Summary

Disallow using jlink with the ALL-MODULE-PATH special value in --add-modules when no --module-path option is given or the --module-path option only points to paths where no modules can be found (empty module paths).

Problem

Previously using this was possible for regular JDK builds:

jlink --add-modules ALL-MODULE-PATH --output image

In essence, it was equivalent to:

jlink --add-modules ALL-MODULE-PATH --module-path $JAVA_HOME/jmods --output image

So the result would have been a JDK with all modules included. However, with JEP 493 this would become problematic since $JAVA_HOME/jmods no longer exists and, thus, jlink --add-modules ALL-MODULE-PATH --output image would link with no modules included resulting in inconsistent behaviour.

Solution

In order to make JEP 493 enabled builds and regular JDK builds consistent in terms of ALL-MODULE-PATH treatment, this special option will need to include an explicit --module-path option. This may include $JAVA_HOME/jmods (if existing) in order to achieve a similar result to the previous jlink --add-modules ALL-MODULE-PATH --output image without --module-path option.

What's more, empty or non-existent module paths result in the same error as when ALL-MODULE-PATH is requested and no --module-path has been provided.

Specification

ALL-MODULE-PATH without an explicit --module-path option is an error:

$ jlink --add-modules ALL-MODULE-PATH --output out-image
Error: --module-path option must be specified with --add-modules ALL-MODULE-PATH

ALL-MODULE-PATH with --module-path option(s), but directories that --module-path point to don't have any observable modules it's also an error:

$ jlink --add-modules ALL-MODULE-PATH --module-path not-existing --output out-image
Error: No module found in module path 'not-existing' with --add-modules ALL-MODULE-PATH

When the module path exists and modules are observable there, ALL-MODULE-PATH works as expected:

$ jlink --add-modules ALL-MODULE-PATH --module-path jmods/ --output out-image --verbose
com.foo.runtime file:///path/to/jmods/com.foo.runtime.jmod
java.base file:///path/to/jdk/jmods/java.base.jmod
jdk.jfr file:///path/to/jdk/jmods/jdk.jfr.jmod

Providers:
  java.base provides java.nio.file.spi.FileSystemProvider used by java.base

Full details in https://github.com/openjdk/jdk/pull/22494