What is purpose of @ConditionalOnProperty annotation?

http://stackoverflow.com/questions/26394778/what-is-purpose-of-conditionalonproperty-annotation

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just modified spring boot configuration, and encountered

@ConditionalOnProperty(prefix = "spring.social.", value = "auto-connection-views") from org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration.java         @Bean(name = { "connect/twitterConnect", "connect/twitterConnected" })        @ConditionalOnProperty(prefix = "spring.social.", value = "auto-connection-views")        public View twitterConnectView() {            return new GenericConnectionStatusView("twitter", "Twitter");        }

I don't understand purpose of this annotation. I guess this might be enable to use bean only if property value exist(e.g. "spring.social", "auto-connection-views").

answers

The annotation is used to conditionally create a Spring bean depending on the configuration of a property. In the usage you've shown in the question the bean will only be created if thespring.social.auto-connection-views property exists and it has a value other thanfalse. This means that, for this View bean to be created, you need to set thespring.social.auto-connection-views property and it has to have a value other than false.

如果property spring.social.auto-connection-views存在,并且值不为false,创建bean.

You can find numerous other uses of this annotation throughout the Spring Boot code base. Another example is:

@ConditionalOnProperty(prefix = "spring.rabbitmq", name = "dynamic", matchIfMissing = true)public AmqpAdmin amqpAdmin(CachingConnectionFactory connectionFactory) {    return new RabbitAdmin(connectionFactory);}

Note the use of matchIfMissing. In this case the AmqpAdmin bean will be created if thespring.rabbitmq.dynamic property exists and has a value other than false or the property doesn't exist at all. This makes the creation of the bean opt-out rather than the example in the question which is opt-in.

如果property spring.rabbitmq.dynamic存在，并且值不为false,创建bean

matchIfMissing = true， 如果改属性条目不存在，创建bean.