ConditionalOnProperty

配置Spring Boot通过@ConditionalOnProperty来控制Configuration是否生效

1、matchIfMissing属性：从application.properties中读取某个属性值，如果该值为空，默认值为true

@Configuration
@ConditionalOnClass({ Feign.class })
@ConditionalOnProperty(value = "feign.oauth2.enabled", matchIfMissing = true)
public class OAuth2FeignAutoConfiguration {

    @Bean
    @ConditionalOnBean(OAuth2ClientContext.class)
    public RequestInterceptor oauth2FeignRequestInterceptor(OAuth2ClientContext oauth2ClientContext) {
        return new OAuth2FeignRequestInterceptor(oauth2ClientContext);
    }
}

2、havingValue属性：通过其两个属性name以及havingValue来实现的，其中name用来从application.properties中读取某个属性值，如果该值为空，则返回false;如果值不为空，则将该值与havingValue指定的值进行比较，如果一样则返回true;否则返回false。如果返回值为false，则该configuration不生效；为true则生效。

@Configuration
//如果synchronize在配置文件中并且值为true
@ConditionalOnProperty(name = "synchronize", havingValue = "true")
public class SecondDatasourceConfig {

    @Bean(name = "SecondDataSource")
    @Qualifier("SecondDataSource")
    @ConfigurationProperties(prefix = "spring.second.datasource")
    public DataSource jwcDataSource() {
        return DataSourceBuilder.create().build();
    }
}