Order metadata to enforce appearance at the end of a sorted collection

[mipo256]

Overview

I kindly want to ask to resume the conversation on this topic.

It seems, that people have to deal with this problem.

What is desired, is that assume we have 3 beans: A, B and NoOp. And I assemble them into a List somewhere via:

@Autowired
private List<CommonInterface> list;

And what I want is the following:

for (var impl : list) {
    if (impl.supports(o)) {
        impl.doWork(o);
    }
}

The NoOp implementation is required to be the last, since it is fallback. But I really do not care about the order of other beans in the List. What I want is to ensure, that the NoOp bean is the last one in the List.

The problem is that simply adding @Order with the lowest priority to the NoOp will not work.

I can, of course, define the @Order with higher priority for each bean except for NoOp, but this requires that every time a new implementation of CommonInterface being added, the developer must not forget about adding the corresponding order marker as well. Otherwise, the NoOp may not be the last, which can lead and probably would lead to bugs.

The deeper problem is that if I deal with already compiled code, and I cannot physically add any @Order/Ordered to the compiled implementations, but I need to add my own to the end of the list.

I hope we can collaboratively find a solution. Thank you.

Related Issues

• Add support for relative ordering #24844

[quaff]

You can annotate NoOp with @Fallback, it's new feature since 6.2.

Currently @Fallback doesn't affect the order, but I think it should.

[jhoeller]

Design-wise, @Primary/@Fallback and @Order are rather orthogonal concerns. The former annotations are used in the dependency resolution algorithm for selecting a single match among multiple candidates, whereas the latter - as part of our ordering/sorting arrangement - is used for multi-element exposure in collections, arrays, and streams.

To me, your request sounds like a variant of the PriorityOrdered interface next to Ordered, just on the opposite end of the spectrum. We could introduce such an opposite marker interface for a whole range of values sorted behind regular Ordered, just like PriorityOrdered provides such a value range before regular Ordered.

However, even PriorityOrdered is not exposed for @Order annotation usage. So if we had an XxxOrdered interface for the other end of the spectrum, your NoOp bean would have to implement that interface directly. I'm not keen on exposing those categories of exposure on the @Order annotation through an enum attribute or the like; ideally, those special categories should remain interface-based.

[sbrannen]

Let's keep in mind that this is related to the following, which we already have in the general backlog.

• Add support for relative ordering #24844