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。
Intro to Inversion of Control and Dependency Injection with Spring
