[JAVA] Introduction to Spring Boot ① ~ DI ~

In advancing the learning of Spring, I will summarize the information I searched for in the teaching materials and online. I'm almost inexperienced in frameworks, so it's super introductory.

Before you start

First of all, what is "Spring Boot"? The scale of "Spring Framework" has become huge, and it has become difficult to understand how to use it. That's where "Spring Boot" came in.

--No need for complicated XML configuration files --Includes web container such as Tomcat

It is a framework that makes it possible to quickly create and execute Spring-based applications by taking advantage of such features. Please understand that it is an easy application creation using "Spring Framework".

Now, let's acquire the knowledge necessary to create a Spring application one by one.

DI(Dependency Injection)

DI is "dependency injection" in Japanese. In terms of meaning, it should be understood as "injecting an object". By injecting dependencies (objects) from outside the class, it has the purpose of resolving the dependencies between classes.

DI container

It is a framework to realize DI. Create and manage instances via DI containers. In addition to resolving class dependencies, you can also get the following side effects:

-The scope of the instance can be controlled. → Whether to generate it every time or to make it a singleton, etc. -Event control is possible for the life cycle of the instance. → Event processing at the time of generation and destruction, etc. -Common processing can be embedded. -Because the objects are loosely coupled, it is easier to test.

Bean definition

Defines a configuration file for the DI container to manage the instance. There are three setting methods.

This time we will look at the settings in annotation.


Bean definition is done by adding annotation to JAVA source. Important annotations

@Component @ComponentScan @Autowired

Will be.


It is an annotation that registers the class with Component annotation in the DI container. Same package as the class with ComponentScan annotation Scans the following classes and registers them in the DI container.


public class SpringSampleApplication {

	public static void main(String[] args) {
		ConfigurableApplicationContext context = SpringApplication.run(SpringSampleApplication.class, args);
		SampleB sampleb = context.getBean(SampleB.class);

@SpringBootApplication is running @Configuration, @EnableAutoConfiguration, @ComponentScan. In the sample code, annotations are intentionally added separately.


Attach it to the class to inject. If you add the Component annotation to a class, the DI container will manage the class for you.


public class SampleA {
	public String getClassName() {
		return this.getClass().getName();

A component that only returns its own class name.


Attach to the field to be injected. The DI container looks for matching objects and injects them into the fields annotated with Autowired.


public class SampleB {

	private SampleA sampleA;
	public void testDi() {
		System.out.println("called " + sampleA.getClassName());

Have the sampleA instance injected from the DI container. Now you can get the class name of sampleA.

Since SampleB also needs to be called by the main method, it is registered as a component.

important point

The following cases do not seem to be DI.

・ Static DI -DI in the newly generated instance

This time, because it is executed by the main method


private static SampleB sampleb;



SampleB sampleb = new SampleB();

I tried, but both did not DI well and I got a NullPointerException.


In addition to Component, there are some annotations that can be scanned. Let's use each annotation properly according to the role of the class.

@Controller(@RestController) → Target MVC Controller @Service → Targeting service layers such as business logic @Repository → Target classes that perform DB access such as DAO

Reference material

[First Spring Boot](https://www.amazon.co.jp/%E3%81%AF%E3%81%98%E3%82%81%E3%81%A6%E3%81%AESpring- Boot% E2% 80% 95% E3% 80% 8CSpring-Framework% E3% 80% 8D% E3% 81% A7% E7% B0% A1% E5% 8D% 98Java% E3% 82% A2% E3% 83% 97% E3% 83% AA% E9% 96% 8B% E7% 99% BA-I% E3% 83% BB-BOOKS / dp / 4777518655) Reintroduction to spring-DI (Dependency Injection) DI / DI container, do you understand properly ...?

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