it looks guava has moved his javadoc...

 curl -I https://google.github.io/guava/releases/27.0.1-jre/api/docs/com/google/common/collect/Multimap.html?is-external=true
HTTP/2 301
server: GitHub.com
content-type: text/html
location: https://guava.dev/releases/27.0.1-jre/api/docs/com/google/common/collect/Multimap.html?is-external=true
Signed-off-by: olivier lamy <[email protected]>

Author: olamy

src/it/projects/MJAVADOC-555_link-automatic-modules/pom.xml

@@ -57,7 +57,7 @@
             </goals>
             <configuration>
               <links>
-                <link>https://google.github.io/guava/releases/${guava.version}/api/docs/</link>
+                <link>https://guava.dev/releases/${guava.version}/api/docs/</link>
               </links>
             </configuration>
           </execution>
@@ -72,4 +72,4 @@
       <version>${guava.version}</version>
     </dependency>
   </dependencies>
-</project>
+</project>

src/it/projects/MJAVADOC-555_link-automatic-modules/verify.groovy

@@ -30,4 +30,4 @@ def p = /<a href="([^"]+)"(?:[^>]+)>Multimap<\/a>/
 def m = classFile.text =~ p
 
 assert m.hasGroup()
-assert m[0][1] == "https://google.github.io/guava/releases/27.0.1-jre/api/docs/com/google/common/collect/Multimap.html?is-external=true" 
+assert m[0][1] == "https://guava.dev/releases/27.0.1-jre/api/docs/com/google/common/collect/Multimap.html?is-external=true"

src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java

@@ -4395,7 +4395,6 @@ private void copyAdditionalJavadocResources( File anOutputDirectory )
 
     /**
      * @param sourcePaths could be null
-     * @param files       not null
      * @return the list of package names for files in the sourcePaths
      */
     private List<String> getPackageNames( Map<Path, Collection<String>> sourcePaths )
@@ -4521,7 +4520,6 @@ private List<String> getPackageNamesRespectingJavaModules( Map<String, Collectio
 
     /**
      * @param sourcePaths could be null
-     * @param files       not null
      * @return a list files with unnamed package names for files in the sourcePaths
      */
     private List<String> getFilesWithUnnamedPackages( Map<Path, Collection<String>> sourcePaths )
@@ -4561,7 +4559,6 @@ private List<String> getFilesWithUnnamedPackages( Map<Path, Collection<String>>
     /**
      * Either return only the module descriptor or all sourcefiles per sourcepath
      * @param sourcePaths could be null
-     * @param files       not null
      * @return a list of files
      */
     private List<String> getSpecialFiles( Map<Path, Collection<String>> sourcePaths )
@@ -4990,7 +4987,7 @@ private void addJavadocOptions( File javadocOutputDirectory,
             for ( Map.Entry<String, Collection<Path>> projectSourcepaths : allSourcePaths.entrySet() )
             {
                 MavenProject aggregatedProject = reactorKeys.get( projectSourcepaths.getKey() );
-                if ( aggregatedProject != null )
+                if ( aggregatedProject != null && !"pom".equals( aggregatedProject.getPackaging() ) )
                 {
                     ResolvePathResult result = null;