Dependency Injection-依賴注入詳解

依賴注入是目前很多優秀框架都在使用的一個設計模式。Java的開發框架如Spring在用，PHP的Laravel/Phalcon/Symfony等也在用。好多不同語言的框架，設計思想大同小異，相互借鑒參考。熟悉了一個語言的開發框架，其它不同的框架甚至不同語言的開發框架，往往也很容易從設計理念和概念上理解。不過，有些語言因為設計特色，一些設計模式反而看似消失不見了。其實是融入了語言里面，不易察覺。我看見過這么一句話：“設計模式是編程語言固有缺陷的產物”。有一個討論在這里：Why is IoC / DI not common in Python?

Dependency Injection 常常簡稱為：DI。它是實現控制反轉（Inversion of Control – IoC）的一個模式。有一本依賴注入詳解的書在這里:Dependency Injection 。它的本質目的是解耦，保持軟件組件之間的松散耦合，為設計開發帶來靈活性。

這里借用一套PHP代碼的演化過程，解釋依賴注入模式的出現過程。代碼來自Phalcon框架文檔。個人感覺，從演化出發，最能達成理解的目標，就如同數學推理一樣讓人信服，自然而然。想當年，我研究Windows時代的COM技術體系，看到有一本書也是這么做的 – Dan Box的《COM本質論》第1-2章，闡述了從Dll到COM組件的設計過程。

假設我們要開發一套組件，這個組件干啥目前并不重要，不過它需要連接數據庫。最簡單的實現，當然是把數據庫的配置信息寫在組件里面。

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();

但是這么干問題很大，屬于“代碼的壞味道”。因為數據庫配置寫死了，完全沒有靈活性可言。這也給后面的測試/部署/安全帶來了隱患。為了解決這個問題，我們試試把配置信息拿出去，從外面傳進來。

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", ????] );

好一點。不過如果我們在很多地方都要用這個組件，那么意味著每次用的時候，都要創建這么一個連接配置對象，不僅冗余，而且難以變更和管理。我們把這個連接配置對象單獨放在一個地方管理，DRY原則。

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。不過有個問題，連接對象每次使用都是重復創建，浪費資源。再改一下，改成共享式，近似于單件模式。

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() );

這就是“依賴注入”模式了，它解決了組件的依賴項和組件之間的過度耦合問題。不過還有個麻煩：如果這個組件依賴項很多怎么辦？每次都要創建并設置一大堆依賴項。

//?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);

每次使用這個組件，都要創建一堆附加的依賴項。如果以后我們修改組件依賴，那么必須挨個改掉。代碼的壞味道又來了。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); ????} }

估計好多人走到這一步就會停下腳步了。代碼用個工廠模式不就行了嘛。可是你對比下開頭的代碼，組件和它的依賴項的耦合不就又來了么？現在，問題又回到開頭了。

一個更好的辦法是使用依賴注入容器。它就如同一個全局的注冊表，像橋一樣獲取依賴項，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();”

問題解決。獲取依賴項只要通過DI容器接口操作，不需要的部分甚至都不會創建，節約了資源。

在Java的Spring框架里面，依賴注入和控制反轉設計思想是近似的，道理相同但是實現不同。因為編程語言各有各的設計特點可以利用。

Spring框架的依賴注入容器接口是：ApplicationContext.

ApplicationContext?context ??=?new?ClassPathXmlApplicationContext("applicationContext.xml");

但是Spring使用DI，有好幾種方法，比如注解式，利用了語言的功能。

具體解釋參考這篇文章，不翻譯了。

Intro to Inversion of Control and Dependency Injection with Spring

容器 Spring

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