Autowiring Dependencies

╰半橙微兮° 2024-02-29 04:51 34阅读 0赞

## Spring bean autowiring 定义： ##

> The Spring framework enables automatic dependency injection. In other words, **by `declaring` all the bean dependencies in a Spring configuration file, Spring container can `autowire` relationships between collaborating beans.** This is called `Spring bean autowiring`.

### Enabling @Autowired Annotations ###

@Autowired 的使用是有前置条件的 ☞ activate the dependency injection annotations

### Spring项目 ☞ 可选方式1 ###

To use `Java-based configuration` in our application, let’s `enable annotation-driven injection` to load our Spring configuration:

@Configuration
    @ComponentScan("com.autowire.sample")
    public class AppConfig {
        }

### Spring项目 ☞ 可选方式2 ###

The `<context:annotation-config>` annotation is mainly used to `activate the dependency injection annotations` **in Spring XML files**.

### SpringBoot项目 ###

**Spring Boot** introduces the `@SpringBootApplication` annotation. This single annotation is equivalent to using `@Configuration`, `@EnableAutoConfiguration`, and `@ComponentScan`.

As a result, when we run this Spring Boot application, it will `automatically scan` the componentsin the current package and its sub-packages. Thus it will `register` them in Spring’s Application Context, and allow us to `inject` beans using @Autowired.

> **阅读更多**：  
> [Spring ＜context:annotation-config＞ and ＜context:component-scan＞][Spring _context_annotation-config_ and _context_component-scan]  
> [Spring Component Scanning][]  
> [Spring @ComponentScan – Filter Types][Spring _ComponentScan _ Filter Types]

![在这里插入图片描述][475edd291cca40729a91087b25123f92.png_pic_center]

There are four [modes][] of autowiring a bean using an `XML configuration`

1、 **no**: the default value – this means no autowiring is used for the bean and we have to `explicitly name the dependencies`.  
2、 **byName**: autowiring is done `based on the name of the property`, therefore Spring will look for a bean with the same name as the property that needs to be set.  
3、 **byType**: similar to the byName autowiring, only `based on the type of the property`. This means Spring will look for a bean with the same type of the property to set. If there’s `more than one` bean of that type, the framework `throws an exception`.  
3、**constructor**: autowiring is done based on constructor arguments, meaning Spring will look for beans with `the same type as the constructor arguments`.

## by type ##

@Bean(autowire = Autowire.BY_TYPE)
    public class Store {
        
        private Item item;
        public setItem(Item item){
        
            this.item = item;    
        }
    }
    
    public class Store {
        
        @Autowired
        private Item item;
    }
    
    <bean id="store" class="org.store.Store" autowire="byType"> </bean>

## By name ##

# more than one bean of the same type,use the @Qualifier to reference a bean by name
    public class Store {
        
        @Autowired
        @Qualifier("item1")
        private Item item;
    }
    
    <bean id="item" class="org.store.ItemImpl1" />
    <bean id="store" class="org.store.Store" autowire="byName"> </bean>

We can also override the autowiring by defining dependencies explicitly through `constructor arguments` or `setters`.

**参考：[Intro to Inversion of Control and Dependency Injection with Spring][]**

[Spring _context_annotation-config_ and _context_component-scan]: https://blog.csdn.net/weixin_37646636/article/details/129575494
[Spring Component Scanning]: https://blog.csdn.net/weixin_37646636/article/details/129598990
[Spring _ComponentScan _ Filter Types]: https://blog.csdn.net/weixin_37646636/article/details/129601176
[475edd291cca40729a91087b25123f92.png_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/02/29/607d2cb99de84a0d86039de5d25cb5e5.png
[modes]: https://dict.youdao.com/w/modes%20/#keyfrom=dict2.top
[Intro to Inversion of Control and Dependency Injection with Spring]: https://www.baeldung.com/inversion-control-and-dependency-injection-in-spring