@ConditionalOnBean、@ConditionalOnProperty、@ConditionalOnClass、@Conditional和Condition接口的使用

一、@ConditionalOnBean、@ConditionalOnProperty、@ConditionalOnClass

@ConditionalOnBean是指当spring容器中有某个bean时才装配。@ConditionalOnMissingBean当spring容器中不存在某个bean时才装配

@ConditionalOnClass是指当classpath下有某个类时才装配。@ConditionalOnMissingClass是指当classpath下没有某个类时才装配

@ConditionalOnProperty是指当某个配置项为某个值时才装配。@ConditionalOnMissingProperty是指当某个配置项不等于某个值时才装配

例如：@ConditionalOnBean(name="user")表示当容器中有名字为user的bean时才装配。

下面做下验证测试：

第一步：当没有任何注解时，spring容器里会有三个Runnable类型的bean，名字分别为createRable1，createRable2，createRable3

package com.lsl;import org.springframework.boot.SpringBootConfiguration;
import org.springframework.context.annotation.Bean;@SpringBootConfiguration
public class MyConfig {@Beanpublic Runnable createRable1(){System.out.println("=======default======");return ()->{};}@Beanpublic Runnable createRable2(){System.out.println("=======dev======");return ()->{};}@Beanpublic Runnable createRable3(){System.out.println("=======user======");return ()->{};}
}

package com.lsl;import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;@SpringBootApplication
public class SpringbootProviderApplication {public static void main(String[] args) {ConfigurableApplicationContext content = SpringApplication.run(SpringbootProviderApplication.class, args);System.out.println("springname=" + content.getEnvironment().getProperty("springboot.name"));System.out.println("ds.name="  + content.getEnvironment().getProperty("ds.name"));System.out.println("ds.password="  + content.getEnvironment().getProperty("ds.password"));//从容器中获取类型为Runnable的所有beanSystem.out.println(content.getBeansOfType(Runnable.class));}}

输出结果：

第二步：当分别加上注解时

package com.lsl;import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.context.annotation.Bean;@SpringBootConfiguration
public class MyConfig {@Bean@ConditionalOnBean(name="user")public Runnable createRable1(){System.out.println("=======default======");return ()->{};}@Bean@ConditionalOnClass(name="user")public Runnable createRable2(){System.out.println("=======dev======");return ()->{};}@Bean@ConditionalOnProperty(name="opFlag",havingValue = "save")public Runnable createRable3(){System.out.println("=======user======");return ()->{};}
}

结果输出：由于没有user的bean所以没有装配

第三步：创建User类，并交给spring容器托管。

package com.lsl;import org.springframework.stereotype.Component;@Component
public class User {
}

输出结果：由于spring容器托管了User类，名字且为user，所以createRable1装配了

在默认配置文件加上配置项，createRable3也装配了

由于classpath下没有user的bean，所有createRable2没有被装配。

二、@Conditional和Condition接口

上面的三个注解时spring提供的比较常用的几个注解，当然spring还提供了其他类似注解，

比如@ConditionalOnJava，当jdk为某个版本时才装配。

除此之外spring还提供了扩展，利用Condition接口和@Conditional注解自定义扩展功能

一般情况下Condition接口和@Conditional注解要配合使用。

首先是实现Condition接口。

package com.lsl;import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;import java.util.Arrays;
import java.util.List;public class MyCondition implements Condition {@Overridepublic boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {String opFlag = conditionContext.getEnvironment().getProperty("opFlag");String[] beanDefinitionNames = conditionContext.getRegistry().getBeanDefinitionNames();List<String> strings = Arrays.asList(beanDefinitionNames);if (strings.contains("user" ) && "save".equals(opFlag)){return true;}return false;}
}

createRable4（）方法上加了 @Conditional(MyCondition.class)注解，表示只有当MyCondition中的方法返回true时才装配。

package com.lsl;import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;@SpringBootConfiguration
public class MyConfig {@Bean@ConditionalOnBean(name="user")public Runnable createRable1(){System.out.println("=======default======");return ()->{};}@Bean@ConditionalOnClass(name="user")public Runnable createRable2(){System.out.println("=======dev======");return ()->{};}@Bean@ConditionalOnProperty(name="opFlag",havingValue = "save")public Runnable createRable3(){System.out.println("=======user======");return ()->{};}@Bean@Conditional(MyCondition.class)public Runnable createRable4(){System.out.println("=======MyCondition======");return ()->{};}
}

输出结果如下：MyCondition的功能是当配置项opFlag=save且容器中已经注册了user名字的bean时才返回true

这种配合使用的方法非常灵活，可以多看看Condition接口中matchs方法的两个参数，这2个参数能获取好多信息，可以根据信息进行相关逻辑判断是返回true还是false.