SpringFramework之@Conditional

克虏伯

发布于 2019-06-21 14:53:47

5180

发布于 2019-06-21 14:53:47

文章被收录于专栏：软件开发-青出于蓝软件开发-青出于蓝

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Springboot的版本2.0.5.release

先上代码吧，如下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注解很重要，是Springboot自动化配置的基础，它会根据指向的condition实现类，在SpringIOC的时候调用其matches方法，如果返回true则这个bean注册到beanFactory中。

Condition是在什么地方被调用呢，在org.springframework.context.annotation.ConditionEvaluator中，如下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;
}

会查找所有Conditional注解的value值，之后实例化、排序，然后调用matches方法。我发现ConditionEvaluator.shouldSkip()被很多地方调用到，但是基本都是在BeanDefinitionReader等扫描的时候调用，如下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来实现的，不过在实现上更为复杂。

Reference

源码

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