docx4j 8.1.2 is now in Maven Central.
Its a minor bugfix release, but essential if two threads might seek to initialise NamespacePrefixMapper.
To use it via Maven, just add one and only one of docx4j-JAXB-Internal, docx4j-JAXB-MOXy,
or docx4j-JAXB-ReferenceImpl to your pom. https://search.maven.org/search?q=g:org.docx4j
Otherwise, see https://www.docx4java.org/downloads.html
Minimum supported Java version is 8.0