Add support for relative ordering

[snicoll]

Overview

Our current support for @Order/Ordered uses an absolute number with a default value of "lowest precedence" (i.e. Integer.MAX_VALUE). We also have PriorityOrdered to override such ordering in certain scenarios.

One problem with the current default value is that it is not natural for customizers where you'd want the "default" ones to be at a given order with the ability of custom instance to run before, or after. Such a default must provide a value as there is no way to run "after" with the current default value. A natural value for that is zero with negative values being processed before and positive values being processed after.

A better way to express ordering is to express it relative to another concept. You may need to run/customize/execute before or after another "instance". In Spring Boot, we've essentially implemented that with @AutoConfigureBefore and @AutoConfigureAfter to order auto-configurations.

This issue is to discuss if there is an appetite to provide such constructs in the core framework, with an SPI that would let us order things (potentially being honored in ObjectProvider#orderedStream().

Related Issues

• Order metadata to enforce appearance at the end of a sorted collection #34670

[andersonkyle]

This would be particularly nice for servlet filter registration.

[mjustin]

As a user, I can confirm this would be useful. The current approach I have to follow is open the bean, see if there's an order defined, and then order relative to that (hoping that it's not at the max/min in a way that would prevent going before/after it). When there isn't an order defined, I need to pull open @Order.value() to see what the default is (LOWEST_PRECEDENCE) and make sure I order relative to that.

Then I write something like the following, where the reasoning is in the comments, rather than being declarative in the code itself:

@Order(Ordered.LOWEST_PRECEDENCE - 1) // Higher precedence than SomeOtherThing

[rstoyanchev]

Team Decision: this is worth doing to provide sufficient control over ordering. There are some design decisions to be thought through such as relative vs explicit ordering, how to refer to others for relative ordering, etc.