"Unzip" tool installer does not know anything specific about Java or any other tool. It just unpacks the archive to a tool cache and then sets home to the root of the archive. So it's not a bug.

In this case it might be helpful to create a new version of the installer in the JDK Tool Plugin. It can be done by copying or extending the https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/tools/DownloadFromUrlInstaller.java in the plugin

CC dnusbaum who maintains the plugin now

Question: why isn't a default installer for open-jdk ?

Cause there is a plugin, but no more upgraded (openJDK-native-plugin) :/

Historical reasons. I believe that part of the code was originally written for Sun JVMs, maybe before OpenJDK was formalized and established

Weird, I thought the unzip installers removed a top level directory if there's only one.

There's an additional option to extract a subdir.

https://github.com/jenkinsci/jenkins/blob/master/core/src/main/resources/hudson/tools/ZipExtractionInstaller/help-subdir.html

Optional subdirectory of the downloaded and unpacked archive to use as the tool's home directory

isn't the opposite that we want ? no subdirectory ?

That's exactly what this does, see the help.

ha ! I've finally understood ! I first understood that it add a subdirectory to the final path...
Yes, it's was it does, so now the JAVA_HOME is correctly set.
But still, I had to open the archive to know what the inner directory was, not very friendly use
...

so now, I just have the following error :

/var/jenkins_home/tools/hudson.tasks.Maven_MavenInstallation/Default/bin/mvn: exec: line 199: /var/jenkins_home/tools/hudson.model.JDK/Java_10/jdk-10.0.2/bin/java: not found

event if the java file is present
I will update the SO question with all this.

Thanks a lot

Not executable? Or on a volume marked noexec?

Reducing priority due to workaround/alternative present.

as it is a automatic installed jdk, it's on the java_home volume for the container. There is no specifics rights/permissions set as I have installed following the documentation.
Here is what I have

root@vps536376:~# docker volume inspect jenkins_home
[
    {
        "CreatedAt": "2018-12-05T15:41:37+01:00",
        "Driver": "local",
        "Labels": null,
        "Mountpoint": "/var/lib/docker/volumes/jenkins_home/_data",
        "Name": "jenkins_home",
        "Options": null,
        "Scope": "local"
    }
]
root@vps536376:~# ls -l /var/lib/docker/volumes/jenkins_home/_data/tools/hudson.model.JDK/Java_10/jdk-10.0.2/bin/
total 592
-rwxr-xr-x 1 root ubuntu   9920 Jun 28 02:54 appletviewer
-rwxr-xr-x 1 root ubuntu   9864 Jun 28 02:54 idlj
-rwxr-xr-x 1 root ubuntu  11112 Jun 28 02:54 jaotc
-rwxr-xr-x 1 root ubuntu   9864 Jun 28 02:54 jar
-rwxr-xr-x 1 root ubuntu   9872 Jun 28 02:54 jarsigner
-rwxr-xr-x 1 root ubuntu   9056 Jun 28 02:54 java
...

• automatically installed JDK (when it worked):
  hudson.model.JDK/JDK8u152/bin/
• manually downloaded and locally used binary archive (jdk-8u212-linux-x64.tar.gz in this example):
  hudson.model.JDK/JDK8/jdk1.8.0_212/bin/

Although I can set the Subdirectory of extracted archive to "jdk1.8.0_212" (for the above example) which sets the java home correctly, I'd like to get the same result as with the automatically installed JDK - e.g. without that additional subdirectory in the filesystem path.