Dependency injection

第一次接触Dependency injection，是在学习spring的时候，正如Spring is flexible中所说

Spring’s flexible and comprehensive set of extensions and third-party libraries let developers build almost any application imaginable. At its core, Spring Framework’s Inversion of Control (IoC) and Dependency Injection (DI) features provide the foundation for a wide-ranging set of features and functionality. Whether you’re building secure, reactive, cloud-based microservices for the web, or complex streaming data flows for the enterprise, Spring has the tools to help.

SOLID中的D所指为Dependency inversion principle，本文将要进行描述的是Dependency injection。

维基百科Dependency injection

In software engineering, dependency injection is a technique in which an object receives other objects that it depends on. These other objects are called dependencies. In the typical "using" relationship[1] the receiving object is called a client and the passed (that is, "injected") object is called a service. The code that passes the service to the client can be many kinds of things and is called the injector. Instead of the client specifying which service it will use, the injector tells the client what service to use. The "injection" refers to the passing of a dependency (a service) into the object (a client) that would use it.

NOTE: 上面这段话已经描述清楚了dependency injection的含义了。