public synchronized Set<ServerName> copyServerNames() {
    Set<ServerName> clone = new HashSet<>(deadServers.size());
    clone.addAll(deadServers.keySet());
    return clone;
  }
@param serverName server name.
@return true if this server is on the dead servers list false otherwise
hbase-server/src/main/java/org/apache/hadoop/hbase/master/DeadServer.java