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

Mon, 03 Aug 2026 12:10:14 -0700 

Hi,
Agree with Matthias. Earlier ideas on "troubleshooting common errors through Maven Core" have been rejected for reasons like "people should read the error messages and take corresponding action, rather than Maven trying to diagnose or even repair it". It would be strange to now include such features. I would prefer having clear(er) error messages if that is the source of problem. 


Also, I see many people blindly ignore warnings, so that would 
contribute to the scenario that Matthias outlined: "it works on my 
machine", while in fact I ignored the warning that says Maven switched 
to an auto-discovered JDK 11 that someone else on my project does not have.


Thanks,

Maarten

On August 3, 2026 at 17:14, Matthias Bünger wrote:

Hi,


I'm not convinced in trying to "fix" configuration errors in users 
system/project from the "shadows". I think a clear error (and if 
possible giving information what's wrong/how to fix) supports the user 
more in really fixing the problem. Otherwise we support "it worked on 
my machine". I can also think that it's massively increasing 
maintenance effort on our side due question why it works here, but not 
there or paths to look at on different OS.


Matthias

Am 03.08.2026 um 14:18 schrieb Guillaume Nodet:

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 — which dropped

--source 6 support — 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 
build.

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 — first in configured toolchains (toolchains.xml), 
then by
lazily discovering JDK installations on the filesystem. If a 
compatible JDK

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 — The discoverer scans well-known locations
(SDKMAN, IntelliJ .jdks/, Gradle, jEnv, JBang, asdf, mise, OS-specific
paths like /usr/lib/jvm). Version is read from the JDK release file — no
java processes are spawned. This is essentially what

maven-toolchains-plugin's ToolchainDiscoverer does, but moved into 
core and
made lazy. Does this make the plugin's auto-discovery redundant? 
Should we

deprecate it?


2. Source level detection — We read the source level from multiple 
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? Are

there other places we should check?


3. "Newest compatible" strategy — When multiple compatible JDKs are 
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 — The v3 ToolchainManagerFactory bridge now passes 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 × JDK 17/21/25).

Feedback and reviews welcome.

Guillaume



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