SpringFramework之@Conditional

    Springboot的版本2.0.5.release

    先上代碼吧，以下List-1

    List-1

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;

@Configuration
public class ConditionConfig {

    @Bean
    @Conditional(LinuxCondition.class)
    public OsService linuxOs(){
        return new LinuxService();
    }

    @Bean
    @Conditional(DefaultCondition.class)
    public OsService defaultOs(){
        return new DefaultService();
    }
}

public class DefaultCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        boolean defaultOs = !context.getEnvironment().getProperty("os.name").contains("Linux");
        System.out.println("defaultOs:" + defaultOs);
        return defaultOs;
    }
}

public class LinuxCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        boolean linux = context.getEnvironment().getProperty("os.name").contains("Linux");
        System.out.println("linux:" + linux);
        return linux;
    }
}

public class DefaultService implements OsService{
    @Override
    public void printName() {
        System.out.println("Default");
    }
}

public class LinuxService implements OsService{
    @Override
    public void printName() {
        System.out.println("Linux");
    }
}

public interface OsService {

    void printName();
}

@EnableAutoConfiguration
public class OsTest {

    public static void main(String[] args) {
        ConfigurableApplicationContext context =
                new SpringApplicationBuilder(ConditionConfig.class)
                .web(WebApplicationType.NONE)
                .run(args);
        OsService bean = context.getBean(OsService.class);
        bean.printName();
    }
}

    Conditional註解很重要，是Springboot自動化配置的基礎，它會根據指向的condition實現類，在SpringIOC的時候調用其matches方法，若是返回true則這個bean註冊到beanFactory中。

    Condition是在什麼地方被調用呢，在org.springframework.context.annotation.ConditionEvaluator中，以下List-2

    List-2

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
    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();
        }
        if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
            return true;
        }
    }
    return false;
}

    會查找全部Conditional註解的value值，以後實例化、排序，而後調用matches方法。我發現ConditionEvaluator.shouldSkip()被不少地方調用到，可是基本都是在BeanDefinitionReader等掃描的時候調用，以下List-3

    List-3

<T> void doRegisterBean(Class<T> annotatedClass, @Nullable Supplier<T> instanceSupplier, @Nullable String name,
        @Nullable Class<? extends Annotation>[] qualifiers, BeanDefinitionCustomizer... definitionCustomizers) {

    AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
    if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
        return;
    }
...

    Springboot中的ConditionalOnClass/ConditionalOnMissingClass等都是基本Spring的condition來實現的，不過在實現上更爲複雜。

Reference

1. 源碼