空指针异常是怎么回事？

环境信息

canal version 1.1.4 mysql version 5.7.17

问题描述

Server启动后扫描Instance报空指针异常 2019-10-08 17:32:54.964 [canal-instance-scan-0] ERROR c.a.o.c.deployer.monitor.ManagerInstanceConfigMonitor - scan failed java.lang.NullPointerException: null at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:210) ~[guava-18.0.jar:na] at com.google.common.collect.Lists.newArrayList(Lists.java:110) ~[guava-18.0.jar:na] at com.alibaba.otter.canal.deployer.monitor.ManagerInstanceConfigMonitor.scan(ManagerInstanceConfigMonitor.java:85) ~[canal.deployer-1.1.4.jar:na] at com.alibaba.otter.canal.deployer.monitor.ManagerInstanceConfigMonitor.access$000(ManagerInstanceConfigMonitor.java:29) ~[canal.deployer-1.1.4.jar:na] at com.alibaba.otter.canal.deployer.monitor.ManagerInstanceConfigMonitor$2.run(ManagerInstanceConfigMonitor.java:53) ~[canal.deployer-1.1.4.jar:na] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_112] at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) [na:1.8.0_112] at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) [na:1.8.0_112] at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) [na:1.8.0_112] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_112] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_112] at java.lang.Thread.run(Thread.java:745) [na:1.8.0_112]

实际执行情况

2019-10-08 17:32:54.964 [canal-instance-scan-0] ERROR c.a.o.c.deployer.monitor.ManagerInstanceConfigMonitor - scan failed java.lang.NullPointerException: null at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:210) ~[guava-18.0.jar:na] at com.google.common.collect.Lists.newArrayList(Lists.java:110) ~[guava-18.0.jar:na] at com.alibaba.otter.canal.deployer.monitor.ManagerInstanceConfigMonitor.scan(ManagerInstanceConfigMonitor.java:85) ~[canal.deployer-1.1.4.jar:na] at com.alibaba.otter.canal.deployer.monitor.ManagerInstanceConfigMonitor.access$000(ManagerInstanceConfigMonitor.java:29) ~[canal.deployer-1.1.4.jar:na] at com.alibaba.otter.canal.deployer.monitor.ManagerInstanceConfigMonitor$2.run(ManagerInstanceConfigMonitor.java:53) ~[canal.deployer-1.1.4.jar:na] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_112] at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) [na:1.8.0_112] at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) [na:1.8.0_112] at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) [na:1.8.0_112] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_112] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_112] at java.lang.Thread.run(Thread.java:745) [na:1.8.0_112]

原提问者GitHub用户hunterhy