Re: [DISCUSS] Automatic JDK toolchain selection when running JDK is incompatible (4.1.0)

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Hi,

All good like that for me, I'd maybe ensure there is a warning for 3 (and
not just an info) detailling how to fix it - installing the right version
or setting up toolchain but as soon as we fall in best effort case, no need
to be fancy or 100% accurate IMHO. Even just a "first matching" distro
sounds good enough.

Romain Manni-Bucau
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Le lun. 3 ao=C3=BBt 2026 =C3=A0 14:18, Guillaume Nodet <[email protected]> =
a =C3=A9crit :

> Hi all,
>
> I've opened a draft PR for a quality-of-life feature in Maven 4.1.0:
> automatic JDK toolchain selection when the running JDK cannot compile the
> project's declared source/release level.
>
> PR: https://github.com/apache/maven/pull/12633
>
> The problem
>
> When a project declares <maven.compiler.source>6</maven.compiler.source>
> (or uses <release>, <targetVersion>, or compiler plugin
> <configuration><source>), and you run Maven with JDK 21 =E2=80=94 which d=
ropped
> --source 6 support =E2=80=94 the build fails with a cryptic javac error. =
The user
> has to figure out they need to install a compatible JDK and either
> configure toolchains.xml or add the maven-toolchains-plugin to their buil=
d.
> This is a frequent stumbling block, especially when maintaining older
> projects.
>
> The solution
>
> Maven now automatically detects the incompatibility and searches for a
> compatible JDK =E2=80=94 first in configured toolchains (toolchains.xml),=
 then by
> lazily discovering JDK installations on the filesystem. If a compatible J=
DK
> is found, it's selected as the compilation toolchain and a warning is
> emitted:
>
> [WARNING] Project requires --source 6 which is not supported by JDK 21.
> [WARNING] Automatically selected JDK 11 (discovered at
> /usr/lib/jvm/java-11) for compilation.
>
>
> This is a zero-cost feature: the auto-selection logic only runs when the
> running JDK genuinely cannot handle the project's source level. Normal
> builds are completely unaffected.
>
> Design decisions worth discussing
>
> 1. Filesystem discovery =E2=80=94 The discoverer scans well-known locatio=
ns
> (SDKMAN, IntelliJ .jdks/, Gradle, jEnv, JBang, asdf, mise, OS-specific
> paths like /usr/lib/jvm). Version is read from the JDK release file =E2=
=80=94 no
> java processes are spawned. This is essentially what
> maven-toolchains-plugin's ToolchainDiscoverer does, but moved into core a=
nd
> made lazy. Does this make the plugin's auto-discovery redundant? Should w=
e
> deprecate it?
>
> 2. Source level detection =E2=80=94 We read the source level from multipl=
e places
> in priority order: Model 4.1.0 <source><targetVersion>, then
> maven.compiler.release/maven.compiler.source properties, then compiler
> plugin <configuration><release>/<source>. Is this the right precedence? A=
re
> there other places we should check?
>
> 3. "Newest compatible" strategy =E2=80=94 When multiple compatible JDKs a=
re found,
> we pick the newest one (highest major version that still supports the
> required source level). The rationale is that a newer JDK gives better
> performance and diagnostics. Should this be configurable?
>
> 4. Compat layer =E2=80=94 The v3 ToolchainManagerFactory bridge now passe=
s the
> discoverer through, so plugins using the v3 API also benefit. This felt
> important for the transition period.
>
> CI is green on all platforms (Linux/macOS/Windows =C3=97 JDK 17/21/25).
>
> Feedback and reviews welcome.
>
> Guillaume
>

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