Jenkins not working, can't login, stuck then timeout - jenkins

Question

I installed the Jenkins v.2.6 on CENTOS 6.8 server a few months ago and everything working as expected since last week it just stops working.
If I go http://mydomain:8081, the login page not loads up anymore and give me the HTTP 503 errorHTTP ERROR: 503 Problem accessing /login. Reason: Service Unavailable.
I check the port 8081 and it is listening still, checked the status of the Jenkins service, it is running fine with the assigned PID #.
I did check the Jenkins log and see the message below and I have no idea how to fix it, try add the --prefix/Jenkins as Argument and 127.0.0.1 in the Jenkins config file as some other thread suggested, but none of those working.
Even rebooted the server and still not working.
Could anyone help?
Thanks,
Jenkins Log File
Running from: /usr/lib/jenkins/jenkins.war
Apr 17, 2017 3:10:21 PM Main deleteWinstoneTempContents
WARNING: Failed to delete the temporary Winstone file /tmp/winstone/jenkins.war
Apr 17, 2017 3:10:22 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Logging initialized #1304ms
Apr 17, 2017 3:10:22 PM winstone.Logger logInternal
INFO: Beginning extraction from war file
Apr 17, 2017 3:10:22 PM org.eclipse.jetty.util.log.JavaUtilLog warn
WARNING: Empty contextPath
Apr 17, 2017 3:10:22 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: jetty-9.2.z-SNAPSHOT
Apr 17, 2017 3:10:24 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: NO JSP Support for /, did not find org.eclipse.jetty.jsp.JettyJspServlet
Apr 17, 2017 3:10:25 PM org.eclipse.jetty.util.log.JavaUtilLog warn
WARNING: Failed startup of context w.#faf73d2{/,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:800)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
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 17, 2017 3:10:25 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started ServerConnector#768f310f{HTTP/1.1}{0.0.0.0:8081}
Apr 17, 2017 3:10:25 PM org.eclipse.jetty.util.log.JavaUtilLog info
After install the JDK 1.8 all working fine for few hours, all sudden I got the timeout without any HTTP error message this time.
I checked the JENKINS log again and here is what it showing this time.
Running from: /usr/lib/jenkins/jenkins.war
Apr 18, 2017 8:53:27 PM Main deleteWinstoneTempContents
WARNING: Failed to delete the temporary Winstone file /tmp/winstone/jenkins.war
Apr 18, 2017 8:53:27 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Logging initialized #861ms
Apr 18, 2017 8:53:27 PM winstone.Logger logInternal
INFO: Beginning extraction from war file
Apr 18, 2017 8:53:27 PM org.eclipse.jetty.util.log.JavaUtilLog warn
WARNING: Empty contextPath
Apr 18, 2017 8:53:27 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: jetty-9.2.z-SNAPSHOT
Apr 18, 2017 8:53:28 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: NO JSP Support for /, did not find org.eclipse.jetty.jsp.JettyJspServlet
Jenkins home directory: /data/jenkins found at: SystemProperties.getProperty("JENKINS_HOME")
Apr 18, 2017 8:53:29 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started w.#7e057f43{/,file:/var/cache/jenkins/war/,AVAILABLE}{/var/cache/jenkins/war}
Apr 18, 2017 8:53:29 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started ServerConnector#305ffe9e{HTTP/1.1}{0.0.0.0:8081}
Apr 18, 2017 8:53:29 PM org.eclipse.jetty.util.log.JavaUtilLog info
INFO: Started #2826ms
Apr 18, 2017 8:53:29 PM winstone.Logger logInternal
INFO: Winstone Servlet Engine v2.0 running: controlPort=disabled
Apr 18, 2017 8:53:30 PM jenkins.InitReactorRunner$1 onAttained
INFO: Started initialization
Apr 18, 2017 8:53:30 PM jenkins.InitReactorRunner$1 onAttained
INFO: Listed all plugins
Apr 18, 2017 8:53:30 PM jenkins.bouncycastle.api.SecurityProviderInitializer addSecurityProvider
INFO: Initializing Bouncy Castle security provider.
Apr 18, 2017 8:53:30 PM jenkins.bouncycastle.api.SecurityProviderInitializer addSecurityProvider
INFO: Bouncy Castle security provider initialized.
Apr 18, 2017 8:53:38 PM jenkins.InitReactorRunner$1 onAttained
INFO: Prepared all plugins
Apr 18, 2017 8:53:38 PM jenkins.InitReactorRunner$1 onAttained
INFO: Started all plugins
Apr 18, 2017 8:53:40 PM hudson.ExtensionFinder$GuiceFinder$FaultTolerantScope$1 error
INFO: Failed to instantiate optional component hudson.plugins.build_timeout.operations.AbortAndRestartOperation$DescriptorImpl; skipping
Apr 18, 2017 8:53:40 PM org.jenkinsci.plugins.dockerbuildstep.DockerBuilder$DescriptorImpl <init>
WARNING: Docker URL is not set, docker client won't be initialized
Apr 18, 2017 8:53:41 PM jenkins.InitReactorRunner$1 onAttained
INFO: Augmented all extensions
Apr 18, 2017 8:53:42 PM jenkins.InitReactorRunner$1 onAttained
INFO: Loaded all jobs
Apr 18, 2017 8:53:42 PM hudson.model.AsyncPeriodicWork$1 run
INFO: Started Download metadata
Could anyone help me again? As I said it works after upgraded the JDK to 1.8 and it created more than 10 builds in few hours, then stop working again.
Thanks,

Answer 1

version 52.0 of the java class file means, that the java source code was compiled for jre8 and has to be executed with jre8.
Here the class file versions and theire equivalent java versions:
Java SE 9 = 53 (0x35 hex)
Java SE 8 = 52 (0x34 hex)
Java SE 7 = 51 (0x33 hex)
Java SE 6.0 = 50 (0x32 hex)
Java SE 5.0 = 49 (0x31 hex)
JDK 1.4 = 48 (0x30 hex)
This means you have either to upgrade the jvm or you have to downgrade the jenkins version. You can see the jenkins version right down in the corner of your jenkins installation, when you open it in the webbrowser.
Here is the official blogpost for the upgrade: https://jenkins.io/blog/2017/01/17/Jenkins-is-upgrading-to-Java-8/
Since java 7 doesn't get any public updates since April 2015, the best way is to upgrade your jvm to 8.

Answer 2

The solution for me was:
Install Java8: sudo apt install oracle-java8-jdk/stable
Set Java8 as the default java: sudo update-alternatives --config java
Restart Jenkins: sudo systemctl restart jenkins
After this I haven't had any problem with Jenkins.