依赖注入是目前很多优秀框架都在使用的一个设计模式。Java的开发框架如Spring在用，PHP的Laravel/Phalcon/Symfony等也在用。好多不同语言的框架，设计思想大同小异，相互借鉴参考。熟悉了一个语言的开发框架，其它不同的框架甚至不同语言的开发框架，往往也很容易从设计理念和概念上理解。不过，有些语言因为设计特色，一些设计模式反而看似消失不见了。其实是融入了语言里面，不易察觉。我看见过这么一句话：“设计模式是编程语言固有缺陷的产物”。有一个讨论在这里：Why is IoC / DI not common in Python?

Dependency Injection 常常简称为：DI。它是实现控制反转(Inversion of Control – IoC)的一个模式。有一本依赖注入详解的书在这里:Dependency Injection 。它的本质目的是解耦，保持软件组件之间的松散耦合，为设计开发带来灵活性。

这里借用一套PHP代码的演化过程，解释依赖注入模式的出现过程。代码来自Phalcon框架文档。个人感觉，从演化出发，最能达成理解的目标，就如同数学推理一样让人信服，自然而然。想当年，我研究Windows时代的COM技术体系，看到有一本书也是这么做的 – Dan Box的《COM本质论》第1-2章，阐述了从Dll到COM组件的设计过程。

假设我们要开发一套组件，这个组件干啥目前并不重要，不过它需要连接数据库。最简单的实现，当然是把数据库的配置信息写在组件里面。

class SomeComponent

{

/**

* The instantiation of the connection is hardcoded inside

* the component, therefore it's difficult replace it externally

* or change its behavior

*/

public function someDbTask()

{

$connection = new Connection(

[

"host" => "localhost",

"username" => "root",

"password" => "secret",

"dbname" => "invo",

]

);

// ...

}

}

$some = new SomeComponent();

$some->someDbTask();

但是这么干问题很大，属于“代码的坏味道”。因为数据库配置写死了，完全没有灵活性可言。这也给后面的测试/部署/安全带来了隐患。为了解决这个问题，我们试试把配置信息拿出去，从外面传进来。

class SomeComponent

{

protected $_connection;

/**

* Sets the connection externally

*/

public function setConnection($connection)

{

$this->_connection = $connection;

}

public function someDbTask()

{

$connection = $this->_connection;

// ...

}

}

$some = new SomeComponent();

// Create the connection

$connection = new Connection(

[

"host" => "localhost",

"username" => "root",

"password" => "secret",

"dbname" => "invo",

]

);

好一点。不过如果我们在很多地方都要用这个组件，那么意味着每次用的时候，都要创建这么一个连接配置对象，不仅冗余，而且难以变更和管理。我们把这个连接配置对象单独放在一个地方管理，DRY原则。

class Registry

{

/**

* Returns the connection

*/

public static function getConnection()

{

return new Connection(

[

"host" => "localhost",

"username" => "root",

"password" => "secret",

"dbname" => "invo",

]

);

}

}

class SomeComponent

{

protected $_connection;

/**

* Sets the connection externally

*/

public function setConnection($connection)

{

$this->_connection = $connection;

}

public function someDbTask()

{

$connection = $this->_connection;

// ...

}

}

$some = new SomeComponent();

// Pass the connection defined in the registry

$some->setConnection(Registry::getConnection());

$some->someDbTask();

可行。不过有个问题，连接对象每次使用都是重复创建，浪费资源。再改一下，改成共享式，近似于单件模式。

class Registry

{

protected static $_connection;

/**

* Creates a connection

*/

protected static function _createConnection()

{

return new Connection(

[

"host" => "localhost",

"username" => "root",

"password" => "secret",

"dbname" => "invo",

]

);

}

/**

* Creates a connection only once and returns it

*/

public static function getSharedConnection()

{

if (self::$_connection === null) {

self::$_connection = self::_createConnection();

}

return self::$_connection;

}

/**

* Always returns a new connection

*/

public static function getNewConnection()

{

return self::_createConnection();

}

}

class SomeComponent

{

protected $_connection;

/**

* Sets the connection externally

*/

public function setConnection($connection)

{

$this->_connection = $connection;

}

/**

* This method always needs the shared connection

*/

public function someDbTask()

{

$connection = $this->_connection;

// ...

}

/**

* This method always needs a new connection

*/

public function someOtherDbTask($connection)

{

}

}

$some = new SomeComponent();

// This injects the shared connection

$some->setConnection(

Registry::getSharedConnection()

);

$some->someDbTask();

// Here, we always pass a new connection as parameter

$some->someOtherDbTask(

Registry::getNewConnection()

);

这就是“依赖注入”模式了，它解决了组件的依赖项和组件之间的过度耦合问题。不过还有个麻烦：如果这个组件依赖项很多怎么办？每次都要创建并设置一大堆依赖项。

// Create the dependencies or retrieve them from the registry

$connection = new Connection();

$session = new Session();

$fileSystem = new FileSystem();

$filter = new Filter();

$selector = new Selector();

// Pass them as constructor parameters

$some = new SomeComponent($connection, $session, $fileSystem, $filter, $selector);

// ... Or using setters

$some->setConnection($connection);

$some->setSession($session);

$some->setFileSystem($fileSystem);

$some->setFilter($filter);

$some->setSelector($selector);

每次使用这个组件，都要创建一堆附加的依赖项。如果以后我们修改组件依赖，那么必须挨个改掉。代码的坏味道又来了。再改。

class SomeComponent

{

// ...

/**

* Define a factory method to create SomeComponent instances injecting its dependencies

*/

public static function factory()

{

$connection = new Connection();

$session = new Session();

$fileSystem = new FileSystem();

$filter = new Filter();

$selector = new Selector();

return new self($connection, $session, $fileSystem, $filter, $selector);

}

}

估计好多人走到这一步就会停下脚步了。代码用个工厂模式不就行了嘛。可是你对比下开头的代码，组件和它的依赖项的耦合不就又来了么？现在，问题又回到开头了。

一个更好的办法是使用依赖注入容器。它就如同一个全局的注册表，像桥一样获取依赖项，并解耦。

use Phalcon\Di;

use Phalcon\DiInterface;

class SomeComponent

{

protected $_di;

public function __construct(DiInterface $di)

{

$this->_di = $di;

}

public function someDbTask()

{

// Get the connection service

// Always returns a new connection

$connection = $this->_di->get("db");

}

public function someOtherDbTask()

{

// Get a shared connection service,

// this will return the same connection every time

$connection = $this->_di->getShared("db");

// This method also requires an input filtering service

$filter = $this->_di->get("filter");

}

}

$di = new Di();

// Register a "db" service in the container

$di->set(

"db",

function () {

return new Connection(

[

"host" => "localhost",

"username" => "root",

"password" => "secret",

"dbname" => "invo",

]

);

}

);

// Register a "filter" service in the container

$di->set(

"filter",

function () {

return new Filter();

}

);

// Register a "session" service in the container

$di->set(

"session",

function () {

return new Session();

}

);

// Pass the service container as unique parameter

$some = new SomeComponent($di);

$some->someDbTask();”

问题解决。获取依赖项只要通过DI容器接口操作，不需要的部分甚至都不会创建，节约了资源。

在Java的Spring框架里面，依赖注入和控制反转设计思想是近似的，道理相同但是实现不同。因为编程语言各有各的设计特点可以利用。

Spring框架的依赖注入容器接口是：ApplicationContext.

ApplicationContext context

= new ClassPathXmlApplicationContext("applicationContext.xml");

但是Spring使用DI，有好几种方法，比如注解式，利用了语言的功能。

具体解释参考这篇文章，不翻译了。