ConditionalOnBean 与 ConditionalOnMissingBean 的正确玩法

  之前看到这篇博客,写ConditionalOnClass的用法,自己实践了一下,感觉有点问题,原文如下:

https://blog.csdn.net/lucyTheSlayer/article/details/80430912

 

于是自己更正了一下,代码结构如下:

ConditionalOnBean 与 ConditionalOnMissingBean 的正确玩法

 

贴上完整的代码:

程序入口:ConditionalDemoApplication:
@SpringBootApplication
public class ConditionalDemoApplication implements CommandLineRunner {
    @Autowired
    private Van van;

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

    @Override
    public void run(String... args) throws Exception{
        van.fight();
    }
}

Van.java

@Service
public class Van {
    @Autowired
    private Fighter fighter;

    public void fight(){
        System.out.println("van:boy next door,do you like 玩游戏");
        fighter.fight();
    }
}

Fighter.java及其实现类:

public interface Fighter {
    void fight();
}
@Service
public class Babana implements Fighter {
@Override
public void fight(){
System.out.println("Banana: *的气息,蕉迟但到");
}
}
@Service
public class Billy implements Fighter {
public void fight(){
System.out.println("Billy:吾乃新日暮里的王,三界哲学的主宰。");

}
}

VanConfig:

@Configuration
public class VanConfig {
@Bean
@ConditionalOnBean(Billy.class)
public Fighter fighter(){
return new Billy();
}

@Bean
@ConditionalOnMissingBean
public Fighter fighter2(){
return new Babana();
}
}

1、运行程序,输入结果如下:

ConditionalOnBean 与 ConditionalOnMissingBean 的正确玩法

 

2、如果将Billy Bean的代码注释掉:

@Configuration
public class VanConfig {
/*@Bean
@ConditionalOnBean(Billy.class)
public Fighter fighter(){
return new Billy();
}*/

@Bean
@ConditionalOnMissingBean
public Fighter fighter2(){
return new Babana();
}
}

重新运行,输入结果如下:

 ConditionalOnBean 与 ConditionalOnMissingBean 的正确玩法

 

3、或者,我们将Billy上的@Service注解注释掉,让springboot扫描不到该类:

//@Service
public class Billy implements Fighter {
    public void fight(){
        System.out.println("Billy:吾乃新日暮里的王,三界哲学的主宰。");

    }
}

同时恢复VanConfig里的配置:

@Configuration
public class VanConfig {
    @Bean
    @ConditionalOnBean(Billy.class)
    public Fighter fighter(){
        return new Billy();
    }

    @Bean
    @ConditionalOnMissingBean
    public Fighter fighter2(){
        return new Babana();
    }
}

再次运行,输入结果如下(与第2次试验的效果相同):

 ConditionalOnBean 与 ConditionalOnMissingBean 的正确玩法

 

4、在第3个试验的基础上,我们再做一个试验,把@ConditionalOnBean(Billy.class)注释掉:

@Configuration
public class VanConfig {
    @Bean
//    @ConditionalOnBean(Billy.class)
    public Fighter fighter(){
        return new Billy();
    }

    @Bean
    @ConditionalOnMissingBean
    public Fighter fighter2(){
        return new Babana();
    }
}

重新运行,结果如下:

ConditionalOnBean 与 ConditionalOnMissingBean 的正确玩法

这说明,在不添加ConditionalOnBean条件时,Billy的注入已不再受任何约束的

结论:

1、@ConditionalOnBean(xxx.class)就是为了判断 xxx.class是否存在,并已注释了springboot容器里了;
2、@ConditionalOnMissingBean 则是在第一点不存在的情况下起作用;

当然,我们还继续作一些测试,比如,去掉Babana上的@Service等,再看一看效果会怎样。
好了,分享完毕,该回去干活了...
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