Declared defaults do not get property references replace

Assume you declare a configuration properties class like this:

@ConstructorBinding
@ConfigurationProperties("sample")
class SampleProperties {

  SampleProperties(@DefaultValue("${user.home}/foo") String location) { … }
}

If the application now configures the property sample.location to ${user.home}/bar, the constructor is called with /Users/…/bar, i.e. the dynamic reference to the user home folder has been replaced with the actual value. If the application does not configure an explicit value, the value handed into constructor is literally ${user.home}/foo, i.e. the reference to the user’s home directory is not resolved.

[wilkinsona]

We talked about this and concluded that we’d like to align the behaviour of @DefaultValue with what would happen if you had written the same value in application .properties or application.YAML. You get conversion at the moment (so, for example, "3w" for a Duration would become a Duration of 3 weeks), but you don’t get placeholder replacement. For consistency, we’d like to get both.

We have some concerns about backward-compatibility so it’ll have to wait for 3.x. We’ve reopened #23164 to improve the docs in the meantime.

[wilkinsona]

As things stand, the desired behaviour can be achieved like this with setter-based binding:

@ConfigurationProperties("sample")
public class SampleMutableProperties {
    
    /**
     * Location to use. Defaults to a directory named example in the user's home directory.
     */
    private String location = System.getProperty("user.home") + "/example";

    public String getLocation() {
        return location;
    }

    public void setLocation(String location) {
        this.location = location;
    }

}

The same can be achieved with immutable configuration properties like this:

@ConstructorBinding
@ConfigurationProperties("sample")
public class SampleImmutableProperties {
    
    /**
     * Location to use. Defaults to a directory named example in the user's home directory.
     */
    private final String location;
    
    public SampleImmutableProperties(String location) {
        this.location = (location != null) ? location : System.getProperty("user.home") + "/example";
    }

    public String getLocation() {
        return location;
    }

}

The javadoc for @DefaultValue states that it “can be used to specify the default value when binding to an immutable property”. That’s not really accurate when you’re trying to turn SampleMutableProperties into something immutable. You have to perform the defaulting manually in the constructor as shown above instead.

Knowing that you’ve only got a string to work with, trying to use @DefaultValue("${user.home}/example") (particularly if you’re already familiar with Spring) seems quite logical to me but it won’t work as hoped as ${user.home} will be left as-is. If it worked as @odrotbohm expected and performed property placeholder resolution it would be more concise and also have the benefit over the field initializer of being able to document a sensible default value for the property automatically.