See Stack Overflow question: Can ${...} properties appear in the <parent> section of a POM file?

While Maven's documentation claims that property values "are accessible anywhere within a POM", there are several tags that must be "constant", i.e. you cannot use property values in them. Today (2024/7/21) those restrictions are undocumented and unenforced.

Some restrictions can be discovered by trial-and-error, provided you pay attention to Maven's warnings.

To complicate matters, some properties are allowed in tags that are supposed to be constant. For instance, magic CI-friendly version properties can appear in <version> and <parent> -> <version>. There may be other exceptions as well; I can't find a definitive list of magic properties that Maven treats differently.

• baseline: bog-standard parent-child project setup
• name-contains-property: POM where name depends on a property defined in the same file
• groupid-contains-property: POM where groupId depends on a property defined in the same file
• groupid-contains-property-defined-in-parent: child's groupId depends on a property defined in the parent
• parent-groupid-contains-property-asymmetric: child declares parent's groupId using a property; parent has a constant groupId
• parent-groupid-contains-property-symmetric: child declares parent's groupId using a property; parent declares its own groupId the same way
• parent-version-contains-property-symmetric: child declares parent's version using a property; parent declares its own version the same way
• parent-version-contains-revision-property-asymmetric: child declares parent's groupId using the magic ${revision} property; parent has a constant groupId
• parent-version-contains-revision-property-symmetric: child declares parent's groupId using the magic ${revision} property; parent declares its own groupId the same way

Maven expects these tags to be constant:

• <parent> and everything in it
• <groupId>, <artifactId>, and <version>
• maybe others (see Open Questions below)

A few notes about specific examples I think are interesting:

groupid-contains-property unexpectedly works. Maven issues a warning, but the effective POM has a groupId with the expanded property.

groupid-contains-property-defined-in-parent unexpectedly works. Maven issues a warning, but the effective POM has a groupId with the expanded property.

parent-groupid-contains-property-asymmetric fails. This is presumably expected, but it might be surprising because the child does contain enough information to fully resolve the parent's groupId.

parent-groupid-contains-property-symmetric unexpectedly works. When the groupIds are char-for-char equal before property expansion, Maven seems to accept it (with a warning). Note that this can produce a broken set of effective POMs if the parent and child do not agree on the value of the property.

parent-version-contains-revision-property-symmetric works but produces broken effective POMs. Maven issues no warnings in this case, but the final child POM references parent with version v2.0 while the final parent POM has version v1.0.

• Can properties appear in <repositories>? (A repo might be needed to find the parent, and the parent might have the value for the property.)