Constructor-Based DI in Spring

清疚 2024-03-25 14:06 3阅读 0赞

Constructor-Based DI means that we pass the required components into a class at the time of instantiation.  
Spring resolves each argument primarily by type, followed by name of the attribute, and index for disambiguation.

# To get started, we need to import the spring-context dependency in our pom.xml
    
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context</artifactId>
        <version>5.2.8.RELEASE</version>
    </dependency>

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## Annotation Based Configuration ##

@Configuration
    @ComponentScan("com.spring")
    public class Config {
        
        @Bean
        public Engine engine() {
        
            return new Engine("v8", 5);
        }
        @Bean
        public Transmission transmission() {
        
            return new Transmission("sliding");
        }
    }
    
    @Component
    public class Car {
        
        @Autowired
        public Car(Engine engine, Transmission transmission) {
        
            this.engine = engine;
            this.transmission = transmission;
        }
    }
    
    ApplicationContext context = new AnnotationConfigApplicationContext(Config.class);
    Car car = context.getBean(Car.class);

Spring will encounter our `Car` class while doing a package scan, and will initialize its instance by calling the @Autowired annotated constructor. By calling the @Bean annotated methods of the `Config` class, we will obtain instances of *`Engine`* and *`Transmission`*.

### Implicit Constructor Injection ###

As of Spring 4.3, `classes with a single constructor can omit the @Autowired annotation`.

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## XML Based Configuration ##

1、Constructor-arg can accept `a literal value` or `a reference to another bean`.  
2、We can use `Type` and `index` to resolve ambiguity.

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**参考：**  
[Constructor Dependency Injection in Spring][]

[Constructor Dependency Injection in Spring]: https://www.baeldung.com/constructor-injection-in-spring