Trying to generate Java Docs with Maven and it returns the following error:

org.apache.maven.reporting.MavenReportException:
Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/en/java/javase/14/docs/api/ are in the unnamed module.

Command line was: "C:\Program Files\Java\jdk-14\bin\javadoc.exe" @options @argfile

Refer to the generated Javadoc files in 'C:\projects\HelloWorldApplication\target\apidocs' dir.

    at org.apache.maven.plugins.javadoc.AbstractJavadocMojo.executeJavadocCommandLine (AbstractJavadocMojo.java:5763)
    at org.apache.maven.plugins.javadoc.AbstractJavadocMojo.executeReport (AbstractJavadocMojo.java:2152)
    at org.apache.maven.plugins.javadoc.JavadocJar.doExecute (JavadocJar.java:189)
    at org.apache.maven.plugins.javadoc.AbstractJavadocMojo.execute (AbstractJavadocMojo.java:1911)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:957)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:289)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:193)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:564)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)

Solution:

Add the configuration <javadocExecutable></javadocExecutable> in JavaDoc plugin.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-javadoc-plugin</artifactId>
    <version>3.1.1</version>
    <executions>
        <execution>
            <goals>
                <goal>jar</goal>
            </goals>
            <phase>install</phase>
            <configuration>
                <failOnError>false</failOnError>
                <doclint>none</doclint>
                <javadocExecutable>C:\Program Files\Java\jdk1.8.0\bin\javadoc.exe</javadocExecutable>
            </configuration>
        </execution>
    </executions>
</plugin>

Run the maven command to generate the Java Docs: mvn install