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.router;
  
  import static org.openqa.selenium.remote.HttpSessionId.getSessionId;
  import static org.openqa.selenium.remote.RemoteTags.SESSION_ID;
  import static org.openqa.selenium.remote.RemoteTags.SESSION_ID_EVENT;
  import static org.openqa.selenium.remote.http.Contents.asJson;
  import static org.openqa.selenium.remote.tracing.Tags.EXCEPTION;
  import static org.openqa.selenium.remote.tracing.Tags.HTTP_REQUEST;
  import static org.openqa.selenium.remote.tracing.Tags.HTTP_REQUEST_EVENT;
  import static org.openqa.selenium.remote.tracing.Tags.HTTP_RESPONSE;
  
  import com.google.common.cache.Cache;
  import com.google.common.cache.CacheBuilder;
  import com.google.common.cache.RemovalListener;
  import com.google.common.collect.ImmutableMap;
  import java.net.URL;
  import java.time.Duration;
  import java.util.HashMap;
  import java.util.Map;
  import java.util.concurrent.Callable;
  import java.util.concurrent.Executors;
  import java.util.concurrent.ScheduledExecutorService;
  import java.util.concurrent.TimeUnit;
  import org.openqa.selenium.NoSuchSessionException;
  import org.openqa.selenium.concurrent.GuardedRunnable;
  import org.openqa.selenium.grid.data.Session;
  import org.openqa.selenium.grid.sessionmap.SessionMap;
  import org.openqa.selenium.grid.web.ReverseProxyHandler;
  import org.openqa.selenium.internal.Require;
  import org.openqa.selenium.net.Urls;
  import org.openqa.selenium.remote.SessionId;
  import org.openqa.selenium.remote.http.ClientConfig;
  import org.openqa.selenium.remote.http.HttpClient;
  import org.openqa.selenium.remote.http.HttpHandler;
  import org.openqa.selenium.remote.http.HttpRequest;
  import org.openqa.selenium.remote.http.HttpResponse;
  import org.openqa.selenium.remote.tracing.AttributeKey;
  import org.openqa.selenium.remote.tracing.EventAttribute;
  import org.openqa.selenium.remote.tracing.EventAttributeValue;
  import org.openqa.selenium.remote.tracing.HttpTracing;
  import org.openqa.selenium.remote.tracing.Span;
  import org.openqa.selenium.remote.tracing.Status;
  import org.openqa.selenium.remote.tracing.Tracer;
  
  class HandleSession implements HttpHandler {
  
    private final Tracer tracer;
    private final HttpClient.Factory httpClientFactory;
    private final SessionMap sessions;
    private final Cache<URL, HttpClient> httpClients;
  
    HandleSession(Tracer tracer, HttpClient.Factory httpClientFactory, SessionMap sessions) {
      this.tracer = Require.nonNull("Tracer", tracer);
      this.httpClientFactory = Require.nonNull("HTTP client factory", httpClientFactory);
      this.sessions = Require.nonNull("Sessions", sessions);
  
      this.httpClients =
          CacheBuilder.newBuilder()
              .expireAfterAccess(Duration.ofMinutes(1))
              .removalListener(
                  (RemovalListener<URL, HttpClient>) removal -> removal.getValue().close())
              .build();
  
      ScheduledExecutorService cleanUpHttpClientsCacheService =
          Executors.newSingleThreadScheduledExecutor(
              r -> {
                Thread thread = new Thread(r);
                thread.setDaemon(true);
                thread.setName("HandleSession - Clean up http clients cache");
                return thread;
              });
      cleanUpHttpClientsCacheService.scheduleAtFixedRate(
          GuardedRunnable.guard(httpClients::cleanUp), 1, 1, TimeUnit.MINUTES);
    }
  
    @Override
    public HttpResponse execute(HttpRequest req) {
      try (Span span = HttpTracing.newSpanAsChildOf(tracer, req, "router.handle_session")) {
        Map<String, EventAttributeValue> attributeMap = new HashMap<>();
        attributeMap.put(
            AttributeKey.HTTP_HANDLER_CLASS.getKey(), EventAttribute.setValue(getClass().getName()));
  
        HTTP_REQUEST.accept(span, req);
        HTTP_REQUEST_EVENT.accept(attributeMap, req);
  
        SessionId id =
            getSessionId(req.getUri())
                .map(SessionId::new)
                .orElseThrow(
                    () -> {
                      NoSuchSessionException exception =
                          new NoSuchSessionException("Cannot find session: " + req);
                      EXCEPTION.accept(attributeMap, exception);
                      attributeMap.put(
                          AttributeKey.EXCEPTION_MESSAGE.getKey(),
                          EventAttribute.setValue(
                              "Unable to execute request for an existing session: "
                                  + exception.getMessage()));
                      span.addEvent(AttributeKey.EXCEPTION_EVENT.getKey(), attributeMap);
                      return exception;
                    });
  
        SESSION_ID.accept(span, id);
        SESSION_ID_EVENT.accept(attributeMap, id);
  
        try {
          HttpTracing.inject(tracer, span, req);
          HttpResponse res = loadSessionId(tracer, span, id).call().execute(req);
  
          HTTP_RESPONSE.accept(span, res);
  
          return res;
        } catch (Exception e) {
          span.setAttribute(AttributeKey.ERROR.getKey(), true);
          span.setStatus(Status.CANCELLED);
  
          String errorMessage =
              "Unable to execute request for an existing session: " + e.getMessage();
          EXCEPTION.accept(attributeMap, e);
          attributeMap.put(
              AttributeKey.EXCEPTION_MESSAGE.getKey(), EventAttribute.setValue(errorMessage));
          span.addEvent(AttributeKey.EXCEPTION_EVENT.getKey(), attributeMap);
  
          if (e instanceof NoSuchSessionException) {
            HttpResponse response = new HttpResponse();
            response.setStatus(404);
            response.setContent(
                asJson(
                    ImmutableMap.of(
                        "value",
                        req.getUri(),
                        "message",
                        errorMessage,
                        "error",
                        "invalid session id")));
            return response;
          }
  
          Throwable cause = e.getCause();
          if (cause instanceof RuntimeException) {
            throw (RuntimeException) cause;
          } else if (cause != null) {
            throw new RuntimeException(errorMessage, cause);
          } else if (e instanceof RuntimeException) {
            throw (RuntimeException) e;
          }
          throw new RuntimeException(errorMessage, e);
        }
      }
    }
  
    private Callable<HttpHandler> loadSessionId(Tracer tracer, Span span, SessionId id) {
      return span.wrap(
          () -> {
            Session session = sessions.get(id);
            URL url = Urls.fromUri(session.getUri());
            ClientConfig config = ClientConfig.defaultConfig().baseUrl(url).withRetries();
            HttpClient client = httpClients.get(url, () -> httpClientFactory.createClient(config));
            return new ReverseProxyHandler(tracer, client);
          });
    }
  }
  