When building nashorn with ant, javadoc is built, and this message is emitted:

javadoc:
  [javadoc] Generating Javadoc
  [javadoc] Javadoc execution
  [javadoc] javadoc: warning - Error fetching URL:http://docs.oracle.com/javase/8/docs/api/
  [javadoc] 1 warning

The above warning is turned into an error, and the *ant* build is likely to fail,
with this javadoc change:
https://bugs.openjdk.java.net/browse/JDK-8191625

-linkoffline option may be used instead.