如何将 bean 注入 Spring Condition 类? [英] How to inject a bean into a Spring Condition class?

问题描述

我正在定义条件，稍后我将检查这些条件以动态加载我的服务接口的两个实现之一.

I am defining conditions that I will check to dynamically load one of the two implementations of my service interface later.

@Component
public class IsPolicyEnabled implements Condition {

    @Autowired
    private MyProperties props;

    @Override
    public boolean matches(ConditionContext arg0, AnnotatedTypeMetadata arg1) {
        return props.isPolicyEnabled();
    }

}

还有

@Component
public class MyProperties {...}

和

@Service
@Conditional(IsPolicyEnabled.class)
public class ServiceA implements Service {...}

但是，我遇到了运行时错误.

However, I am running into a runtime error as.

java.lang.IllegalStateException: Failed to load ApplicationContext
Caused by: java.lang.NullPointerException
at com.xyz.utils.IsPolicyEnabled.matches(IsPolicyEnabled.java:9)
at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:108)
at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:88)
at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:71)
at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.isConditionMatch(ClassPathScanningCandidateComponentProvider.java:515)

基本上，它无法初始化已在条件实现中自动连接的道具对象.不允许吗?

Basically, it failed to initialize props object that has been auto-wired inside the condition implementation. Is that not allowed?

如何在条件实现中自动连接另一个依赖项，因为我的条件评估取决于该依赖项提供的值?

How can I auto wire another dependency inside the condition implementation since my condition evaluation depends on a value provided by that dependency?

推荐答案

条件被检查紧接着 bean 定义将被注册 [...]

Conditions are checked immediately before the bean-definition is due to be registered [...]

^{Condition, Spring Framework 5.0.8.RELEASE API 文档}

^{Condition, Spring Framework 5.0.8.RELEASE API documentation}

您不能将 bean 注入 Condition 实例，因为上下文中还没有 bean 定义¹.

You can't inject a bean into a Condition instance because there are no bean-definitions in the context yet¹.

此外，您不应该在 Condition 类中使用 bean:

Moreover, you are not supposed to work with beans within Condition classes:

条件必须遵循与 BeanFactoryPostProcessor 相同的限制，并且注意不要与 bean 实例交互.

Conditions must follow the same restrictions as BeanFactoryPostProcessor and take care to never interact with bean instances.

^{Condition, Spring Framework 5.0.8.RELEASE API 文档}

^{Condition, Spring Framework 5.0.8.RELEASE API documentation}

你应该重新考虑设计，因为

You should rethink the design because

[...] 我的条件评估取决于该依赖项提供的值.

[...] my condition evaluation depends on a value provided by that dependency.

表示不太正确.

^{1 准确的说，Spring 已经注册了一些 bean，以满足其自身的需要.}

^{1 Precisely speaking, there are a few beans already registered by Spring for its own needs.}

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