ConditionalOnBean (Spring Boot API)

```
@Target({TYPE,METHOD})
@Retention(RUNTIME)
@Documented
@Conditional(org.springframework.boot.autoconfigure.condition.OnBeanCondition.class)
public @interface ConditionalOnBean
```
Conditional that only matches when the specified bean classes and/or names are 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 {

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

 }
```
In the sample above the condition will match if a 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.

可选元素概要

可选元素
修饰符和类型	可选元素	说明
`Class<? extends Annotation>[]`	`annotation`	The annotation type decorating a bean that should be checked.
`String[]`	`name`	The names of beans to check.
`SearchStrategy`	`search`	Strategy to decide if the application context hierarchy (parent contexts) should be considered.
`String[]`	`type`	The class type names of bean that should be checked.
`Class<?>[]`	`value`	The class type of bean that should be checked.

- 元素详细资料
  - value
```
Class<?>[] value
```
    The class type of bean that should be checked. The condition matches when any of the classes specified is contained in the ApplicationContext.
    返回:
    the class types of beans to check
    默认值:
    {}
- - type
```
String[] type
```
    The class type names of bean that should be checked. The condition matches when any of the classes specified is contained in the ApplicationContext.
    返回:
    the class type names of beans to check
    默认值:
    {}
- - annotation
```
Class<? extends Annotation>[] annotation
```
    The annotation type decorating a bean that should be checked. The condition matches when any of the annotations specified is defined on a bean in the ApplicationContext.
    返回:
    the class-level annotation types to check
    默认值:
    {}
- - name
```
String[] name
```
    The names of beans to check. The condition matches when any of the bean names specified is contained in the ApplicationContext.
    返回:
    the name of beans to check
    默认值:
    {}
- - search
```
SearchStrategy search
```
    Strategy to decide if the application context hierarchy (parent contexts) should be considered.
    返回:
    the search strategy
    默认值:
    org.springframework.boot.autoconfigure.condition.SearchStrategy.ALL