SpringBoot深入探究@Conditional条件装配的使用

来源：jb51　　时间：2022/6/27 12:53:35　　对本文有异议

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：

注解	描述
@ConditionalOnBean	在某个 bean 存在的时候
@ConditionalOnMissingBean	在某个 bean 不存在的时候
@ConditionalOnClass	当前 classPath 下可以找到某个 class 的时候
@ConditionalOnMissingClass	当前 classPath 下无法找到某个 class 的时候
@ConditionalOnResource	当前 classPath 下否存在某个资源文件的时候
@ConditionalOnProperty	当前 JVM 包含某个属性值的时候
@ConditionalOnWebApplication	当前 Spring context 是 web 应用程序的时候
@ConditionalOnJava	在指定JDK版本的时候
@ConditionalOnExpression	符合表达式的时候
@ConditionalOnJndi	JNDI存在该项的时候

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");
    }
}