Maven: module-info.java:[6,21] module not found: org.slf4j - maven

Question

I'm currently migrating a project to Java 11 (i.e. >= 9) and because JavaFX seems not to work without modules, anymore, I'm currently adding module-info.java files to all sub-projects. But I get the following error:
[INFO] -------------------------------------------------------------
[ERROR] COMPILATION ERROR :
[INFO] -------------------------------------------------------------
[ERROR] /home/mangu/workspace/cloudstore.2/co.codewizards.java9test.project1/src/main/java/module-info.java:[6,21] module not found: org.slf4j
[INFO] 1 error
I'm running OpenJDK 11.0.3, Maven 3.6.1 (embedded in Eclipse 2019-06) and I definitely have the dependency for the module org.slf4j declared in my pom.xml!
In order to simplify things, I created a little, minimal test-project causing the same error.
Here's the test-project's pom.xml:
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>co.codewizards.java9test</groupId>
<artifactId>co.codewizards.java9test.project1</artifactId>
<version>0.1.0-SNAPSHOT</version>
<properties>
<maven.compiler.source>9</maven.compiler.source>
<maven.compiler.target>9</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.8.0-beta4</version>
</dependency>
</dependencies>
</project>
I tried both: 1.8.0-beta4 and 2.0.0-alpha0. Eclipse shows me an existing module-info.class in both JARs (i.e. currently inside slf4j-api-1.8.0-beta4.jar).
Here's the module-info.java:
module co.codewizards.java9test.project1 {
requires java.base;
requires java.se;
requires org.slf4j;
}
And here's the output from mvn clean install -Dmaven.test.skip=true -X (but run inside Eclipse): mvn.log
Any idea what I'm doing wrong?
Update: I just uploaded the little test project: co.codewizards.java9test.project1.7z

Answer 1

Your maven logs show slf4j-api being placed on the classpath, not modulepath.
You need to upgrade the version of maven-compiler-plugin, as modulepath is supported only since version 3.6, whereas your build uses version 3.1.

Answer 2

I was getting the same issue due to older verion of slf4j was coming from one of dependency in pom. So I resolved it by adding below slf4j dependency to my pom :
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.29</version>
</dependency>
module-info:
module my.service {
requires org.slf4j;
}