Spring @ConditionalOnBean注解

@ConditionalOnBean作用：放在 @Bean 注解上，如果@ConditionalOnBean注解参数中指定的bean存在，就实例化当前作用的Bean

#	当给定的在bean存在时,则实例化当前Bean
@ConditionalOnBean         
#	当给定的在bean不存在时,则实例化当前Bean
@ConditionalOnMissingBean  
#	当给定的类名在类路径上存在，则实例化当前Bean
@ConditionalOnClass        
#	当给定的类名在类路径上不存在，则实例化当前Bean
@ConditionalOnMissingClass 

@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class City {
    private String cityName;
    private Integer citySize;
}

@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class User {
    private String name;
    private Integer age;
    private City city;
}

@Configuration
public class ConditionalConfig {

//    @Bean
//    public City city(){
//        City city = new City();
//        city.setCityName("beijing");
//        return city;
//    }

	//当city的bean存在时，才会将user注入到spring中，否则报null
    @ConditionalOnBean(name = "city")
    @Bean
    public User user(){
        City city = new City();
        city.setCityName("shanghai");
        city.setCitySize(20);
        return new User("zhangsan",2,city);
    }
}

@RunWith(SpringRunner.class)
@SpringBootTest
public class ConditionalOnDemo {

    @Autowired(required = false)
    User user;

    @Test
    public void run(){
        System.out.println("user is："+user);
    }

}

当Spring没有注入City的bean时，就会报空

user is：null

当Spring注入City的bean时，就会正常执行，将user的bean注入到spring容器中

user is：User(name=zhangsan, age=2, city=City(cityName=beijing, citySize=20))