context:annotation-config 和 context:component-scan 介绍

旧城等待， 2024-03-25 16:20 44阅读 0赞

Spring provides two ways to define **beans** and **dependencies**: **`XML configuration`** and **`Java annotations`**. We can also categorize Spring’s annotations under two groups: **`DI annotations`** and **`bean annotations`**.

> ![在这里插入图片描述][69d482fd5aad4a2eb1191dc201a13448.png]

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Prior to（在前） annotations, we had to manually define all our beans and dependencies in **XML configuration files**. Now thanks to Spring’s annotations, **it can automatically discover and [wire][] all our beans and dependencies for us**.

However, we should remember that annotations are useless unless we activate them. In order to `activate` them, we can add either `<context:annotation-config>` or `<context:component-scan>` on top of our XML file.

> `context:annotation-config` 和 `context:component-scan` 组合使用，个人理解 ☞ 他们支持的注解不同，彼此狼狈为奸

<?xml version="1.0" encoding="UTF-8"?>
    <beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xmlns:context="http://www.springframework.org/schema/context"
      xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
      
      <context:annotation-config/>
      <context:component-scan base-package="com.annotationconfigvscomponentscan.components" />
    
    </beans>

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## `<context:annotation-config>` ##

The `<context:annotation-config>` **activates the annotations only for the beans already registered in the application context**.  
The `<context:annotation-config>` annotation is mainly used to `activate the DI annotations`.

> @Autowired, @Qualifier, @Resource, @PostConstruct, @PreDestroy are several ones that `<context:annotation-config>` can detect.

public class AccountService {
        }
    public class UserService {
        
        @Autowired
        private AccountService accountService;
    }
    
    <bean id="accountService" class="AccountService"></bean>
    <bean id="userService" class="UserService"></bean>
    
    #-----------------------------------------------------------------------
    
    # If we didn't use @Autowired, then we would need to set the accountService dependency manually
    <bean id="userService" class="UserService">
        <property name="accountService" ref="accountService"></property>
    </bean>

## `<context:component-scan>` ##

`<context:component-scan>` tells Spring which packages need to be scanned to look for `the annotated beans or components`.

> @Component, @Repository, @Service, @Controller, @RestController, @Configuration are several ones that `<context:component-scan>` can detect.

@Component
    public class UserService {
        
        @Autowired
        private AccountService accountService;
    }
    
    @Component
    public class AccountService {
        }

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参考：  
[Difference between context:annotation-config vs context:component-scan][Difference between context_annotation-config vs context_component-scan]

[69d482fd5aad4a2eb1191dc201a13448.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/25/ee018948a1214655a69a54a8923d4c37.png
[wire]: https://dict.youdao.com/w/wire%20/#keyfrom=dict2.top
[Difference between context_annotation-config vs context_component-scan]: https://www.baeldung.com/spring-contextannotation-contextcomponentscan