@Override
        public Object answer(InvocationOnMock invocation) {
          WALKeyImpl walKey = invocation.getArgument(1);
          MultiVersionConcurrencyControl mvcc = walKey.getMvcc();
          if (mvcc != null) {
            MultiVersionConcurrencyControl.WriteEntry we = mvcc.begin();
            walKey.setWriteEntry(we);
          }
          return 01L;
        }
This class attempts to unit test bulk HLog loading.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestBulkLoad.java