...
<validation>
 <check-valid-connection-sql>select 1</check-valid-connection-sql>
 <validate-on-match>false</validate-on-match>
 <background-validation>true</background-validation>
 <background-validation-millis>10000</background-validation-millis>
</validation>
... 

WARN  [org.jboss.jca.core.connectionmanager.pool.validator.ConnectionValidator] (ConnectionValidator) [] IJ000602: ConnectionValidator ignored unexpected runtime exception: java.lang.NullPointerException
	at java.util.concurrent.ConcurrentLinkedDeque.checkNotNull(ConcurrentLinkedDeque.java:798) [rt.jar:1.8.0_292]
	at java.util.concurrent.ConcurrentLinkedDeque.linkLast(ConcurrentLinkedDeque.java:386) [rt.jar:1.8.0_292]
	at java.util.concurrent.ConcurrentLinkedDeque.addLast(ConcurrentLinkedDeque.java:903) [rt.jar:1.8.0_292]
	at org.jboss.jca.core.connectionmanager.pool.mcp.SemaphoreConcurrentLinkedDequeManagedConnectionPool.returnForFrequencyCheck(SemaphoreConcurrentLinkedDequeManagedConnectionPool.java:1580) [ironjacamar-core-impl-1.4.33.Final-redhat-00001.jar:1.4.33.Final-redhat-00001]
	at org.jboss.jca.core.connectionmanager.pool.mcp.SemaphoreConcurrentLinkedDequeManagedConnectionPool.validateConnections(SemaphoreConcurrentLinkedDequeManagedConnectionPool.java:1506) [ironjacamar-core-impl-1.4.33.Final-redhat-00001.jar:1.4.33.Final-redhat-00001]
	at org.jboss.jca.core.connectionmanager.pool.validator.ConnectionValidator$ConnectionValidatorRunner.run(ConnectionValidator.java:285) [ironjacamar-core-impl-1.4.33.Final-redhat-00001.jar:1.4.33.Final-redhat-00001]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [rt.jar:1.8.0_292]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [rt.jar:1.8.0_292]
	at java.lang.Thread.run(Thread.java:748) [rt.jar:1.8.0_292]