After upgrading from Java 9 to 10, links to the JDK no longer work when generating documentation with the Javadoc tool (e.g., for a file importing java.util.Optional
Appropriate bits have been added to Maven Javadoc Plugin in master already, but that won't help due to a bug in javadoc(1) in Java 11. See MJAVADOC-561 for details. The broken links can only be fixed by Oracle.
Edit: The fix is scheduled for Java 11.0.2 by Oracle.