code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC licenses this file
  // to you under the Apache License, Version 2.0 (the
  // "License"); you may not use this file except in compliance
  // with the License.  You may obtain a copy of the License at
  //
  //   http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing,
  // software distributed under the License is distributed on an
  // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied.  See the License for the
  // specific language governing permissions and limitations
  // under the License.
  
  package org.openqa.selenium.grid.session.remote;
  
  import java.io.IOException;
  import org.openqa.selenium.WebDriverException;
  import org.openqa.selenium.grid.session.ActiveSession;
  import org.openqa.selenium.internal.Require;
  import org.openqa.selenium.remote.Command;
  import org.openqa.selenium.remote.CommandExecutor;
  import org.openqa.selenium.remote.DriverCommand;
  import org.openqa.selenium.remote.Response;
  import org.openqa.selenium.remote.http.HttpRequest;
  import org.openqa.selenium.remote.http.HttpResponse;
  
  class ActiveSessionCommandExecutor implements CommandExecutor {
  
    private final ActiveSession session;
    private boolean active;
  
    public ActiveSessionCommandExecutor(ActiveSession session) {
      this.session = Require.nonNull("Session", session);
    }
  
    @Override
    public Response execute(Command command) throws IOException {
      if (DriverCommand.NEW_SESSION.equals(command.getName())) {
        if (active) {
          throw new WebDriverException("Cannot start session twice! " + session);
        }
  
        active = true;
  
        // We already have a running session.
        Response response = new Response(session.getId());
        response.setValue(session.getCapabilities());
        return response;
      }
  
      // The command is about to be sent to the session, which expects it to be
      // encoded as if it has come from the downstream end, not the upstream end.
      HttpRequest request = session.getDownstreamDialect().getCommandCodec().encode(command);
  
      HttpResponse httpResponse = session.execute(request);
  
      return session.getDownstreamDialect().getResponseCodec().decode(httpResponse);
    }
  }
  