@Conditional的使用

@Conditional可以根据条件来判断是否注入某些Bean。

package com.morris.spring.config;import com.morris.spring.condition.LinuxCondition;
import com.morris.spring.condition.WindowsCondition;
import com.morris.spring.entity.User;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;@Configuration
public class ConditionalConfig {// 如果是windows系统就注入bill@Conditional(WindowsCondition.class)@Bean(name = "user")public User bill() {return new User("bill", 22);}// 如果是linux系统就注入linus@Bean(name = "user")@Conditional(LinuxCondition.class)public User linus() {return new User("linus", 20);}}

WindowsCondition和LinuxCondition都需要实现Condition接口。

WindowsCondition

package com.morris.spring.condition;import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;public class WindowsCondition implements Condition {/*** 根据条件判断是否注入对应的Bean* @param conditionContext 应用上下文* @param annotatedTypeMetadata 加了@Conditional注解的方法的元数据信息* @return*/@Overridepublic boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {String osName = conditionContext.getEnvironment().getProperty("os.name");if(osName.contains("Windows")) {return true;}return false;}
}

LinuxCondition

package com.morris.spring.condition;import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;public class LinuxCondition implements Condition {@Overridepublic boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {String osName = conditionContext.getEnvironment().getProperty("os.name");if(osName.contains("linux")) {return true;}return false;}
}

如果要测试LinuxCondition并不需要再linux系统下运行，只需要的启动时设置环境参数：-Dos.name=linux。

Conditional的扩展

ConditionalOnBean

ConditionalOnBeanc.java

package com.morris.spring.condition;import org.springframework.context.annotation.Conditional;import java.lang.annotation.*;@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {Class<?>[] value() default {};
}

OnBeanCondition.java

package com.morris.spring.condition;import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;import java.util.Map;public class OnBeanCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {Map<String, Object> annotationAttributes = metadata.getAnnotationAttributes(ConditionalOnBean.class.getName());Class<?>[] clazz = (Class<?>[]) annotationAttributes.get("value");for (Class<?> aClass : clazz) {Map<String, ?> beans = context.getBeanFactory().getBeansOfType(aClass);if(beans.isEmpty()) {return false;}}return true;}
}

ConditionalOnProperty

ConditionalOnProperty.java

package com.morris.spring.condition;import org.springframework.context.annotation.Conditional;import java.lang.annotation.*;@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {String[] value() default {};
}

OnPropertyCondition.java

package com.morris.spring.condition;import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;import java.util.Map;public class OnPropertyCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {Map<String, Object> annotationAttributes = metadata.getAnnotationAttributes(ConditionalOnProperty.class.getName());String[] propertyArray = (String[]) annotationAttributes.get("value");for (String property : propertyArray) {if(!context.getEnvironment().containsProperty(property)) {return false;}}return true;}
}

源码分析

如果Condition返回的是false，那么spirng就不会对方法或类进行解析。

org.springframework.context.annotation.ConditionEvaluator#shouldSkip

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {// 判断类或方法上面是否有@Conditional注解if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {return false;}if (phase == null) {if (metadata instanceof AnnotationMetadata &&ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);}return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);}List<Condition> conditions = new ArrayList<>();for (String[] conditionClasses : getConditionClasses(metadata)) {for (String conditionClass : conditionClasses) {Condition condition = getCondition(conditionClass, this.context.getClassLoader());conditions.add(condition);}}AnnotationAwareOrderComparator.sort(conditions);for (Condition condition : conditions) {ConfigurationPhase requiredPhase = null;if (condition instanceof ConfigurationCondition) {requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();}// 调用condition.matches方法if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {return true;}}return false;
}

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