Our builds failed with Java 9 build 118 because by default several Java EE features were disabled by default in Java 9. This affects JAXB and CORBA, which are no longer visible by default.
Lucene is clean of those features but Solr uses javax.xml.bind.DatatypeConverter in SimplePostTool, which is not visible by default in Java 9. Interestingly it uses this to encode the username/password to BASE64. Since Java 8 there is java.util.Base64, so we don't need the hack anymore.