无法从 BeanPostProcessor 中的占位符获取值

Question

我正在将 Spring 与 Spring Boot BOM 2.4.0 附带的遗留 Tomcat 应用程序（不是 Spring Boot）一起使用，问题类似于 Spring Expression Language not working，但我有一个具体案例。

如果我有一个 @Value("${spring.kafka.maximumRequestSize:15728640}") 和以下的依赖类

@Configuration
@Order
public class KafkaTracingDecorator implements  ApplicationContextAware {
    private KafkaTracing kafkaTracing;

    @Override
    public void setApplicationContext(final ApplicationContext applicationContext) throws BeansException {

        kafkaTracing = applicationContext.getBean(KafkaTracing.class);
    }
}

它有效。以下也有效，但我没有在后处理器上做任何事情

@Configuration
@Order
public class KafkaTracingDecorator implements BeanPostProcessor, ApplicationContextAware {

    @Override
    public Object postProcessAfterInitialization(final Object bean, final String beanName) throws BeansException {

        return bean;
    }

    @Override
    public void setApplicationContext(final ApplicationContext applicationContext) throws BeansException {

//        kafkaTracing = applicationContext.getBean(KafkaTracing.class);
    }
}

但是当我尝试像这样实现BeanPostProcessor 到implement the decorator pattern 时

@Configuration
@Order
public class KafkaTracingDecorator implements BeanPostProcessor, ApplicationContextAware {

    private KafkaTracing kafkaTracing;

    @Override
    public Object postProcessBeforeInitialization(final Object bean, final String beanName) throws BeansException {

        if (bean instanceof KafkaProducer) {
            return kafkaTracing.producer((KafkaProducer) bean);
        } else if (bean instanceof KafkaConsumer) {
            return kafkaTracing.consumer((KafkaConsumer) bean);
        } else {
            return bean;
        }
    }

    @Override
    public void setApplicationContext(final ApplicationContext applicationContext) throws BeansException {

        kafkaTracing = applicationContext.getBean(KafkaTracing.class);
    }
}

在创建 WebApplicationContext 时我得到Caused by: java.lang.NumberFormatException: For input string: "${spring.kafka.maximumRequestSize:15728640}"。

即使我不使用该值也会发生这种情况，因此它不会进入方法

@Configuration
@Order
public class KafkaTracingDecorator implements BeanPostProcessor, ApplicationContextAware {

    private KafkaTracing kafkaTracing;

    @Override
    public Object postProcessBeforeInitialization(final Object bean, final String beanName) throws BeansException {

        return bean;
    }

    @Override
    public void setApplicationContext(final ApplicationContext applicationContext) throws BeansException {

        kafkaTracing = applicationContext.getBean(KafkaTracing.class);
    }
}

这是一个非常简化的示例，但也以同样的方式失败：

@Configuration
public class KafkaTracingDecorator implements BeanPostProcessor {

    @Value("${spring.kafka.maximumRequestSize:15728640}")
    private int maxRequestSize;

}

（请注意，这在 Spring Boot 上按预期工作，因此可能与 Spring 的非引导使用有关）

导致错误的部分堆栈跟踪未列出任何自定义代码：

    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:660)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1413)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:601)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:213)
    at org.springframework.context.support.PostProcessorRegistrationDelegate.registerBeanPostProcessors(PostProcessorRegistrationDelegate.java:270)
    at org.springframework.context.support.AbstractApplicationContext.registerBeanPostProcessors(AbstractApplicationContext.java:761)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:566)
    at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:401)
    at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:292)
    at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:103)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4689)

MVCE 添加：https://github.com/trajano/spring-beanpostproc-mvce 链接到 GitHub 问题，以防这是一个错误 https://github.com/spring-projects/spring-framework/issues/26571，而不是使用问题。

Answer 1

您需要通过注册PropertySourcesPlaceholderConfigurer bean 来更新Decorator 类来解决您的问题。

根据Spring Docs on PropertyPlaceholderConfigurer：

PropertyPlaceholderConfigurer 是 PlaceholderConfigurerSupport 的子类。此子类有助于针对本地属性和/或系统属性和环境变量解析 ${...} 占位符。

因此，配置此 bean 将从类路径中加载 application.properties，它将能够解析 @Value 注释中使用的 ${..} 以获取实际值集。

更新了 Decorator 类：

@Configuration
public class Decorator implements BeanPostProcessor {
    
    @Value("${spring.kafka.maximumRequestSize}")
    private int maxRequestSize;
    
    @Bean
    public static PlaceholderConfigurerSupport placeholderConfigurerSupport() {
        PlaceholderConfigurerSupport support = new PropertySourcesPlaceholderConfigurer();
        support.setLocations(new ClassPathResource("application.properties"));
        return support;
    }

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println(maxRequestSize);
        return bean;
    }
}

application.properties：

spring.kafka.maximumRequestSize=15728640

Tomcat 8.5.57 在 AdoptedOpenJDK 11.0.6 上运行的控制台日志：

...
.....
INFO: Initializing Spring root WebApplicationContext
Feb 20, 2021 9:21:41 AM org.springframework.web.context.ContextLoader initWebApplicationContext
INFO: Root WebApplicationContext: initialization started
15728640
Feb 20, 2021 9:21:41 AM org.springframework.web.context.ContextLoader initWebApplicationContext
INFO: Root WebApplicationContext initialized in 518 ms
Feb 20, 2021 9:21:41 AM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["http-nio-8080"]
Feb 20, 2021 9:21:41 AM org.apache.catalina.startup.Catalina start
INFO: Server startup in 1419 ms