Errors in module-info.java
Even if the compliance level of the project has been set to Java9, there might be shown misleading errors for module-info.java in Eclipse.
I expected that an update of the maven project would only be required if I change the pom.xml file. Now I learned that changing the module-info.java also requires a manual update of the maven project.
=> update the maven project (Alt+F5)
After that update my errors vanished.
I also learned that it is better to first update the versions in the pom.xml file and then generate the module-info.java. Otherwise the module-info.java will include non-existing modules like "requires log4j" instead of "requires log4j.api"
Another misleading error in module-info.java might occur due to pom packaging, see below.
Unresolved imports
For the case that an import can not be resolved the question might be "Which corresponding (new) module do I need for that (old) import?". What might help here:
Search at following page for the required page:
http://cr.openjdk.java.net/~mr/jigsaw/ea/module-summary.html
(lists all exported packages of the JDK-modules)
Use JDeps on the (old) *.jar file to get a list of required jar files, e.g.
jdeps --class-path "lib" -recursive MY-OLD.jar >output.txt
Use jar to find the module for a jar file
jar --describe-module --file REQUIRED-JAR-FILE.jar
Also see:
Re-Export dependencies
In order to automatically make log4j visible for a grandparent project
grandparent => parent => log4j
you might want to use
requires transitive log4j.api
in parent instead of
requires log4j.api
in grandparent. Also see:
What's the difference between requires and requires transitive statements in Java 9 module declaration
POM packaging
My main issue seems to be that my Java8 pom.xml file used pom packaging:
<packaging>pom</packaging>
If I remove that line, no errors are shown in module-info.java
Also see this extra question: How to use maven with Java9.0.1 and pom packaging in Eclipse Oxygen 1a Release (4.7.1a)?
New log4j version
A. In addition to the change "requires log4j" => "requires log4j.api" I had to adapt the calling code for the new log4j version that is compatible to Java9:
private static Logger sysLog = Logger.getLogger(Main.class);
to
private static Logger sysLog = LogManager.getLogger(Main.class);
B. log4j 2 does not have PropertyConfigurator. Also see this related question:
PropertyConfigurator in log4j2
C. log4j 2 does not support log4j.properties files. Previously I used
src/main/resources/META-INF/log4j.properties
for configuration. Now I use
src/main/resources/log4j2.xml
Also see
Working example as a reference
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>Log4JWithJava9</groupId>
<artifactId>Log4JWithJava9</artifactId>
<version>0.0.1-SNAPSHOT</version>
<build>
<sourceDirectory>src</sourceDirectory>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.7.0</version>
<configuration>
<release>9</release>
</configuration>
</plugin>
</plugins>
</build>
<dependencies>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-api</artifactId>
<version>2.9.1</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>2.9.1</version>
</dependency>
</dependencies>
</project>
module-info.java:
module Log4JWithJava9 {
requires javafx.base;
requires log4j.api;
}
Main.java:
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
public class Main {
private static Logger sysLog = LogManager.getLogger(Main.class);
public static void main(String[] args) {
}
}
Eclipse plugins
In order to add log4j to an eclipse plugin, I copied the file log4j-api-2.10.0.jar in a folder "lib" and added the jar file to Java Build Path=>Libraries=>ModulePath
Instead of requires log4j.api I had to use requires org.apache.logging.log4j
