@ConditionalOnMissingBean

org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean

@Conditional(value={OnBeanCondition.class})
@Target(value={METHOD, TYPE})
@Retention(value=RUNTIME)
@Documented

Conditional that only matches when the specified bean classes and/or names are not already contained in the BeanFactory. 

When placed on a @Bean method, the bean class defaults to the return type of the factory method: 
 @Configuration
 public class MyAutoConfiguration {

     @ConditionalOnMissingBean
     @Bean
     public MyService myService() {
         ...
     }

 }

In the sample above the condition will match if no bean of type MyService is already contained in the BeanFactory. 

The condition can only match the bean definitions that have been processed by the application context so far and, as such, it is strongly recommended to use this condition on auto-configuration classes only. If a candidate bean may be created by another auto-configuration, make sure that the one using this condition runs after.
Author:Phillip WebbAndy Wilkinson