需求

  在产品对接过程中，一个公共的服务对接多个产品，其中产品A使用RocketMQ进行消费，产品B无需消费。此时，代码共用了，如何做到产品B可以不消费？
  研究了一些方法，首先想到的是如何使RocketMQ的监听消费@RocketMQMessageListener注解失效。研究了一会儿，注解本身没有开关，也没有找到合适的方法去使指定注解失效。换了另一个方向：条件注解。亲测有效！

@ConditionalOnProperty注解

介绍

1. 在SpringBoot中可以通过该注解来控制@Configuration是否生效。同时，我们可以通过该注解判断一个property属性，是否符合我们设定的配置值，符合则使该注解修饰的类或方法生效，否则不生效。
2. 该注解是@Conditional的扩展注解。

源码解析

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.springframework.boot.autoconfigure.condition;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Conditional;

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
@Documented
@Conditional({OnPropertyCondition.class})
public @interface ConditionalOnProperty {
	//数组，获取property对应的值，与name属性不可以同时使用
    String[] value() default {};
	//属性名称值的前缀
    String prefix() default "";
	//数组，配置属性值的名称，可与prefix组合使用，不可与value属性同时使用
    String[] name() default {};
	//比较和name属性值获取到的属性值是否相同，与name组合使用，若true，则加载配置
    String havingValue() default "";
	//缺少配置是否匹配加载，若为true，则表示无该name相关属性值，也加载。反之，不加载。
    boolean matchIfMissing() default false;
}

使用示例

新建bean类

@Data
public class PersonBean {
    /**
     * name
     */
    private String name;
    /**
     * age
     */
    private int age;
}

需测试的注解类

/**
 * @Description 测试bean配置
 * @Author LiaoJy
 * @Date 2023/4/17
 */
@Slf4j
@Configuration
@ConditionalOnProperty(prefix = "conditional", name = "configuration.switch", havingValue = "true", matchIfMissing = true)
public class BeanConfigTest {

    @Bean
    public PersonBean personBean() {
        PersonBean personBean = new PersonBean();
        personBean.setAge(30);
        personBean.setName("andya");
        log.info("========person bean init========");
        return personBean;
    }

}

配置场景一

conditional:
  configuration:
    switch: false

控制台日志结果

  .   ____          _            __ _ _
 /\ / ___'_ __ _ _(_)_ __  __ _    
( ( )\___ | '_ | '_| | '_ / _` |    
 \/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v2.6.9)

... ...
2023-04-17 14:14:11.620  INFO 25494 --- [           main] c.test.selfcoding.SelfCodingApplication  : No active profile set, falling back to 1 default profile: "default"
2023-04-17 14:14:12.175  INFO 25494 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8001 (http)
2023-04-17 14:14:12.182  INFO 25494 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2023-04-17 14:14:12.182  INFO 25494 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.64]
2023-04-17 14:14:12.259  INFO 25494 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2023-04-17 14:14:12.259  INFO 25494 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 609 ms
2023-04-17 14:14:12.445  INFO 25494 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8001 (http) with context path ''
2023-04-17 14:14:12.451  INFO 25494 --- [           main] c.test.selfcoding.SelfCodingApplication  : Started SelfCodingApplication in 1.123 seconds (JVM running for 1.387)

结果未打印========person bean init========日志，类未生效。

配置场景二

不配置，或者配置如下

conditional:
  configuration:
    switch: true

控制台日志结果

  .   ____          _            __ _ _
 /\ / ___'_ __ _ _(_)_ __  __ _    
( ( )\___ | '_ | '_| | '_ / _` |    
 \/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v2.6.9)

... ...
2023-04-17 14:17:35.172  INFO 25577 --- [           main] c.test.selfcoding.SelfCodingApplication  : No active profile set, falling back to 1 default profile: "default"
2023-04-17 14:17:35.771  INFO 25577 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8001 (http)
2023-04-17 14:17:35.777  INFO 25577 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2023-04-17 14:17:35.777  INFO 25577 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.64]
2023-04-17 14:17:35.859  INFO 25577 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2023-04-17 14:17:35.859  INFO 25577 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 655 ms
2023-04-17 14:17:35.899  INFO 25577 --- [           main] c.test.selfcoding.config.BeanConfigTest  : ========person bean init========
2023-04-17 14:17:36.134  INFO 25577 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8001 (http) with context path ''
2023-04-17 14:17:36.141  INFO 25577 --- [           main] c.test.selfcoding.SelfCodingApplication  : Started SelfCodingApplication in 1.421 seconds (JVM running for 1.775)

可以看到打印了========person bean init========日志，类生效。

@ConditionalOnExpression

介绍

1. 上述讲解的@ConditionalOnProperty只能精准的匹配havingValue中的值进行控制，无法根据更多的属性值进行匹配（即使它有数组的value值，也只能配合havingValue进行与校验）
2. 我们可以通过@ConditionalOnExpression注解进行其他属性值的表达式来校验。
3. @ConditionalOnExpression是执行Spel表达式，根据返回的布尔值判断是否符合条件。

源码

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.springframework.boot.autoconfigure.condition;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Conditional;

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
@Documented
@Conditional({OnExpressionCondition.class})
public @interface ConditionalOnExpression {
	//Spel表达式
    String value() default "true";
}

使用示例

需测试的注解类一

/**
 * @Description 测试bean配置
 * @Author LiaoJy
 * @Date 2023/4/17
 */
@Slf4j
@Configuration
//@ConditionalOnProperty(prefix = "conditional", name = "configuration.switch", havingValue = "true", matchIfMissing = true)
@ConditionalOnExpression("!'${conditional.configuration.switch}'.equals('false')")
public class BeanConfigTest {

    @Bean
    public PersonBean personBean() {
        PersonBean personBean = new PersonBean();
        personBean.setAge(30);
        personBean.setName("andya");
        log.info("========person bean init========");
        return personBean;
    }

}

此时只要配置conditional.configuration.switch不为false值，此类都会生效。

需测试的注解类二

/**
 * @Description 测试bean配置
 * @Author LiaoJy
 * @Date 2023/4/17
 */
@Slf4j
@Configuration
//@ConditionalOnProperty(prefix = "conditional", name = "configuration.switch", havingValue = "true", matchIfMissing = true)
@ConditionalOnExpression("${conditional.configuration.switch:true}")
public class BeanConfigTest {

    @Bean
    public PersonBean personBean() {
        PersonBean personBean = new PersonBean();
        personBean.setAge(30);
        personBean.setName("andya");
        log.info("========person bean init========");
        return personBean;
    }

}

此时只有配置conditional.configuration.switch为false值，此类才不会生效。

其他使用场景示例

1. 布尔属性

@ConditionalOnExpression("${conditional.configuration.switch:true}

2. 多个布尔属性，只要满足其一

@ConditionalOnExpression("${conditional.configuration.switch1:true} || ${conditional.configuration.switch2:true}")

3. 配置值等于数字

@ConditionalOnExpression("${conditional.configuration.switch} == 1")
@ConditionalOnExpression("${conditional.configuration.switch} != 1")

4. 配置值与指定字符串相同

@ConditionalOnExpression("'${conditional.configuration.switch}'.equals('yes')")

5. 多个配置值相同

@ConditionalOnExpression("'${conditional.configuration.str1}'.equals('${conditional.configuration.str2}')")

6. … …

总结

  本来只想用用@ConditionalOnProperty，用完@ConditionalOnExpression更爽一些！