@Conditional
官方文档定义：“Indicates that a component is only eligible for registration when all specified conditions match”，意思是只有满足一些列条件之后创建一个bean。
@Conditional定义
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface Conditional {
Class<? extends Condition>[] value();
}
@Conditional注解主要用在以下位置：
- 类级别可以放在注标识有@Component（包含@Configuration）的类上
- 作为一个meta-annotation,组成自定义注解
- 方法级别可以放在标识由@Bean的方法上
spring-boot的@Conditional系列
spring-boot利用Conditional来确定是不是要创建Bean实例，如下是官方的说明，大概是说boot是启用@Conditional注解来确定是否要加载该实例
<blockquote>No. Boot is enabled in Spring Framework 4.0 by the @Conditional annotation infrastructure. Your perception of loaded is accurate otherwise - JRebel is much more comprehensive, works with multiple IDEs, etc. </blockquote>
- ConditionalOnBean: 当且仅当指定的bean classes and/or bean names在当前容器中,才创建标记上该注解的类的实例
- ConditionalOnBean: 当且仅当指定的bean classes and/or bean names不存在当前容器中,才创建标记上该注解的类的实例，有指定忽略ignored的参数存在，可以忽略Class、Type等
- ConditionalOnClass：当且仅当ClassPath存在指定的Class时，才创建标记上该注解的类的实例
- ConditionalOnMissingClass：当且仅当ClassPath不存在指定的Class时，创建标记上该注解的类的实例
- ConditionalOnProperty：当且仅当Application.properties存在指定的配置项时，创建标记上了该注解的类的实例
- ConditionalOnJava：指定JDK的版本
- ConditionalOnExpression：表达式用${..}=false等来表示
- ConditionalOnJndi：JNDI存在该项时创建
- ConditionalOnResource：在classpath下存在指定的resource时创建
- ConditionalOnSingleCandidate：Conditional
that only matches when the specified bean class is already contained in the BeanFactory
and a single candidate can be determined.The condition will also match if multiple matching bean instances are already contained in the BeanFactory
but a primary candidate has been defined; essentially, the condition match if auto-wiring a bean with the defined type will succeed. - ConditionalOnWebApplication：在web环境下创建
