As far as I understand, Bazel by default provides an embedded JDK (or at least some rules to get it), so we don't have to have one locally.

rules_jvm_external use Coursier, which is invoked by running java:

https://github.com/bazelbuild/rules_jvm_external/blob/aed0eb3747682e64611bf98bb1ca590a44b470de/coursier.bzl#L109-L129

This assumes java is available at JAVA_HOME, which is unset with no local JDK. If I understand correctly Bazel sets JAVABASE to the location of the embedded JDK.

So is it possible to use the embedded JDK with rules_jvm_external? Maybe it is possible now and I am missing something? And if it's not currently possible, wouldn't it be sufficient to just check if JAVABASE is set too? I guess the order of checking is important because someone can have a local JDK, while still wanting to use the embedded one.

For now, a cheap workaround is to just set JAVA_HOME relatively, knowing that both maven and jdk are in the external workspace. I can confirm that adding this line:

build --repo_env=JAVA_HOME=../bazel_tools/jdk

to the .bazelrc file works and the embedded JDK is used.