code: @Override
      @FFDCIgnore({ IOException.class })
      public void flushHeaders() throws IOException {
          if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled()) {
              Tr.debug(tc, "flushHeaders: committed=" + this.isc.getResponse().isCommitted());
          }
          validate();
          this.ignoreFlush = false;
          if (!this.isc.getResponse().isCommitted()) {
              this.isc.getResponse().setCommitted();
          } else {
              // response headers already committed (written)
              return;
          }
          try {
              this.isc.sendResponseHeaders();
          } catch (MessageSentException mse) {
              FFDCFilter.processException(mse, getClass().getName(),
                                          "flushHeaders", new Object[] { this, this.isc });
              if (TraceComponent.isAnyTracingEnabled() && tc.isEventEnabled()) {
                  Tr.event(tc, "Invalid state, message-sent-exception received; " + this.isc);
              }
              this.error = new IOException("Invalid state");
              throw this.error;
          } catch (IOException ioe) {
              // no FFDC required
              this.error = ioe;
              if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled()) {
                  Tr.debug(tc, "Received exception during write: " + ioe);
              }
              throw ioe;
          }
      }
docs: /*
  (non-Javadoc)
  
  @see com.ibm.ws.http.channel.internal.outbound.HttpOutputStream#flushHeaders()