Dependency injection

依赖注入

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This article is about article is about the computing process. For other uses,see DI.

Dependency injection (DI) in Computer programming refers to the process of supplying an external dependency to a software component. It is a specific form of inversion of control where the concern being inverted is the process of obtaining the needed dependency.

@H_452_502@计算机程序设计中的依赖注入（DI）@H_452_502@指的是将一个外部依赖项提供给一个软件组件的过程。这是一种特殊形式的控制反转，其中反转的关注点是获得需要的依赖项的处理。

Conventionally,if an object needs to gain access to a particular service,the object takes responsibility to get hold of that service: either it holds a direct reference to the location of that service,or it goes to a known 'service locator' and requests that it be passed back a reference to an implementation of a specified type of service. By contrast,using dependency injection,the object simply provides a property that can hold a reference to that type of service; and when the object is created a reference to an implementation of that type of service will automatically be injected into that property - by an external mechanism.

@H_452_502@通常，如果一个对象要获得对某个特定服务的访问，对象自己将负责获得这个服务：要么此对象有对这个服务位置的直接引用，要么此对象向一个已知的‘服务定位器’发出请求以得到一个对于特定类型服务实现的引用。与之相对，使用依赖注入，对象只需要简单的提供一个类型，这个类型包含找到某种服务的引用，当对象被创建时，这个引用就可以被一个外部机制自动注入此对象的这个属性里.

When the dependency injection technique is used to decouple high-level modules from low-level services,the resulting design guideline is called the Dependency inversion principle.

在依赖注入技术开始被用于将高层模块与底层服务解耦的过程中，产生了被称为“依赖倒置原则”的设计准则

The dependency injection approach offers more flexibility because it becomes easier to create alternative implementations of a given service type,and then to specify which implementation is to be used via a configuration file,without any change to the objects that use the service. This is especially useful in unit testing,because it is easy to inject a mock implementation of a service into the object being tested. On the other hand,excessive use of dependency injection can make applications more complex and harder to maintain: in order to understand the application's behavIoUr the developer needs to look at the configuration as well as the code,and the configuration is "invisible" to IDE-supported reference analysis and refactoring unless the IDE specifically supports the dependency injection framework.

@H_452_502@依赖注入使设计更加灵活。我们可以更容易创建给定服务类型的可选实现，并无需改动使用此服务的对象，仅通过配置文件就可以指定使用哪一个实现。由于可以非常容易的把一个模仿实现注入到被测试对象中，依赖注入技术在单元测试中非常有用。但是，过多的使用依赖注入将会令应用程序更加复杂而难于维护：为了理解应用程序的行为，除了阅读代码，开发人员还必须察看配置文件，然而，配置文件往往对于IDE@H_452_502@提供的引用分析和重构机制不可见（除非IDE@H_452_502@对依赖注入框架提供了特别的支持）。

Contents

目录

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A code illustration using Java

JAVA@H_452_502@代码示例

Suppose that IFoo is an interface:

@H_452_502@假设IFoo@H_452_502@是一个接口

public interface IFoo
{
void bar(); // Perform bar
void baz(); // Perform baz
}

There exist also a number of implementation classes,each of them implementing IFoo in some way:

@H_452_502@还有一些实现类，每一个都用不同方式实现了IFoo@H_452_502@：

public class DatabaseFoo
implements IFoo
{
void bar()
{
Database.selectBar().execute(); // Use the database to do bar
}

void baz()
{
Database.selectBaz().run(); // Use the database to do baz
}
}

public class PixieDustFoo
implements IFoo
{
void bar()
{
Spell.cast("bar"); // Magic!
}

void baz()
{
Spell.cast("baz"); // Magic!
}
}

public class EnterpriseFoo
implements IFoo
{
void bar()
{
EnterpriseFactoryObserverFactoryCreator efofc = new EnterpriseFactoryObserverFactoryCreator("bar");
efofc.creatify();
efofc.preparify();
efofc.configurise();
efofc.makeAwardWinning();
efofc.opportunities.leverage();
}

void baz()
{
EnterpriseFactoryObserverFactoryCreator efofc = new EnterpriseFactoryObserverFactoryCreator("baz");
efofc.creatify();
efofc.preparify();
efofc.configurise();
efofc.makeAwardWinning();
efofc.opportunities.leverage();
}
}

IFoo only specifies the operations available in its interface,but doesn't itself provide any implementation,instead leaving that to other implementer classes. This way a user wishing to use the IFoo functionality can use any implementation,not knowing anything more about them than that they conform to the Foo interface.

IFoo@H_452_502@仅仅指定了这个接口可以进行的操作，但并没有提供任何实现，而是留给了其它实现者的类。这样当一个用户希望使用IFoo@H_452_502@功能，就可以使用任何一个实现，除了知道它们都遵守IFoo@H_452_502@接口外，不需要事前知道任何更多的细节。

An object needing the services defined by IFoo needs to get an instance of a class that implements IFoo:

@H_452_502@一个需要使用IFoo@H_452_502@定义的服务的对象，需要得到一个实现IFoo@H_452_502@的类的实例：

public class ImportantClass {
IFoo foo;

public ImportantClass()
{
this.foo = new EnterpriseFoo();
}

void doReallyImportantStuff()
{
this.foo.bar();
}
}

However,this defeats the entire point of using an interface instead of a concrete implementation. To fix that,it's enough to let the outside caller provide the desired implementation:

但是，这完全破坏了使用接口而不是一个具体实现的目标。为修正这个问题，让外部调用者提供所需的实现就足可以满足目标了。

public ImportantClass(IFoo foo)
{
this.foo = foo;
}

When using dependency injection there is usually a configuration mechanism or architecture for deciding which implementation gets injected into an object.

当使用依赖注入时，通常会有一个配置机制或是某个特定架构，以决定哪个实现将被注入到一个对象中。

Forms of Dependency Injection

依赖注入的几种形式

Martin Fowler identifies three ways in which an object can get a reference to an external module,according to the pattern used to provide the dependency:

@H_452_502@依照提供依赖项时使用的模式，Martin Fowler@H_452_502@定义了三种对象可以从得到一个外部模块引用的方式：

The open source Yan Container[1] provides support for arbitrary types of injection[2] besides the common ones defined above.

@H_452_502@除了前列的几种普通形式外，开源项目Yan Container@H_452_502@还对任意形式的注入提供了支持。

Existing frameworks

现有框架

Dependency injection frameworks exist for a number of platforms and languages including:

在一些平台和语言中都有依赖注入框架，包括：

ActionScript

C++

ColdFusion

Delphi

Java

Java 2 Micro Edition

.NET

PHP4

PHP5

Perl

Python

Ruby

See also

参见

External links

外部链接

Retrieved from "http://en.wikipedia.org/wiki/Dependency_injection"

Categories: Software components | Software architecture

Hidden categories: Articles to be merged since June 2008 | Articles lacking in-text citations | All articles with unsourced statements | Articles with unsourced statements since July 2008

源文档 <http://en.wikipedia.org/wiki/Dependency_injection>