Oh yes, these two modules "pmd-java8" and "pmd-ui" are for java8 only and pmd is released with java7 - hence the the parent pom version has not been adjusted during the release.
You can disable the modules for now by executing this maven command:
mvn -P!jdk8-modules clean package //使用这个命令就能正常编译
This disables the profile "jdk8-modules" so that the two modules are not built.
解决方法来自：https://sourceforge.net/p/pmd/discussion/188193/thread/5f30e7cf/