In Apache hadoop 2.x. Does the current UserGroupInformation lose credentials in a child thread?

Question

Here is the simple experiment I used to demonstrate the problem, written in scala, with Hadoop 2.7.7 as the only dependency:

import org.apache.hadoop.security.UserGroupInformation

import scala.concurrent.duration.{Duration, MINUTES}
import scala.concurrent.{Await, ExecutionContext, Future}

object UGITest {
        val ugi = UserGroupInformation.getCurrentUser
        val credential = ugi.getCredentials

        val ff = Future {

          val _ugi = UserGroupInformation.getCurrentUser
          val _credential = _ugi.getCredentials

          require(ugi == _ugi, s"UGI is lost, before: $ugi, now ${_ugi}")

          require(credential == _credential, s"credential is lost, before: $credential, now ${_credential}")

        }(ExecutionContext.global)

        Await.result(ff, Duration.apply(1, MINUTES))
}

The first requirement ugi==_ugi passed successfully, indicating that closure of the Future was successfully launched in a child thread.

The second requirement credential==_credential fail with the following information:

java.lang.IllegalArgumentException: requirement failed: credential is lost, before: org.apache.hadoop.security.Credentials@cb6e68f, now org.apache.hadoop.security.Credentials@6b746674
    at scala.Predef$.require(Predef.scala:281)
    at ...

It appears that the same UserGroupInformation is used, but all credentials are lost. What's the purpose of this design?

The above experiment was just executed on a single computer not in any cluster. I have't tested it with any hadoop authentication framework (e.g. kerberos) enabled. But I think the result will be more or less the same.