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.support;
  
  import java.lang.reflect.InvocationHandler;
  import java.lang.reflect.InvocationTargetException;
  import java.lang.reflect.Method;
  import java.util.Arrays;
  import java.util.HashSet;
  import java.util.Set;
  import org.openqa.selenium.WebDriver;
  import org.openqa.selenium.WebDriverException;
  
  /**
   * Multithreaded client code should use this to assert that it accesses webdriver in a thread-safe
   * manner.
   *
   * <p>Wrap WebDriver instances as follows:
   *
   * <pre class="code">
   * WebDriver driver = ThreadGuard.protect(new FirefoxDriver());
   * </pre>
   *
   * Threading issues related to incorrect client threading may have mysterious and hard to diagnose
   * errors. Using this wrapper prevents this category of errors. It is recommended for all
   * multithreaded usage. This class has no overhead of any importance.
   */
  public class ThreadGuard {
  
    public static WebDriver protect(WebDriver actualWebDriver) {
      WebDriverInvocationHandler invocationHandler = new WebDriverInvocationHandler(actualWebDriver);
      return (WebDriver)
          java.lang.reflect.Proxy.newProxyInstance(
              actualWebDriver.getClass().getClassLoader(),
              getInterfaces(actualWebDriver),
              invocationHandler);
    }
  
    private static Class<?>[] getInterfaces(Object target) {
      Class<?> base = target.getClass();
      Set<Class<?>> interfaces = new HashSet<>();
      if (base.isInterface()) {
        interfaces.add(base);
      }
      while (base != null && !Object.class.equals(base)) {
        interfaces.addAll(Arrays.asList(base.getInterfaces()));
        base = base.getSuperclass();
      }
      return interfaces.toArray(new Class[0]);
    }
  
    static class WebDriverInvocationHandler implements InvocationHandler {
  
      private final long threadId;
      private final Object underlying;
      private final String threadName;
  
      public WebDriverInvocationHandler(Object underlyingWebDriver) {
        Thread thread = Thread.currentThread();
        this.threadId = thread.getId();
        this.threadName = thread.getName();
        this.underlying = underlyingWebDriver;
      }
  
      @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
          if (Thread.currentThread().getId() != threadId) {
            Thread currentThread = Thread.currentThread();
            throw new WebDriverException(
                String.format(
                    "Thread safety error; this instance of WebDriver was constructed on "
                        + "thread %s (id %d) and is being accessed by thread %s (id %d)"
                        + "This is not permitted and *will* cause undefined behaviour",
                    threadName, threadId, currentThread.getName(), currentThread.getId()));
          }
          return invokeUnderlying(method, args);
        } catch (InvocationTargetException e) {
          throw e.getTargetException();
        }
      }
  
      protected Object invokeUnderlying(Method method, Object[] args)
          throws IllegalAccessException, InvocationTargetException {
        return method.invoke(underlying, args);
      }
    }
  }
  