@Configuration
@PropertiesValueSource("classpath:com/acme/db.properties")
public class AppConfig {
    @ExternalValue("datasource.username") String username;

    @Bean
    public TestBean testBean() {
        return new TestBean(username);
    }
}
            

com/acme/db.properties will be read from the classpath and the value associated with key datasource.username will be injected into the username field. The contents of db.properties might be as follows:

datasource.username=scott
datasource.password=tiger
...
                

	Note
	An array of properties file locations may be supplied to @PropertiesValueSource, and along with classpath:, all of the standard Spring resource-loading prefixes are supported, such as file: and http:. @Configuration @PropertiesValueSource({"classpath:com/acme/a.properties", "file:/opt/acme/b.properties"}) public class AppConfig { // ... }

@ExternalValue may also be used as a method-level annotation

@Configuration
@PropertiesValueSource("classpath:com/acme/db.properties")
public abstract class AppConfig {
    @ExternalValue("datasource.username")
    abstract String username();

    @Bean
    public TestBean testBean() {
        return new TestBean(username());
    }
}
                

The primary advantage to using @ExternalValue methods is that rather than injecting the external value just once (as is done in the case of @ExternalValue fields), @ExternalValue methods are evaluated every time they're referenced. As this is not usually required, @ExternalValue fields are the preferred method. A downside of @ExternalValue methods is that they should be abstract, requiring you to declare the entire @Configuration class abstract, and this is not in alignment with the semantics users typically associate with using the abstract keyword.