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https://github.com/calvin-l/obscure-maven-examples

Empirical observations of poorly-documented Maven behavior
https://github.com/calvin-l/obscure-maven-examples

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Empirical observations of poorly-documented Maven behavior

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# Examples with Obscure Uses of Maven Properties



See Stack Overflow question:

[Can `${...}` properties appear in the `` section of a POM file?](https://stackoverflow.com/questions/78752180/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"](https://maven.apache.org/pom.html#Properties),

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](https://maven.apache.org/maven-ci-friendly.html)

can appear in `` and ` -> `.  There may be other

exceptions as well; I can't find a definitive list of magic properties that

Maven treats differently.



## What's Here?



 - `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



## Observations (Maven 3.9.7, JDK 17)



Maven expects these tags to be constant:



 - `` and everything in it

 - ``, ``, and ``

 - 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`.



## Open Questions



 - Can properties appear in ``? (A repo might be needed to find

   the parent, and the parent might have the value for the property.)