是否有“恢复”到Spring @DependsOn注释？

Question

我需要先初始化一个组件。使用@DependsOn，它看起来像这样：

@Component("beana")
public class BeanA{

    @PostConstruct
    void init(){
       // do smth
    }
}

@Component("beanb")
@DependsOn("beana")
public class BeanB{

    @PostConstruct
    void init(){
       // do smth
    }
}

我现在必须告诉BeanB，它取决于BeanA的初始化。我的问题是我不希望BeanB知道BeanA的存在（例如，当BeanB在初始化时在EventBus中发布事件，而BeanA处理这些事件时）。我想在BeanA上使用注释，说明它应该在BeanB之前初始化。所以会是这样的：

@Component("beana")
@RequiredBy("beanb") 
public class BeanA{

    @PostConstruct
    void init(){
       // do smth
    }
}

@Component("beanb")
public class BeanB{

    @PostConstruct
    void init(){
       // do smth
    }
}

是否有任何Spring注释或像这样处理它的可能性？

Answer 1

我相信对此没有开箱即用的 spring 注释，但您可以轻松制作自己的注释。

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface RequiredBy {
    String[] value();
}

然后可以遍历所有 bean 定义并将dependsOn 设置为该requiredbean。

@Component
public static class RequiredByBeanDefinitionPostProcessor implements BeanDefinitionRegistryPostProcessor {

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
        for (String beanName : registry.getBeanDefinitionNames()) {
            final BeanDefinition beanDefinition = registry.getBeanDefinition(beanName);
            if (beanDefinition.getBeanClassName() == null) {
                continue;
            }
            try {
                final Class<?> beanClass = Class.forName(beanDefinition.getBeanClassName());
                if (beanClass.isAnnotationPresent(RequiredBy.class)) {
                    final String[] dependantBeanNames = beanClass.getAnnotation(RequiredBy.class).value();
                    for (String dependantBeanName : dependantBeanNames) {
                        BeanDefinition dependantBeanDefinition = registry.getBeanDefinition(dependantBeanName);
                        dependantBeanDefinition.setDependsOn(beanName);
                    }
                }
            }
            catch (ClassNotFoundException e) { throw new RuntimeException(e); }
        }
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException { }
}

然后像在你的例子中一样使用它：

@Component("beanA")
public static class BeanA {
    @PostConstruct
    private void init() {
        System.out.println(this.getClass().getSimpleName());
    }
}

@Component("beanB")
@RequiredBy({ "beanC", "beanA" })
public static class BeanB {
    @PostConstruct
    private void init() {
        System.out.println(this.getClass().getSimpleName());
    }
}

@Component("beanC")
@RequiredBy("beanA")
public static class BeanC {
    @PostConstruct
    private void init() {
        System.out.println(this.getClass().getSimpleName());
    }
}

=>

BeanB
BeanC
BeanA