xupeng created HDFS-11827:
-----------------------------

             Summary: NPE is thrown when log level changed in 
BlockPlacementPolicyDefault#chooseRandom() method 
                 Key: HDFS-11827
                 URL: https://issues.apache.org/jira/browse/HDFS-11827
             Project: Hadoop HDFS
          Issue Type: Bug
    Affects Versions: 2.6.0
         Environment: hadoop-version: 2.6.0-cdh5.4.4
            Reporter: xupeng


This issue was found by my colleague when changing log-level of 
BlockPlacementPolicy using "hadoop daemonlog" command. 

The exception stack trace is below:
2017-05-16 13:36:51,053 ERROR 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager: ReplicationMonitor 
thread received Runtime exception.
java.lang.NullPointerException
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseRandom(BlockPlacementPolicyDefault.java:666)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseRemoteRack(BlockPlacementPolicyDefault.java:607)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:375)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:446)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:446)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:240)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:119)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyWithMultiDC.chooseTargetInternal(BlockPlacementPolicyWithMultiDC.java:263)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyWithMultiDC.chooseTarget(BlockPlacementPolicyWithMultiDC.java:214)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$ReplicationWork.chooseTargets(BlockManager.java:3836)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$ReplicationWork.access$200(BlockManager.java:3801)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeReplicationWorkForBlocks(BlockManager.java:1394)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeReplicationWork(BlockManager.java:1300)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeDatanodeWork(BlockManager.java:3764)
        at 
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$ReplicationMonitor.run(BlockManager.java:3711)
        at java.lang.Thread.run(Thread.java:745)



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