[MJAVADOC-748] detectOfflineLinks URL generation for modules is not helpful

**[Adam Gent](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=agentgt)** opened **[MJAVADOC-748](https://issues.apache.org/jira/browse/MJAVADOC-748?redirect=false)** and commented

EDIT:

OK there is an easy workaround. After looking at the code I just didn't know that project.url gets automagically appended with the artifact path for submodules that inherit from a parent.

The workaround is to just set `project.url` for each module to the base url.

So I suppose project.url is useful as the offlineLink URL.

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For a multi module modular project (e.g. a project with multiple module-info submodules)
I have yet to see a correct usage of `detectOfflineLinks`.

Here is the use case I expect most people including myself is when performing the following

1. Generate non-aggregated javadoc jars for each module for maven central
2. Generate an aggregated exploded javadoc and host it

The hope is in step 1 the cross module links will use step 2 aggregated html.
The idea is someone clicks on a javadoc.io hosted javadoc and then clicks on a class that is in a separate module the link will not break but takes us to the aggregate hosted doc.

Here is the problem the detectOfflineLinks does a whole bunch incorrect heuristics to guess the online URL that do not map at all to the aggregate javadoc.

In an aggregate javadoc you get a directory of all the module-info modules. I stress module-info and not maven artifact names.

You can see an example of that here: https://github.com/jstachio/jstachio.github.io/tree/d036294/p/jstachio/v0.11.0

Now if we go to javadoc.io : https://javadoc.io/doc/io.jstach/jstachio/0.11.0/io.jstach.jstachio/module-summary.html

And look at JStache link (search in page) it has the following URL:

https://github.com/jstachio/jstachio/jstachio-api-parent/jstachio-annotation/apidocs/io.jstach.jstache/io/jstach/jstache/JStache.html

Now ignoring the whole `project.url` (which btw ... why is this is not a property? Like it really doesn't make since because a project.url is going to have multiple versions on its page.)

The project.url is: https://github.com/jstachio/jstachio but it should be https://jstach.io/p/jstachio/v0.11.0 (so for now mentally replace that).

For the JStache link we have `jstachio-api-parent/jstachio-annotation/apidocs/io.jstach.jstache/io/jstach/jstache/JStache.html`.

(BTW the directory `jstachio-api-parent` is actually just `api` so even if I just host the built project it still would not work).

The above should be (ignoring project.url prefix): `io.jstach.jstache/io/jstach/jstache/JStache.html`

As the module name is `io.jstach.jstache`.

I set this issue as minor because I assume the work around is it manually put in all the offlineLinks. I haven't tried that but I assume it will work albeit laborious.

I have serious doubts anyone uses detectOfflineLinks with success. Please show me an example if I'm wrong. For one it requires doing something like this with project.url:

`https://mysite.com/project/${project.version}`

I have looked around and I don't see the above much.

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With project.url of https://jstach.io/p/jstachio/v1.0.0-SNAPSHOT:

`code`
[DEBUG] Added Javadoc offline link: https://jstach.io/p/jstachio/v1.0.0-SNAPSHOT/jstachio-api-parent/jstachio-annotation/apidocs for the module: io.jstach:jstachio-annotation:jar:1.0.0-SNAPSHOT
`code`

it should be:

`code`
[DEBUG] Added Javadoc offline link: https://jstach.io/p/jstachio/v1.0.0-SNAPSHOT for the module: io.jstach:jstachio-annotation:jar:1.0.0-SNAPSHOT
`code`

That is the offline link should just be the `project.url` and should not have the module artifact names anywhere in the URL.

I assume this is a happening because the submodules project.url (project.getUrl()) uses the artifact name including the parent.

https://github.com/apache/maven-javadoc-plugin/blob/931164d0c5d11b4ac4559ada68ee5fba1d0df4f6/src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java#L5853

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**Affects:** 3.5.0

Start in src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java around line 5853, where detectOfflineLinks is linked in the report. Reproduce the multi-module case described with project.url and module-info names, then inspect the generated offline-link debug output. Done means links resolve to the aggregate Javadoc base URL without Maven artifact-path segments.

Written by the indexing model from the issue text.