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https://issues.apache.org/jira/browse/CASSANDRA-21096?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18053757#comment-18053757
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rstest commented on CASSANDRA-21096:
------------------------------------
Send a simple fix for this!
> queryNames might cause NPE
> --------------------------
>
> Key: CASSANDRA-21096
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21096
> Project: Apache Cassandra
> Issue Type: Bug
> Components: Test/dtest/java
> Reporter: rstest
> Priority: Normal
> Time Spent: 10m
> Remaining Estimate: 0h
>
> The `MapMBeanWrapper.queryNames()` method in `TableMetricTest` returns `null`
> instead of an empty `Set`, violating the `MBeanWrapper` interface contract.
> This causes a `NullPointerException` when `GCInspector` is instantiated
> during nodetool operations.
> {code:java}
> @Override
> public Set<ObjectName> queryNames(ObjectName name, QueryExp query)
> {
> return null;
> }
> {code}
> With this, we might meet NPE if queryNames() is called, for example, if we
> restart a node and create a new GCInspector, then the GCInspector constructor
> queries MBeans.
>
> {code:java}
> // GCInspector.java:144-146
> ObjectName gcName = new
> ObjectName(ManagementFactory.GARBAGE_COLLECTOR_MXBEAN_DOMAIN_TYPE + ",*");
> for (ObjectName name : MBeanWrapper.instance.queryNames(gcName, null)) //
> NPE HERE {code}
>
> h2. StackTrace:
>
> {code:java}
> java.lang.NullPointerException: Cannot invoke "java.util.Set.iterator()"
> because the return value of
> "org.apache.cassandra.utils.MBeanWrapper.queryNames(javax.management.ObjectName,
> javax.management.QueryExp)" is null
> at org.apache.cassandra.service.GCInspector.<init>(GCInspector.java:145)
> at
> org.apache.cassandra.distributed.mock.nodetool.InternalNodeProbe.connect(InternalNodeProbe.java:79)
> at
> org.apache.cassandra.distributed.mock.nodetool.InternalNodeProbe.<init>(InternalNodeProbe.java:58)
> at
> org.apache.cassandra.distributed.impl.Instance$DTestNodeTool.<init>(Instance.java:1079)
> at
> org.apache.cassandra.distributed.impl.Instance.lambda$nodetoolResult$51(Instance.java:986)
> {code}
> A better way would be:
>
> Change `MapMBeanWrapper.queryNames()` to return an empty set instead of null:
> {code:java}
> @Override
> public Set<ObjectName> queryNames(ObjectName name, QueryExp query)
> {
> return Collections.emptySet(); // FIX: Return empty set, not null
> } {code}
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