使用自定义注解结合@Conditional注解实现ConditionalOnMissingBean

红太狼 2024-03-22 15:55 30阅读 0赞

> bean不通过beanName的方式做ConditionalOnMissingBean，可以使用自定义注解结合@Conditional注解实现。

> 1. 创建自定义注解，如@ConditionalOnMissingMyBean。

@Target({ElementType.TYPE, ElementType.METHOD})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Conditional(OnMissingMyBeanCondition.class)
    public @interface ConditionalOnMissingMyBean {
         String[] value() default {};
         Class<?>[] type() default {};
     }

> 2. 创建注解处理类OnMissingMyBeanCondition，并实现Condition接口的matches方法。

public class OnMissingMyBeanCondition implements Condition {
         @Override
        public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
            Map<String, Object> attributes = metadata.getAnnotationAttributes(ConditionalOnMissingMyBean.class.getName());
            String[] beanNames = (String[]) attributes.get("value");
            Class<?>[] types = (Class<?>[]) attributes.get("type");
             ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
            BeanDefinitionRegistry registry = context.getRegistry();
             for (Class<?> type : types) {
                if (beanFactory.getBeanNamesForType(type).length > 0) {
                    return false;
                }
            }
             for (String beanName : beanNames) {
                if (registry.containsBeanDefinition(beanName)) {
                    return false;
                }
            }
             return true;
        }
     }

> 3. 在需要使用@ConditionalOnMissingMyBean注解的地方使用该注解即可。

@Configuration
    public class MyConfiguration {
         @Bean
        public MyBean myBean() {
            return new MyBean();
        }
         @Bean
        @ConditionalOnMissingMyBean(type = MyBean.class)
        public MyAnotherBean myAnotherBean() {
            return new MyAnotherBean();
        }
     }

> **总结：这样，当MyBean存在时，MyAnotherBean就不会被创建；反之，当MyBean不存在时，MyAnotherBean就会被创建。**