[JAVA] About DI of Spring ①

Purpose of this article

DI: What is Dependency Injection? Continued Explanation of how Spring realizes DI

Scope of this article

Outline explanation of DI in Spring

DI review

--Reason for using DI ... I want to reduce the dependency between classes and improve reusability / testability. --How to realize ... Pass the class you want to use in a certain class with the interface type → Define the class to be actually passed separately.



--Bean ... Injection class --Bean definition ... Various properties set in the bean when injecting the bean --Bean definition file / Configuration ... A collection of Bean definitions (Java class, XML file) --DI container ... Set various beans and pass them to the other party when using them.

Outline of DI realization in Spring

Next time, detailed movements of each class ApplicationContext -** The central figure in realizing DI in Java ** --A class that has the function of a DI container --A class created by extending the BeanFactory class --There is a class that is further extended by the corresponding Configuration etc. --Multiple DI containers can be created for one application

Configuration --The Bean definition file --Pass this to the ApplicationContext class and generate a bean in the DI container -There are three methods: [Java-based], [xml-based], and [annotation-based]. ――Three can be used alone, but setting becomes easier by using the annotation base and others in combination. --Write the Bean definition in this

Bean --General class, basically no special description is required If --is an annotation-based Configuration, write an annotation separately. --You can also inject and nest beans inside the bean

ex) Describe the initialization condition for the bean of HogeService in hogeConfig and get the bean via ApplicationContext (Java base)


ApplicationContext ctx = new AnnotationConfigApplicationContext(HogeConfig.class);
HogeService service = ctx.getBean(HogeService.class)


public class HogeConfig {
	HogeService hogeService() {
		return new HogeService(); 

Is it like the following when expressed in a conceptual diagram? SpringにおけるDIの概要.jpg

I think it's relatively easy to understand if you swallow the concept.

Next time preview

We are planning to explain the specifications etc. when actually trying it.

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