[FIXED] "Unknown" error in eclipse while adding maven-jar-plugin maven plugin

Issue

I have multiple maven project. To change where the compiled jar is exported, I'm using maven-jar-plugin plugin like that :

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <version>3.2.0</version>
    <configuration>
        <outputDirectory>${export_dir}</outputDirectory>
    </configuration>
</plugin>

I use "Maven -> Update project". Now, I can run the mvn install and it will works.

BUT, Eclipse show this error:

The TOP of the file is :

The full pom.xml file :

<?xml version="1.0" encoding="UTF-8"?>
<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>my.package</groupId>
    <artifactId>myproject</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>my-project</name>

    <properties>
        <java.version>1.8</java.version>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.1</version>
                <configuration>
                    <source>${java.version}</source>
                    <target>${java.version}</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>3.2.4</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <createDependencyReducedPom>false</createDependencyReducedPom>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.2.0</version>
                <configuration>
                    <outputDirectory>${export_dir}</outputDirectory>
                </configuration>
            </plugin>
        </plugins>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

    <repositories>
        <repository>
            <id>spigot-repo</id>
            <url>https://hub.spigotmc.org/nexus/content/repositories/snapshots/</url>
        </repository>
        <repository>
            <id>sonatype</id>
            <url>https://oss.sonatype.org/content/groups/public/</url>
        </repository>
    </repositories>

    <dependencies>
        <dependency>
            <groupId>org.spigotmc</groupId>
            <artifactId>spigot-api</artifactId>
            <version>1.8.8-R0.1-SNAPSHOT</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>
</project>

And if I remove this plugin, then update project, the error disappear.

My eclipse version: Version: Oxygen.3a Release (4.7.3a), Build id: 20180405-1200 (because it was the last without Java 16 required)

Do you know how can I remove it ?

Solution

Before I was running Eclipse with Java 8.

So, I :

• Update Eclipse as latest (2021-12)
• Use Java 16 instead of Java 8
• Made some changes of Eclipse options because of upgrades

And now, I don't know exactly why but it disappear. I'm thinking it's because of the Java version, but it's not sure.

Answered By - Elikill58
Answer Checked By - Willingham (JavaFixing Volunteer)