这里只讲述遇到的一种原因：
1. 服务停止
最初以为是jvm导致的，但是在/var/log并没有找到jvm致命错误日志（hs_err_pid.log）
又排除了GC情况，使用 jstat -gc pid 5000查看也没发现问题
最后还是从日志入手
原因：日志中存在不明进程销毁日志，如下：
29-Nov-2018 13:28:53.355 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["http-nio-0.0.0.0-8080"]
29-Nov-2018 13:28:53.370 INFO [main] org.apache.catalina.startup.Catalina.start Server startup in 26456 ms
29-Nov-2018 14:02:02.816 INFO [Thread-9] org.apache.coyote.AbstractProtocol.pause Pausing ProtocolHandler ["http-nio-0.0.0.0-8080"]
29-Nov-2018 14:02:02.872 INFO [Thread-9] org.apache.catalina.core.StandardService.stopInternal Stopping service Catalina
29-Nov-2018 14:02:02.875 INFO [localhost-startStop-2] org.apache.catalina.core.ApplicationContext.log Destroying Spring FrameworkServlet 'skynet'
29-Nov-2018 14:02:02.923 INFO [localhost-startStop-2] org.apache.catalina.core.ApplicationContext.log Shutting down log4j
29-Nov-2018 14:02:02.933 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [skynet] registered the JDBC driver [com.alibaba.druid.proxy.DruidDriver] but failed to unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered.
29-Nov-2018 14:02:02.933 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [skynet] registered the JDBC driver [com.mysql.jdbc.Driver] but failed to unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered.
29-Nov-2018 14:02:02.934 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [ ] appears to have started a thread named [DubboRegistryFailedRetryTimer-thread-1] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
sun.misc.Unsafe.park(Native Method)
java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:226)
java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2082)
java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1090)
java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:807)
java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1068)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
java.lang.Thread.run(Thread.java:745)可见tomcat不是通过正常脚本关闭
还有一种情况是执行kill -9 pid，但是测试了一下，这种情况并没有产生日志
最后怀疑与ssh会话有关，因为我用的是windows+xshell，下班时直接关闭xshell，而恰巧此时tomcat也随之停止了服务。
可能是用户直接关闭了ssh会话导致了java进程的强制退出。
下面针对ssh可能引发的问题做了测试：
先关闭服务器
1.使用命令：sh bin/startup.sh 启动
此时强制关闭xshell，再登录xshell,使用grep查看运行的tomcat，发现tomcat依然运行。
2.使用命令：sh bin/startup.sh & tail -f logs/catalina.out 启动并查看日志
此时强制关闭xshell，再登录xshell,使用grep查看运行的tomcat，发现tomcat已关闭。