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https://issues.apache.org/jira/browse/MSOURCES-95?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Karl Heinz Marbaise reopened MSOURCES-95:
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> When one of the source folders is empty the source jar is recreated even when 
> sources not changed
> -------------------------------------------------------------------------------------------------
>
>                 Key: MSOURCES-95
>                 URL: https://issues.apache.org/jira/browse/MSOURCES-95
>             Project: Maven Source Plugin
>          Issue Type: Bug
>            Reporter: Plamen Totev
>            Priority: Minor
>         Attachments: reproduce-project.zip
>
>
> There is issue opened in Plexus Archive project - 
> https://github.com/codehaus-plexus/plexus-archiver/issues/24
> In short if you download the attached zip and build it several times you'll 
> see that the source jar is build every time no matter that the source are not 
> changed. That's because {{target/generated-sources/annotations}} is empty. I 
> think the reason for this behavior is that the default inclusion pattern in 
> {{AbstractSourceJarMojo}} is {{\*\*/\*}}. If it's {{\*\*/\*\*}} then the jar 
> will not be rebuilded. Is there any reason why it's {{\*\*/\*}} and not 
> {{\*\*/\*\*}} ?
> In the next paragraphs I'll try to explain why the second patter work and the 
> first not.
> Let's say we have the following directory:
> {code}
> + src
> +--+ main
>    +--+ java
>        +--+ app
>        |  +--+ App.java
>        |
>        +--+ empty_dir
> {code}
> when added to {{JarArchiver}} with prefix "" (empty string) this is how 
> Plexus Archiver "sees" them:
> {code}
> src/main/java -> "" (empty string)
> src/main/java/app -> "app"
> src/main/java/app/App.java -> "app/App.java"
> src/main/java/empty_dir -> "empty_dir"
> {code}
> To decide which entries to add Plexus Archives applies inclusion and 
> exclusion patterns. Internally it uses 
> {{org.codehaus.plexus.util.DirectoryScanner}} 
> (https://github.com/codehaus-plexus/plexus-utils/blob/master/src/main/java/org/codehaus/plexus/util/DirectoryScanner.java).
>  The JavaDoc contains nice explanation how the patterns are applied but in 
> the next paragraph I'll try to explain in short what happens.
> When the pattern is set to {{\*\*/\*}} it will match all but /src/main/java 
> ("app", "app/App.java", "empty_dir"). This is because the pattern matches 
> zero or more path segments, followed by any path segment. src/main/java is 
> mapped to "" so the pattern will not match it (it's empty path segment 
> followed by nothing). So far so good. But what if the source directory is 
> empty(src/main/java). Then nothing will be matched and we'll end with empty 
> resource collection to add to the archive. And because it's empty there is no 
> way to know when it was last modified so Plexus Archiver will do the safe 
> thing and recreate the archive. After all when the archive was created the 
> pattern may have matched something and the archive is not empty so it's 
> outdated.
> What if the pattern is set to {{\*\*/\*\*}} it will match "", "app", 
> "app/App.java" and "empty_dir". That's because it matches zero or more path 
> segments, followed by zero or more path segments - in other words everything 
> (my understanding is that it's the same as {{\*\*}} - match everything). So 
> if /src/main/java is empty we'll end up with collection containing only "" - 
> file resource with empty name. Because of the empty name it won't be added to 
> the archive(and that's exactly what we want) but there is at least one 
> resource with last modification date so now Plexus Archiver knows if the 
> archive is up to date or not. If there were files in the directory and now 
> there is none then the directory last modification date will be newer than 
> the one of the archive.



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