Provide a way to bind classes with multiple prefixes

I’ve a use case where I need to bind configuration properties based on two prefixes, one of which is determined at runtime. Let’s say the constant prefix is foo and the runtime prefix is bar.

Given a new instance of Java Bean class FooBar, the code should bind all environment variables FOO_, then overwrite with all environment variables BAR_.

public class FooBar {
    private Duration latency = Duration.ofMillis(500L);
    // other properties
    // getters, setters
}

If there are no environment variables FOO_LATENCY or BAR_LATENCY, FooBar.getLatency() is 500 ms. If only one of FOO_LATENCY and BAR_LATENCY is present, FooBar.getLatency() takes its value. If both FOO_LATENCY and BAR_LATENCY are present, FooBar.getLatency() takes the value of BAR_LATENCY.

This SO answer shows a way to do it by repeated binding. It works, but it confusing.

It’d be nice to be able to merge BindResults. Some ideas off the top of my head:

1. binder.bind accepts multiple prefixes, not just one, latter progressively overwriting the former.
2. bindResult.mergeWith(anotherBindResult), latter overwriting the former.

[philwebb]

Spring Cloud would also like this feature.

[wilkinsona]

One benefit of the multiple prefixes approach is that it would support constructor binding to an immutable type.