WARNING [Finalizing set up] hudson.model.UpdateCenter.updateDefaultSite Upgrading Jenkins. Failed to update the default Update Site 'default'. Plugin upgrades may fail.
 javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
    at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
    at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1949)
    at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:302)
    at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:296)
    at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1509)
    at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:216)
    at sun.security.ssl.Handshaker.processLoop(Handshaker.java:979)
    at sun.security.ssl.Handshaker.process_record(Handshaker.java:914)
    at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1062)
    at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1375)

jenkins会显示该jenkins实例似乎已离线

原因是就是升级站点的链接https://updates.jenkins.io/update-center.json默认是https的

解决方案：

输入网址http://ip:8080/jenkins/pluginManager/advanced。 最底下有个【升级站点】，把其中的链接由https改成http的就好了，http://updates.jenkins.io/update-center.json， tomcat重新启动后就可以正常使用了。