I've been using Jenkins on this machine for a long time, it worked perfectly before.

I don't know what happened to this machine, I suspect it may be cause by system update, since I always do Ubuntu system update every few days.

When I open jenkins from browser  by localhost:8080 as normal, I see this error:

HTTP ERROR: 503

Problem accessing /. Reason:

Service Unavailable

Powered by Jetty://

And when looking into the log file /var/log/jenkins, I got this:

=========================

Apr 14, 2017 5:20:40 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started ServerConnector@3274ae84{HTTP/1.1}{0.0.0.0:8080}
Apr 14, 2017 5:20:40 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started @1333ms
Apr 14, 2017 5:20:40 PM winstone.Logger logInternal
INFO: Winstone Servlet Engine v2.0 running: controlPort=disabled
Apr 14, 2017 5:24:18 PM winstone.Logger logInternal
INFO: JVM is terminating. Shutting down Winstone
Running from: /usr/share/jenkins/jenkins.war
Apr 14, 2017 5:24:20 PM Main deleteWinstoneTempContents
WARNING: Failed to delete the temporary Winstone file /tmp/winstone/jenkins.war
Apr 14, 2017 5:24:20 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Logging initialized @401ms
Apr 14, 2017 5:24:20 PM winstone.Logger logInternal
INFO: Beginning extraction from war file
Apr 14, 2017 5:24:20 PM org.eclipse.jetty.util.log.JavaUtilLog warn
WARNING: Empty contextPath
Apr 14, 2017 5:24:20 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: jetty-9.2.z-SNAPSHOT
Apr 14, 2017 5:24:21 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: NO JSP Support for /, did not find org.eclipse.jetty.jsp.JettyJspServlet
Apr 14, 2017 5:24:21 PM org.eclipse.jetty.util.log.JavaUtilLog warn
WARNING: Failed startup of context w.@6593c96b{/,file:/var/cache/jenkins/war/,STARTING}{/var/cache/jenkins/war}
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.eclipse.jetty.webapp.IterativeDescriptorProcessor.visit(IterativeDescriptorProcessor.java:85)
at org.eclipse.jetty.webapp.IterativeDescriptorProcessor.process(IterativeDescriptorProcessor.java:72)
at org.eclipse.jetty.webapp.MetaData.resolve(MetaData.java:408)
at org.eclipse.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1340)
at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:741)
at org.eclipse.jetty.webapp.WebAppContext.doStart(WebAppContext.java:505)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:132)
at org.eclipse.jetty.server.Server.start(Server.java:387)
at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:114)
at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:61)
at org.eclipse.jetty.server.Server.doStart(Server.java:354)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at winstone.Launcher.<init>(Launcher.java:152)
at winstone.Launcher.main(Launcher.java:352)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at Main._main(Main.java:264)
at Main.main(Main.java:112)

Caused by: java.lang.UnsupportedClassVersionError: jenkins/util/SystemProperties : Unsupported major.minor version 52.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:803)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:442)
at java.net.URLClassLoader.access$100(URLClassLoader.java:64)
at java.net.URLClassLoader$1.run(URLClassLoader.java:354)
at java.net.URLClassLoader$1.run(URLClassLoader.java:348)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:347)
at org.eclipse.jetty.webapp.WebAppClassLoader.findClass(WebAppClassLoader.java:510)
at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:441)
at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:403)
at org.eclipse.jetty.server.handler.ContextHandler.loadClass(ContextHandler.java:1583)
at org.eclipse.jetty.webapp.StandardDescriptorProcessor.visitListener(StandardDescriptorProcessor.java:1956)
... 25 more

Apr 14, 2017 5:24:21 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started ServerConnector@3274ae84{HTTP/1.1}{0.0.0.0:8080}
Apr 14, 2017 5:24:21 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started @1355ms
Apr 14, 2017 5:24:21 PM winstone.Logger logInternal
INFO: Winstone Servlet Engine v2.0 running: controlPort=disabled