是否有重新注入/更新注入的bean字段的方法？

Question

public class MyBean {

    @Inject
    @DbConfigValue(MyConfig.HOST)
    String host;
}

@Singleton
@Lock(LockType.READ)
public class Configuration {

    @Produces
    @Dependent
    @DbConfigValue
    public String getDbConfigValue(InjectionPoint point) {
        // get key for the config-value from qualifier-annotation of the injected field
        String key = point.getAnnotated().getAnnotation(DbConfigValue.class).value();
        // i have read+cached database config values in @PostConstruct before
        return cachedConfigValues.get(key);
    }
}

这对于初始注入/bean构建非常有效。一些网络教程建议采用这种方法。

现在，我认为可以合理地假设配置值（如果存储在DB中）可能会在运行时发生变化。因此，每当管理员更改数据库配置值时，我就会触发CDI-Event。

问题：有什么方法可以将值重新注入到已经初始化的bean实例的字段中吗？还是注入总是只与实例创建相关？

public class MyEventListener {

    @Inject
    BeanManager beanManager;

    @Asynchronous
    public void onDbConfigValueChangedEvent (@Observes(during = TransactionPhase.AFTER_SUCCESS) DbConfigValueChangedEvent event) {
        try {
            // could be filtered by custom qualifier:
            Set<Bean<?>> beans = beanManager.getBeans(Object.class,new AnnotationLiteral<Any>() {});

            for (Bean<?> bean : beans) {
                Set<InjectionPoint> points = bean.getInjectionPoints();
                // What now? javax.enterprise.inject.spi.Bean is the 
                // bean-representation only. 
                // Can I somehow resolve the actual bean-instances here?
                // Then update Field via Reflection?
            }
        }
        catch(Exception e){
            // ...
        }
    }
}

public class MyBean {

    @Inject
    Configuration config;

    public void someMethod(){
        String host = config.getConfig(MyConfig.HOST);
        // ...
    }
}

然而，我想知道一个普遍的问题：是否有任何支持重新注射？或者如果不是，规范（CDI或Java EE）是否禁止它？

Answer 1

根据db的快/慢程度，这可能是昂贵的。您可能可以利用producer方法中的一些缓存机制。利用instance注入机制，它会懒洋洋地加载实际注入的bean。

您的生产者（可能会利用一些缓存来避免所有的db调用）

@Singleton
@Lock(LockType.READ)
public class Configuration {

    @Produces
    @RequestScoped //May fail if not in a request-context, but for ejb-calls, it is guaranteed to work as CDI has EJB Request Context
    @DbConfigValue
    public String getDbConfigValue(InjectionPoint point) {
        // get key for the config-value from qualifier-annotation of the injected field
        String key = point.getAnnotated().getAnnotation(DbConfigValue.class).value();
        // i have read+cached database config values in @PostConstruct before
        return cachedConfigValues.get(key);
    }
}

和注射要点：

@SessionScoped
public class MyBean {

    @Inject
    @DbConfigValue(MyConfig.HOST)
    private Instance<String> host;

    public void doSomething() {
        String myHost = host.get(); // of course will throw exception if value is failing. It will be resolved with every request.
    }
}