首先我们来一个实例,上代码
<?phpclass A{public function test(){echo 'this is A!<br>';$b = new B();$b->test();}}class B{public function test(){echo 'this is B!<br>';$c = new C();$c->test();}}class C{public function test(){echo 'this is C!<br>';}}$obj = new A();$obj->test();结果是:
this is A!this is B!this is C!
从代码分析,A类依赖B类，B类依赖C类。这是我们最原始的实现思路.这种实现思路很明显会有问题
假如我们现在B类修改下，代码如下:
class B{public $name;public function __construct($name){$this->name = $name;}public function test(){echo 'this is B'.$this->name.'!<br>';$c = new C();$c->test();}}此时再看我们原来A类test方法直接调用明显会有问题,于是此时我们需要将原来代码:
class A{public function test(){echo 'this is A!<br>';$b = new B();$b->test();}}修改成:
class A{public function test(){echo 'this is A!<br>';$b = new B('(class B)');//构造的时候多加了一个参数$b->test();}}如果此时C类构造方法也变动了呢，B的方法里面也同样受影响,很明显可用性非常低
为了解耦，此时我们用到第二种思路:构造器注入,直接上代码:
<?phpclass A{public $obj;public function __construct(B $b){$this->obj = $b;}public function test(){echo 'this is A!<br>';$this->obj->test();}}class B{public $obj;public function __construct(C $c){$this->obj = $c;}public function test(){echo 'this is B!<br>';$this->obj->test();}}class C{public function test(){echo 'this is C!<br>';}}$c = new C();$b = new B($c);$obj = new A($b);$obj->test();这种方法可以解决第一种方法，如果依赖的类构造方法（比如B类）有所改动,A类不需要改动任何代码。但是久而久之，这种方法毛病也出来了，我们首先必须要先实例化C类,再实例化B类，最后再实例化A类。
了解到第二种方案需要优化，于是出现了基本成型的第三种方案，此时我们用到了container(容器)和instance(实例),直接上代码:
class Container{public static $_classes = [];public static $_definations = [];public function get($class){if(isset(self::$_classes[$class]))return self::$_classes[$class];//当前类所依赖的类$depends = [];$tc = new ReflectionClass($class);//得到构造方法$constructor = $tc->getConstructor();//得到构造方法的参数if($constructor !== NULL){foreach($constructor->getParameters() as $parameter){if($parameter->isDefaultValueAvailable()){$depends[] = $parameter->getDefaultValue();}else{$pc = $parameter->getClass();$instance = Instance::getInstance($pc == NULL ? NULL : $pc->getName());$depends[] = $instance;}}}foreach($depends as $k => $v){if($v instanceof Instance){if($v->id !== NULL){$depends[$k] = $this->get($v->id);}}}$tm_instance = $tc->newInstanceArgs($depends);self::$_classes[$class] = $tm_instance;return $tm_instance;}}class Instance{/** * @var 类唯一标示 */public $id;/** * 构造函数 * @param string $id 类唯一ID * @return void */public function __construct($id){$this->id = $id;}/** * 获取类的实例 * @param string $id 类唯一ID * @return Object Instance */public static function getInstance($id){return new self($id);}}class Base{/** * 魔术方法 * @param string $name * @param string $value * @return void */public function __set($name, $value){$this->{$name} = $value;}}class A extends Base{private $instanceB;public function __construct(B $instanceB){$this->instanceB = $instanceB;}public function test(){echo 'this is A!<br/>';$this->instanceB->test();}}class B extends Base{private $instanceC;public function __construct(C $instanceC){$this->instanceC = $instanceC;}public function test(){echo 'this is B!<br/>';return $this->instanceC->test();}}class C extends Base{public function test(){echo 'this is C!';}}$container = new Container();$obj_a = $container->get('A');$obj_a->test();此方法有参考yii2中yii2\di\container实现，只是将它简化了，更容易看懂。重点看看container的get方法
php依赖注入
原文地址：http://blog.51cto.com/chinalx1/2106072