@Priority(LOWEST_PRECEDENCE) still higher than no priority

[FyiurAmron]

Affects: all Spring Framework versions (spring-beans module)

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@Priority(LOWEST_PRECEDENCE) is still higher than no priority - which makes it impossible to lower the priority of selected @Component via this annotation.

(see #26241 for a similar issue, although this one proposes a simpler solution for a particular subcase)

Let's assume this:

@Component
@Priority(HIGHEST_PRECEDENCE)
class ComponentHighestPrecedence extends SomeComponent {}

@Component
@Priority(LOWEST_PRECEDENCE)
class ComponentLowestPrecedence extends SomeComponent {}

@Component
class ComponentNoExplicitPrecedence extends SomeComponent {}

In this case, ComponentHighestPrecedence will be always selected, as expected. If we remove it, however, ComponentLowestPrecedence will be always selected, due to how https://github.com/spring-projects/spring-framework/blob/main/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java#L1787 is currently implemented.

Although that's probably by design, there is a much bigger issue here: there is no way to say that a component should have priority lower than or equal to "no explicit priority". LOWEST_PRECEDENCE is thus a misnomer in this case, it's "lower than any explicit but higher than all implicit".

The use case when this is problematic is very simple: let's say we create a library that provides a bean that needs to be used only if the library consumer doesn't provide his own implementation. The workaround is to force the consumer to use @Primary, but that forces maintenance on the consumer. OTOH, if we would have a default reasonable priority value for components with no @Priority, we could just declare @Priority(DEFAULT_PRECEDENCE - 1) on the library component, and be done with it.

Changing this is obviously a breaking change, but maybe there is some reasonable way to provide this as a configuration option somewhere, that would make e.g. getPriority return 0 (or even better, some DEFAULT_PRECEDENCE value) instead of null for the beans that have no priority given explicitly?

[snicoll]

Although that's probably by design

Yes.

LOWEST_PRECEDENCE is thus a misnomer in this case, it's "lower than any explicit but higher than all implicit".

I disagree. Not having a priority (or an @Order) at all means that you don't express any preference. The Javadoc for OrderComparator states:

Any object that does not provide its own order value is implicitly assigned a value of Ordered.LOWEST_PRECEDENCE, thus ending up at the end of a sorted collection in arbitrary order with respect to other objects with the same order value.

If you implement Ordered then you must provide a default value. You can even do so with a default method whose value is 0 if you feel like it's a good default. Unfortunately, we can't change the default behavior of a component that doesn't express a priority.

[FyiurAmron]

@snicoll

Any object that does not provide its own order value is implicitly assigned a value of Ordered.LOWEST_PRECEDENCE, thus ending up at the end of a sorted collection in arbitrary order with respect to other objects with the same order value.

but this is not the case here! - if that was the case, there would be an exception thrown here due to two beans having Ordered.LOWEST_PRECEDENCE (one explicit, one implicit) - instead, the component has precedence lower than LOWEST_PRECEDENCE, which is contrary to what you wrote and what's in the doc you linked to (putting aside it's a misnomer). If having no @Priority would be equal to LOWEST_PRECEDENCE, that would be understandable here - but, as I wrote, this is not the case, it's actually lower due to current implementation.

[snicoll]

Please review the doc once again.

thus ending up at the end of a sorted collection in arbitrary order with respect to other objects with the same order value.

You can have as many component with the same priority value as you want. If they have the same value, the order is arbitrary. There's no after or before as you claim.

[FyiurAmron]

You can have as many component with the same priority value as you want.

@snicoll with the same @Order, yes. Not with the same @Priority:

https://github.com/spring-projects/spring-framework/blob/main/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java#L1791

						if (candidatePriority.equals(highestPriority)) {
							throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
									"Multiple beans found with the same priority ('" + highestPriority +
									"') among candidates: " + candidates.keySet());
						}

I'm under the impression you're mistaking @Order-handling code with @Priority-handling code.

[FyiurAmron]

@snicoll even the docs say that it's not what you say:

	 * @return the priority assigned to that bean or {@code null} if none is set
	 */
	@Nullable
	protected Integer getPriority(Object beanInstance) {

Point here, it's not that "If you implement Ordered then you must provide a default value." - there is no Ordered implementation here, it's about how the default/implicit priority values are handled - i.e. null is returned from getPriority(), component is ignored in comparison and thus always ends at below LOWEST_PRECEDENCE.

[snicoll]

That's just an internal detail of how this is implemented and we're mixing the two indeed but I am trying to explain why this is the expected behavior. Your third component does not have a priority so it passes the check above. At the end of the day, for the ordering purpose, a component that does not have a priority (order) comes "at the end". From an ordering perspective, it means assigning it LOWEST_PRECEDENCE and then the order of the components that have the same order value is arbitrary in the sense that you can't expect one component with the same order to be invoked before another one.

If having no @priority would be equal to LOWEST_PRECEDENCE, that would be understandable

Behind the scenes, that's exactly what's happening.

Looking at the spec not much is said about what is happening if a component does not have a value but it's totally logical to me that it as a lower priority than the lowest priority that is defined explicitly.

[FyiurAmron]

@snicoll TBH, for me it would be logical if the term "lowest priority" would imply that a thing would have lowest priority of all, and not of just a subset of cases, but I'm not at liberty to argue about logics here. Thank you for your prompt response; if it's by design, then it's by design.

[snicoll]

LOWEST_PRECEDENCE is a concept of @Order and has nothing to do with JakartaEE's priority. It just happens that both annotations accept an integer as the value of the order or the priority, respectively. I don't really understand why you're arguing about the semantic of a separate concept.

I believe the current design is consistent. If you don't have an order, then you're invoked after every component that have one. Same for priority.