laravel的自动注入

laravel里，当我们在controller 的构造方法，或者实例方法中用类型指定一个参数时，系统可以自动帮你把该参数的实例注入进去。

那么内部是如何实现的呢？

先列出几个技术点

1.类ReflectionClass

   该类用于对指定类进行反射，提取类信息。

2.ReflectionClass.getConstructor

   获取指定类的构造方法，返回ReflectionMethod

3.ReflectionMethod.getParameters

   获取方法的参数信息

4.ReflectionClass.newInstanceArgs

  创建类实例

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当，laravel通过路由寻找到controller的类名时，会先创建实例，如果构造函数里包含指定了类型的参数，并且router设定里并没有传递时，

就会创建这个参数的实例，当然这个参数的构造方法如果还包含指定类型的参数时，还是会创建，一直递归到没有参数指定为止。

入口代码在类Illuminate\Container\Containerd的方法make里

    public function make($abstract, array $parameters = [])
    {
        $abstract = $this->getAlias($this->normalize($abstract));

        // If an instance of the type is currently being managed as a singleton we'll
        // just return an existing instance instead of instantiating new instances
        // so the developer can keep using the same objects instance every time.
        if (isset($this->instances[$abstract])) {
            return $this->instances[$abstract];
        }

        $concrete = $this->getConcrete($abstract);

        // We're ready to instantiate an instance of the concrete type registered for
        // the binding. This will instantiate the types, as well as resolve any of
        // its "nested" dependencies recursively until all have gotten resolved.
        if ($this->isBuildable($concrete, $abstract)) {
            $object = $this->build($concrete, $parameters);
        } else {
            $object = $this->make($concrete, $parameters);
        }

        // If we defined any extenders for this type, we'll need to spin through them
        // and apply them to the object being built. This allows for the extension
        // of services, such as changing configuration or decorating the object.
        foreach ($this->getExtenders($abstract) as $extender) {
            $object = $extender($object, $this);
        }

        // If the requested type is registered as a singleton we'll want to cache off
        // the instances in "memory" so we can return it later without creating an
        // entirely new instance of an object on each subsequent request for it.
        if ($this->isShared($abstract)) {
            $this->instances[$abstract] = $object;
        }

        $this->fireResolvingCallbacks($abstract, $object);

        $this->resolved[$abstract] = true;

        return $object;
    }

这个地方的核心代码是在类Illuminate\Container\Containerd的方法build里

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 (! $reflector->isInstantiable()) {
            if (! empty($this->buildStack)) {
                $previous = implode(', ', $this->buildStack);

                $message = "Target [$concrete] is not instantiable while building [$previous].";
            } else {
                $message = "Target [$concrete] is not instantiable.";
            }

            throw new BindingResolutionException($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);
    }