Running java with --add-reads doesn't recognize a module even though it appears when running with --list-modules

Question

Given two modules m1 and m2 on the module path:

java -version
openjdk version "11.0.2" 2019-01-15

java -p M2/bin:M1/bin --list-modules
...
m1 <path>M1/bin/
m2 <path>M2/bin/

When running module m2, and specifying --add-reads from m2 to m1:

java -p M2/bin:M1/bin --add-reads m2=m1 -m m2/p2.C2
WARNING: Unknown module: m1 specified to --add-reads
...

Java cannot see the module m1, even though when using --list-modules it is clearly visible.

How to make --add-reads recognize the module?

Answer 1

Unlike requires in module-info.java, --add-reads will not add the module to the module graph, even though the documentation for --add-reads says: This is, essentially, a command-line form of a requires clause in a module declaration. Turns out that the actual equivalent is --add-modules combined with --add-reads:

java -p M2/bin:M1/bin --add-modules m1 --add-reads m2=m1 -m m2/p2.C2