SpringBoot深入探究@Conditional条件装配的使用

1.相关介绍

@Conditional注解可以用在任何类型或者方法上面，通过@Conditional注解可以配置一些条件判断，当所有条件都满足的时候，被@Conditional标注的目标才会被spring容器处理。

比如可以通过@Conditional来控制bean是否需要注册，控制被@Configuration标注的配置类是需要需要被解析等。

Conditional相关的其它注解：

2.举例测试

@ConditionalOnBean, 在某个 bean 存在的时候被标注的组件才被注册到IOC容器中

下面是其详细代码

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional({OnBeanCondition.class})
public @interface ConditionalOnBean {
   Class<?>[] value() default {};
   String[] type() default {};
   Class<? extends Annotation>[] annotation() default {};
   String[] name() default {};
   SearchStrategy search() default SearchStrategy.ALL;
   Class<?>[] parameterizedContainer() default {};
}

我们就拿它来测试一下

1.指定组件不存在时

MyConfig

package com.limi.config;
import com.limi.entity.User;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.context.annotation.*;
@Configuration
public class MyConfig {
//    @Bean
//    public User user01(){
//        return new User(1, "andy", "123456");
//    }
   @Bean
   @ConditionalOnBean(name = "user01") //当user01存在时才注册user02
   public User user02(){
       return new User(2, "bob", "666666");
   }
}

MainApplication

package com.limi;
import com.limi.entity.User;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
@SpringBootApplication
public class MainApplication {
   public static void main(String[] args) {
       //1、返回我们IOC容器
       ConfigurableApplicationContext run = SpringApplication.run(MainApplication.class, args);
       if(run.containsBean("user01"))
           System.out.println((User)run.getBean("user01"));
           System.out.println("user01 不存在!\n");
       if(run.containsBean("user02"))
           System.out.println((User)run.getBean("user02"));
       else
           System.out.println("user02 不存在!\n");
   }
}

运行结果

2.指定组件存在时

MyConfig

package com.limi.config;
import com.limi.entity.User;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.context.annotation.*;
@Configuration
public class MyConfig {
   @Bean
   public User user01(){
       return new User(1, "andy", "123456");
   @Bean
   @ConditionalOnBean(name = "user01") //当user01存在时才注册user02
   public User user02(){
       return new User(2, "bob", "666666");
   }
}

MainApplication

package com.limi;
import com.limi.entity.User;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
@SpringBootApplication
public class MainApplication {
   public static void main(String[] args) {
       //1、返回我们IOC容器
       ConfigurableApplicationContext run = SpringApplication.run(MainApplication.class, args);
       if(run.containsBean("user01"))
           System.out.println((User)run.getBean("user01"));
       else
           System.out.println("user01 不存在!\n");
       if(run.containsBean("user02"))
           System.out.println((User)run.getBean("user02"));
       else
           System.out.println("user02 不存在!\n");
   }
}

运行结果

3.完整代码

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.limi</groupId>
   <artifactId>springboot-test1</artifactId>
   <version>1.0-SNAPSHOT</version>
   <packaging>jar</packaging>
   <parent>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-parent</artifactId>
       <version>2.3.4.RELEASE</version>
   </parent>
   <dependencies>
       <dependency>
           <groupId>org.springframework.boot</groupId>
           <artifactId>spring-boot-starter-web</artifactId>
       </dependency>
   </dependencies>
   <!--打包插件, 使用它可以把项目打包为jar包-->
   <build>
       <plugins>
           <plugin>
               <groupId>org.springframework.boot</groupId>
               <artifactId>spring-boot-maven-plugin</artifactId>
           </plugin>
       </plugins>
   </build>
</project>

User

package com.limi.entity;
public class User {
   private Integer id;
   private String userName;
   private String passWord;
   public User(){}
   public User(Integer id, String userName, String passWord) {
       this.id = id;
       this.userName = userName;
       this.passWord = passWord;
   }
   @Override
   public String toString() {
       return "User{" +
               "id=" + id +
               ", userName='" + userName + '\'' +
               ", passWord='" + passWord + '\'' +
               '}';
   }
   public Integer getId() {
       return id;
   }
   public void setId(Integer id) {
       this.id = id;
   }
   public String getUserName() {
       return userName;
   }
   public void setUserName(String userName) {
       this.userName = userName;
   }
   public String getPassWord() {
       return passWord;
   }
   public void setPassWord(String passWord) {
       this.passWord = passWord;
   }
}

MyConfig

package com.limi.config;
import com.limi.entity.User;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.context.annotation.*;
@Configuration
public class MyConfig {
   @Bean
   public User user01(){
       return new User(1, "andy", "123456");
   }
   @Bean
   @ConditionalOnBean(name = "user01") //当user01存在时才注册user02
   public User user02(){
       return new User(2, "bob", "666666");
   }
}

MainApplication

package com.limi;
import com.limi.entity.User;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
@SpringBootApplication
public class MainApplication {
   public static void main(String[] args) {
       //1、返回我们IOC容器
       ConfigurableApplicationContext run = SpringApplication.run(MainApplication.class, args);
       if(run.containsBean("user01"))
           System.out.println((User)run.getBean("user01"));
       else
           System.out.println("user01 不存在!\n");
       if(run.containsBean("user02"))
           System.out.println((User)run.getBean("user02"));
       else
           System.out.println("user02 不存在!\n");
   }
}

来源：https://blog.csdn.net/qq_41865229/article/details/125185114