code: void setParent(HTTPContext context) {
          if (context.getHost() == null) {
              throw new IllegalArgumentException(context.getClass().getSimpleName() + " host must not be null");
          }
          this.host = context.getHost();
          this.port = context.getPort();
      }
docs: Set the {@link HTTPContext} for this servlet.  This is required for
  <code>Servlet</code> to be fully initialized.  Don't use
  {@link #equals(Object)} prior to setting the <code>HTTPContext</code>,
  which most likely implies not adding this to a collection beforehand
  either.
  
  @param context
  the context to set
  
  @throws IllegalArgumentException
  if the context host is null
  @see {@link HTTPContext#add(Servlet)}