@Override
  public void addURL(URL url) {
    Preconditions.checkState(!isClosed, getClass().getSimpleName() + " is already closed");
    super.addURL(url);
  }
{@link UDFClassLoader} is used to dynamically register
udf (and related) jars

This was introducted to fix HIVE-11878

Each session will have its own instance of {@link UDFClassLoader}
This is to support HiveServer2 where there can be multiple
active sessions. Addition/removal of jars/resources in one
session should not affect other sessions.
ql/src/java/org/apache/hadoop/hive/ql/exec/UDFClassLoader.java