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https://issues.apache.org/jira/browse/HBASE-4386?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13103711#comment-13103711
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Todd Lipcon commented on HBASE-4386:
------------------------------------
I think the issue is that items are added to the {{tasks}} list without
synchronization. So the ArrayList can get into an inconsistent state where
iterating it returns null.
> NPE in TaskMonitor
> ------------------
>
> Key: HBASE-4386
> URL: https://issues.apache.org/jira/browse/HBASE-4386
> Project: HBase
> Issue Type: Bug
> Affects Versions: 0.92.0
> Reporter: Todd Lipcon
> Priority: Critical
> Fix For: 0.92.0
>
>
> Saw the following hitting /rs-status
> <pre> INTERNAL_SERVER_ERROR</pre></p><h3>Caused
> by:</h3><pre>java.lang.NullPointerException
> at
> org.apache.hadoop.hbase.monitoring.TaskMonitor.purgeExpiredTasks(TaskMonitor.java:97)
> at
> org.apache.hadoop.hbase.monitoring.TaskMonitor.getTasks(TaskMonitor.java:127)
> at
> org.apache.hbase.tmpl.common.TaskMonitorTmplImpl.renderNoFlush(TaskMonitorTmplImpl.java:50)
> at
> org.apache.hbase.tmpl.common.TaskMonitorTmpl.renderNoFlush(TaskMonitorTmpl.java:170)
> at
> org.apache.hbase.tmpl.regionserver.RSStatusTmplImpl.renderNoFlush(RSStatusTmplImpl.java:70)
> at
> org.apache.hbase.tmpl.regionserver.RSStatusTmpl.renderNoFlush(RSStatusTmpl.java:176)
> at
> org.apache.hbase.tmpl.regionserver.RSStatusTmpl.render(RSStatusTmpl.java:167)
> at
> org.apache.hadoop.hbase.regionserver.RSStatusServlet.doGet(RSStatusServlet.java:48)
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