springboot報錯無法讀取配置檔案中的屬性：Could not resolve placeholder ‘xxxx‘ in value “${xxxx}

現象:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'constantPropertiesUtils': Injection of autowired dependencies failed; nested exception is java.lang.IllegalArgumentException: Could not resolve placeholder 'custom.filehost' in value "${custom.filehost}"
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:403) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1429) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:594) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:879) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:878) ~[spring-context-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'custom.filehost' in value "${custom.filehost}"
	at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:178) ~[spring-core-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:124) ~[spring-core-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.core.env.AbstractPropertyResolver.doResolvePlaceholders(AbstractPropertyResolver.java:236) ~[spring-core-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.core.env.AbstractPropertyResolver.resolveRequiredPlaceholders(AbstractPropertyResolver.java:210) ~[spring-core-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.lambda$processProperties$0(PropertySourcesPlaceholderConfigurer.java:175) ~[spring-context-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.resolveEmbeddedValue(AbstractBeanFactory.java:908) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1228) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1207) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:636) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:116) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:397) ~[spring-beans-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	... 17 common frames omitted

環境：

系統：windows 10
IDE：vs code
編譯工具：gradle

解決：

因為發現application.properties檔案裡其他屬性都能被@Value註解識別，只有新增的屬性不能，於是想到可能是某些快取問題導致的吧。

首先，嘗試執行‘gradle clean build’命令清理工程並重新構建，結果發現仍然報相同的錯誤。

接著嘗試在vs code中開啟命令皮膚輸入"clean the java language server workspace"並執行，重新開啟vs code後再執行此錯誤就不再出現了。

總結：

最終還是沒弄清楚這個問題的原因，只是憑藉經驗取巧解決的，希望有知道問題原因的大神能分享一下。

PS：使用vs code開發經常遇到各種奇奇怪怪的問題，在花費大量時間分析問題或嘗試各種解決方案無效後，使用命令"clean the java language server workspace"重啟vs code往往都能解決問題。這也算一個收穫吧，以後遇到類似問題先用clean這招試一下，說不定就解決了，還能節省時間。