com.google.apphosting.utils.config.AppEngineConfigException: appengine-web.xml does not contain a element

Appengine thread troubleshoot:
com.google.apphosting.utils.config.AppEngineConfigException: appengine-web.xml does not contain a element.
See http://code.google.com/appengine/docs/java/config/appconfig.html#Using_Concurrent_Requests for more information.
You probably want to enable concurrent requests.
at com.google.apphosting.utils.config.AppEngineWebXmlReader.readAppEngineWebXml(AppEngineWebXmlReader.java:87)
at com.google.apphosting.utils.config.EarHelper.readWebModule(EarHelper.java:166)
at com.google.appengine.tools.development.ApplicationConfigurationManager$WarServerConfigurationHandle.readConfiguration(ApplicationConfigurationManager.java:399)
at com.google.appengine.tools.development.ApplicationConfigurationManager.(ApplicationConfigurationManager.java:146)
at com.google.appengine.tools.development.ApplicationConfigurationManager.newWarConfigurationManager(ApplicationConfigurationManager.java:88)
at com.google.appengine.tools.development.DevAppServerImpl.(DevAppServerImpl.java:138)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at com.google.appengine.tools.development.DevAppServerFactory.createDevAppServer(DevAppServerFactory.java:165)
at com.google.appengine.tools.development.DevAppServerFactory.createDevAppServer(DevAppServerFactory.java:69)
at com.google.appengine.tools.development.DevAppServerFactory.createDevAppServer(DevAppServerFactory.java:53)
at com.google.appengine.tools.development.gwt.AppEngineLauncher.start(AppEngineLauncher.java:84)
at com.google.gwt.dev.DevMode.doStartUpServer(DevMode.java:500)
at com.google.gwt.dev.DevModeBase.startUp(DevModeBase.java:1055)
at com.google.gwt.dev.DevModeBase.run(DevModeBase.java:804)
at com.google.gwt.dev.DevMode.main(DevMode.java:309)
   [ERROR] Unable to start App Engine server
Resolution:
Add threadsafe tag value as true to the appengine-web.xml in war\WEB-INF folder.