Spring Boot Gradle应用运行报错：无法解析占位符，外部配置如何引用主配置变量？

问题场景 #

运行Spring Boot Gradle应用时，外部配置文件无法引用application.properties中的变量，抛出占位符无法解析的错误。

报错信息 #

Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'default.user' in value "${default.user}"
    at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:180) ~[spring-core-5.3.24.jar:5.3.24]
    at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:126) ~[spring-core-5.3.24.jar:5.3.24]
    at org.springframework.core.env.AbstractPropertyResolver.doResolvePlaceholders(AbstractPropertyResolver.java:239) ~[spring-core-5.3.24.jar:5.3.24]
    at org.springframework.core.env.AbstractPropertyResolver.resolveRequiredPlaceholders(AbstractPropertyResolver.java:210) ~[spring-core-5.3.24.jar:5.3.24]
    at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.lambda$processProperties$0(PropertySourcesPlaceholderConfigurer.java:191) ~[spring-context-5.3.24.jar:5.3.24]
    at org.springframework.beans.factory.support.AbstractBeanFactory.resolveEmbeddedValue(AbstractBeanFactory.java:936) ~[spring-beans-5.3.24.jar:5.3.24]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1332) ~[spring-beans-5.3.24.jar:5.3.24]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1311) ~[spring-beans-5.3.24.jar:5.3.24]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:657) ~[spring-beans-5.3.24.jar:5.3.24]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640) ~[spring-beans-5.3.24.jar:5.3.24]
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119) ~[spring-beans-5.3.24.jar:5.3.24]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399) ~[spring-beans-5.3.24.jar:5.3.24]
    ... 17 common frames omitted

相关配置文件 #

• src/main/resources/application.properties

app.user=testaccount

• src/main/resources/config/testing.properties

default.user=${app.user}

代码片段 #

@Value("${default.user}") private String defaultUser;

已尝试的无效配置 #

在application.properties中添加以下配置，但问题未解决：

spring.config.location=/config/testing.properties
spring.config.additional-location=/config/testing.properties

解决方案 #

1. 修正配置文件加载路径 #

使用spring.config.additional-location时，需指定正确的类路径路径（testing.properties位于src/main/resources/config下），正确配置应为：

spring.config.additional-location=classpath:/config/testing.properties

注意：spring.config.location会覆盖Spring Boot默认配置文件加载路径，导致application.properties无法被加载，因此必须用spring.config.additional-location追加配置文件，而非替换。

2. 通过@PropertySource注解加载配置 #

在配置类上添加@PropertySource注解显式加载外部配置文件，Spring会自动利用已加载的application.properties变量解析占位符：

@Configuration
@PropertySource(value = "classpath:/config/testing.properties", encoding = "UTF-8")
public class AppConfig {
}

3. 验证配置加载顺序 #

启动应用时，查看日志中包含Loading property source的条目，确认application.properties和testing.properties均被加载，且application.properties先于外部配置文件加载，确保占位符能被正确解析。

内容的提问来源于stack exchange，提问作者Lee