Spring Dependency Injection

-

DI in Spring

The Spring Framework IoC (Inversion of Control) container handles Dependency Injection by registering (learning about) Spring beans and providing them to satisfy dependencies as needed.

@Configuration

Used for Java-based configuration
Creates a configuration Spring Bean
@Bean-annotated methods define other beans to be registered
Pull in other configuration beans with @Import

Spring Beans

Managed by Spring
Created as singletons
- Use @Scope to change this
- Subsequent requests produce the same instance

@ComponentScan

Finds and registers components in subdirectories of annotated class
- @Component, @Controller, @Service, @Repository
default in @SpringBootApplication

- from Spring Boot Reference Guide:

package com.example.myproject;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

// same as @Configuration @EnableAutoConfiguration @ComponentScan
@SpringBootApplication 
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

}

@Autowired

Autowired dependencies will be filled by registered beans
Only unambiguous dependencies will be fulfilled
Use @Qualifier or @Profile to narrow down choices

@Autowired: Disambiguation - @Qualifier

Spring resolves @Autowired entries by type. If multiple beans of the same type are available, a fatal exception will be thrown. (ie, if the bean share a common SuperClass or Interface)

@Component("tigerTony")
public class Tony implements Tiger {
    public String speak() {
        return "Grrrrreat!";
    }
}

@Component("tigerTigger")
public class Tigger implements Tiger {
    public String speak() {
        return "hoo-hoo-hoo-hoo-oo-oo-oo!";
    }
}

@Autowired: Disambiguation - @Qualifier

public class TiggerService {
    @Autowired
    private Tiger tiger;
}

will result in the following error:

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: 
No qualifying bean of type [com.autowire.sample.Tiger] is defined: 
expected single matching bean but found 2: tigerTigger,tigerTony

@Autowired: Disambiguation - @Qualifier

User the @Qualifier annotation to avoid that error:

public class TiggerService {
    @Autowired
    @Qualifier("tigerTigger")
    private Tiger tiger;
 
}

@Autowired: Disambiguation - @Profile

Using @Profile annotation, you can apply conditional usage on beans or components of the same name:

@Component("dbJawn")
@Profile("prod")
public class ProdDB implements DBJawn {
    @Override
    public boolean isLive() {
        return true;
    }
}

@Component("dbJawn")
@Profile("dev")
public class DevDB implements DBJawn {
    @Override
    public boolean isLive() {
        return false;
    }
}

@Autowired: Disambiguation - @Profile

Refine the @Autowired behavior by applying the desired @Profile to the component that uses it:

@Component
@Profile("dev")
public class DBJawnUser {
    @Autowired
    DBJawn dbJawn;
    ...
}