mavenlog4jjava-9java-platform-module-systemeclipse-oxygen

How to use log4j with maven and java9 in Eclipse Oxygen?

I try to migrate a Java8 project to Java9. The auto generated module-info.java contains an entry

 requires log4j;

and an error is shown:

log4j cannot be resolved to a module

=> How do I correctly include log4j as a module dependency with Java9?

(I have the same issue for following dependencies: requires hibernate.core; requires hibernate.jpa.2.1.api; requires jcommander; requires junit; requires reflections; )

What I did so far:

Solution

  • 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:

    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

    enter image description here

    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

    enter image description here