thank you @jens for your answer
I started by upgrading gwt Only from version 2.8.2 To 2.10.0 and I kept java 8 .
I my jetty-env.xml file i had org.eclipse.jetty.webapp.WebAppContext as a configurationClass, so It cause fd ollowing error when launching java.lang.ClassNotFoundException: org.eclipse.jetty.webapp.WebAppContext
at java.lang.ClassLoader.findClass(ClassLoader.java:523)
at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:487)
at com.google.gwt.dev.shell.jetty.JettyLauncher$WebAppContextWithReload$WebAppClassLoaderExtension.loadClass(JettyLauncher.java:458)
at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
at org.eclipse.jetty.util.Loader.loadClass(Loader.java:64)
at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.nodeClass(XmlConfiguration.java:477)
at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.configure(XmlConfiguration.java:417)
at org.eclipse.jetty.xml.XmlConfiguration.configure(XmlConfiguration.java:364)
at org.eclipse.jetty.plus.webapp.EnvConfiguration.lambda$configure$0(EnvConfiguration.java:120)
at org.eclipse.jetty.webapp.WebAppClassLoader.runWithServerClassAccess(WebAppClassLoader.java:138)
at org.eclipse.jetty.plus.webapp.EnvConfiguration.configure(EnvConfiguration.java:118)
at org.eclipse.jetty.webapp.WebAppContext.configure(WebAppContext.java:498)
at org.eclipse.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1409)
at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:910)
at org.eclipse.jetty.servlet.ServletContextHandler.doStart(ServletContextHandler.java:288)
at org.eclipse.jetty.webapp.WebAppContext.doStart(WebAppContext.java:524)
at com.google.gwt.dev.shell.jetty.JettyLauncher$WebAppContextWithReload.doStart(JettyLauncher.java:568)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:73)
at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:169)
at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:110)
at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:97)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:73)
at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:169)
at org.eclipse.jetty.server.Server.start(Server.java:423)
at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:110)
at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:97)
at org.eclipse.jetty.server.Server.doStart(Server.java:387)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:73)
at com.google.gwt.dev.shell.jetty.JettyLauncher.start(JettyLauncher.java:776)
at com.google.gwt.dev.DevMode.doStartUpServer(DevMode.java:636)
at com.google.gwt.dev.DevModeBase.startUp(DevModeBase.java:898)
at com.google.gwt.dev.DevModeBase.run(DevModeBase.java:705)
at com.google.gwt.dev.DevMode.main(DevMode.java:432)
Suppressed: java.lang.ClassNotFoundException: org.eclipse.jetty.webapp.WebAppContext
at java.net.URLClassLoader.findClass(URLClassLoader.java:387)
at org.eclipse.jetty.webapp.WebAppClassLoader.findClass(WebAppClassLoader.java:629)
at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:511)
... 31 more