Document that placeholders in `@DefaultValue` annotations are not resolved

[hwolf]

My application uses @DefaultValue annotation with a SpEl expression as default value. I assumed SpringBoot resolves the expression, but SpringBoot doesn't.

Example:

@SpringBootApplication
@EnableConfigurationProperties(SampleApplication.SampleProperties.class)
public class SampleApplication {
  @ConstructorBinding
  @ConfigurationProperties("sample")
  public static class SampleProperties {
    public SampleProperties(String prop1, @DefaultValue("${sample.prop1}") String prop2) {
      System.out.println("prop1=" + prop1);
      System.out.println("prop2=" + prop2);
    }
  }

  public static void main(String[] args) {
    System.setProperty("sample.prop1", "Hello World");
    SpringApplication.run(SampleApplication.class, args);
  }
}

Actual output:

prop1=Hello World
prop2=${sample.prop1}

Expected output:

prop1=Hello World
prop2=Hello World

It would be nice, if SpEl expressions in the @DefaultValue annotations would be resolved as for other Spring annotations, for example, the@Value annotation.

[wilkinsona]

Thanks for the suggestion.

The @DefaultValue annotation is intended to provide a default value for an immutable property, in the same way that you can provide a default value for a JavaBean-based property in the field declaration:

private String example = "defaultValue";

Property placeholders are intentionally not supported in either scenario. For simplicity we recommend that a property's default value is constant. This, among other things, allows it to be included in the configuration property metadata and displayed as part of auto-completion in your IDE.