What is dependency injection (DI)

DI The full name is Dependency Injection, It's called dependency injection in Chinese .

It is related to the reversal of control (IoC) It means the same thing , It's just that these two terms describe the same concept from two perspectives . For one Spring For beginners , These two terms are difficult to understand , Now we will describe these two concepts in simple language .

When a Java object ( caller ) You need to call another Java object ( Callees , That is, the dependent object ) when , In the traditional mode , Callers usually use “new Callees ” Code to create objects , Pictured 1 Shown . This approach will lead to increased coupling between the caller and the callee , It is not conducive to the upgrading and maintenance of later projects .

chart 1 The caller creates the callee object

In the use of Spring After the framework , An instance of an object is no longer created by the caller , But by the Spring Container to create ,Spring The container is responsible for controlling the relationship between programs , Instead of being directly controlled by the caller's program code . such , Control has shifted from application code to Spring Containers , There is a reversal of control , This is it. Spring The reversal of control .

from Spring From the perspective of containers ,Spring The container is responsible for assigning the dependent object to the caller's member variable , This is equivalent to injecting the caller with the instance it depends on , This is it. Spring Dependency injection of , Pictured 2 Shown .

chart 2 Inject the callee object into the caller object