so we need to bail out.
    if (! $reflector->isInstantiable()) {
        $message = "Target [$concrete] is not instantiable.";
 
        throw new BindingResolutionContractException($message);
    }
 
    $this->buildStack[] = $concrete;
 
    $constructor = $reflector->getConstructor();
 
    // If there are no constructors, that means there are no dependencies then
    // we can just resolve the instances of the objects right away, without
    // resolving any other types or dependencies out of these containers.
    if (is_null($constructor)) {
        array_pop($this->buildStack);
 
        return new $concrete;
    }
 
    $dependencies = $constructor->getParameters();
 
    // Once we have all the constructor's parameters we can create each of the
    // dependency instances and then use the reflection instances to make a
    // new instance of this class, injecting the created dependencies in.
    $parameters = $this->keyParametersByArgument(
        $dependencies, $parameters
    );
 
    $instances = $this->getDependencies(
        $dependencies, $parameters
    );
 
    array_pop($this->buildStack);
 
    return $reflector->newInstanceArgs($instances);
}

具体从容器中获取实例的方法：

protected function resolveClass(ReflectionParameter $parameter)
{
    try {
        return $this->make($parameter->getClass()->name);
    }
 
    // If we can not resolve the class instance, we will check to see if the value
    // is optional, and if it is we will return the optional parameter value as
    // the value of the dependency, similarly to how we do this with scalars.
    catch (BindingResolutionContractException $e) {
        if ($parameter->isOptional()) {
            return $parameter->getDefaultValue();
        }
 
        throw $e;
    }
}

框架底层通过Reflection反射为开发节省了很多细节，实现了自动依赖注入。这里不做继续深入研究了。

写了一个模拟这个过程的类测试：

 
class kulou
{
    //
}
 
class junjun
{
    //
}
 
class tanteng
{
    private $kulou;
    private $junjun;
    public function __construct(kulou $kulou,junjun $junjun)
    {
        $this->kulou = $kulou;
        $this->junjun = $junjun;
    }
}
 
//$tanteng = new tanteng(new kulou(),new junjun());
$reflector = new ReflectionClass('tanteng');
$constructor = $reflector->getConstructor();
$dependencies = $constructor->getParameters();
print_r($dependencies);exit;
原理是通过ReflectionClass类解析类的构造函数，并且取出构造函数的参数，从而判断依赖关系，从容器中取，并自动注入