[Java] Spring DI ③

What are you doing with DI?

• ** Find a DI managed class (component scan) **
• ** Instance creation and injection **

Component scan

• When Spring is started, the process of searching for the class with the annotation to be managed by DI runs and registers it in the DI container.
• The class managed on the DI container is called ** Bean **

[Annotation to be scanned]

• @Component
• @Controller
• @Service
• @Repository
• @Configuration
• @RestController
• @ControllAdvice
• @ManagedBean
• @Named

Instance creation and injection

• ** Instantiate and inject beans into DI container **
• Collect DI target classes (Beans) and DI creates instances of them (new)
• Inject the generated instance into the field with the following ** @ Autowired annotation **
• Field variables
• Constructor arguments
• setter argument
• You can pass an instance as an argument by prefixing the constructor with @Autowired.
• The image is "Create an instance of each class in the DI container, get the instance with getter, and call the getter of the DI container in the field with @Autowired".

//image
public class DependencyInjection{
    //Create an instance of each class
    private SampleComponent component = new SampleComponent();
    private SampleService service = new SampleService();
    //Getter for SampleComponent instance
    public static SampleComponent getSampleComponent(){
        return component;
    }
    //Getter for SampleService instance
    public static SampleService getSampleService(){
        return service;
    }
}

• ** You can save the trouble of creating Factory class by just adding annotations such as @Component! ** **

So how do you implement it?

• Annotation base
• JavaConfig
• xml
• JavaConfig + annotation
• xml + annotation

Annotation base

• Spring automatically registers classes with @Component, @Controller, etc. in the DI container

JavaConfig

• Prepare a class with @Configuration annotation
• Prepare a getter method with @Bean annotation in the class
• The return value of getter is registered in the DI container as a bean.
• In the DI container, prepare JavaConfig class and Bean instances, and get each instance with getter at @Autowired location.
• Unlike annotation base, the return value of getter with @Bean annotation is managed by DI container.
• In other words, it is necessary to prepare methods for instances managed by DI container
• Detailed settings and switching are possible

JavaConfig + annotation

• Annotate the class you want to manage with DI such as @Controller
• Set only the instance you want to make detailed settings with JavaConfig
• If you want to create a small application based on annotations and increase the scale, or switch settings, implement it with JavaConfig + annotations.

xml based

• Do the same in xml as you set in JavaConfig