@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
@Documented
@Conditional({OnPropertyCondition.class})
public @interface ConditionalOnProperty {
    // NAME和VALUE的互為別名，在使用時(shí)是互斥的
    String[] value() default {};
    String[] name() default {};
	// 配置項(xiàng)前綴，如果完整配置為：servicex.swagger.config.enabled，則prefix為：servicex.swagger.config
    String prefix() default ""; 
	// 用havingValue與配置項(xiàng)的值進(jìn)行對(duì)比，一致返回TRUE，不一致返回FALSE.
    String havingValue() default "";
	// 如果配置文件中, 沒(méi)有該配置項(xiàng), 判斷是否加載BEAN, 默認(rèn)為false。
    boolean matchIfMissing() default false;
}

// 假設(shè)YML中的配置如下：
servicex.swagger.config.enabled=true

// 即使配置文件中沒(méi)有"servicex.swagger.config.enabled"該配置，
// 依然加載SwaggerAutoConfiguration，因?yàn)閙atchIfMissing = true。
@Configuration
@EnableAutoConfiguration
@ConditionalOnProperty(name = "servicex.swagger.config.enabled", matchIfMissing = true)
public class SwaggerAutoConfiguration {
}

// 既指定prefix也指定name，因?yàn)榕渲庙?xiàng)為true，所以會(huì)加載SwaggerAutoConfiguration。
@Configuration
@EnableAutoConfiguration
@ConditionalOnProperty(prefix = "servicex.swagger.config", name = "enabled")
public class SwaggerAutoConfiguration {
}

// 只指定name或者value，因?yàn)榕渲庙?xiàng)為true，所以會(huì)加載SwaggerAutoConfiguration。
@Configuration
@EnableAutoConfiguration
@ConditionalOnProperty(name = "servicex.swagger.config.enabled")
public class SwaggerAutoConfiguration {
}

// 只指定name或者value，因?yàn)榕渲庙?xiàng)為true，havingValue也為true，所以會(huì)加載SwaggerAutoConfiguration。
@Configuration
@EnableAutoConfiguration
@ConditionalOnProperty(name = "servicex.swagger.config.enabled", havingValue = "true")
public class SwaggerAutoConfiguration {
}

// 只指定name或者value，因?yàn)榕渲庙?xiàng)為true，havingValue為false，所以不會(huì)加載SwaggerAutoConfiguration。
@Configuration
@EnableAutoConfiguration
@ConditionalOnProperty(name = "servicex.swagger.config.enabled", havingValue = "false")
public class SwaggerAutoConfiguration {
}

ConditionalOnProperty
ConditionalOnResource
// 當(dāng)某個(gè)Bean在應(yīng)用上下文存在時(shí)，才會(huì)加載
ConditionalOnBean
@ConditionalOnBean(MyAutoConfiguration.class)
public class SwaggerAutoConfiguration {
}
// 當(dāng)某個(gè)Bean在應(yīng)用上下文不存在時(shí)，才會(huì)加載
ConditionalOnMissingBean
// 當(dāng)某個(gè)Bean在CLASS-PATH存在時(shí)，才會(huì)加載
ConditionalOnClass
// 當(dāng)某個(gè)Bean在CLASS-PATH不存在時(shí)，才會(huì)加載
ConditionalOnMissingClass
// 當(dāng)表達(dá)式的返回值為TRUE時(shí)，才會(huì)加載
ConditionalOnExpression
@ConditionalOnExpression("${servicex.swagger.config.enabled:true} and ${swagger.config.enabled:true}")
public class SwaggerAutoConfiguration {
}
// 指定資源在CLASS-PATH存在時(shí)，才會(huì)加載
ConditionalOnResource
@ConditionalOnResource(resources = "/banner.txt")
public class SwaggerAutoConfiguration {
}