Annotations cannot be both Scope annotations and Qualifier annotations: this causes confusion when trying to use them.


The problem

Qualifiers and Scoping annotations have different semantic meanings and a single annotation should not be both a qualifier and a scoping annotation.

If an annotation is both a scoping annotation and a qualifier, unless great care is taken with its application and usage, the semantics of objects annotated with the annotation are unclear.

Take a look at this example:

@interface DayScoped {}

static class Allowance {}
static class DailyAllowance extends Allowance {}
static class Spender {
  Spender(Allowance allowance) {}

static class BindingModule extends AbstractModule {
  Allowance providesAllowance() {
    return new DailyAllowance();

Here, the Allowance instance used by Spender isn’t actually scoped to a single day, as the @Provides method applies the DayScoped scoping only to the @DayScoped Allowance. Instead, the default constructor of Allowance is used to create a new instance every time a Spender is created.

If @DayScope wasn’t a Qualifier, the provider method would do the right thing: the un-annotated Allowance binding would be scoped to DayScope, implemented by a single DailyAllowance instance per day.


Suppress false positives by adding the suppression annotation @SuppressWarnings("OverlappingQualifierAndScopeAnnotation") to the enclosing element.