wangzhixiang created HDFS-15115:
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             Summary: Namenode crash caused by NPE in 
BlockPlacementPolicyDefault when dynamically change logger to debug
                 Key: HDFS-15115
                 URL: https://issues.apache.org/jira/browse/HDFS-15115
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: wangzhixiang
         Attachments: 
0001-fix-up-the-Namenode-crash-bug-caused-by-dynamically-.patch

To get debug info, we dynamically change the logger of 
BlockPlacementPolicyDefault to debug when namenode is running. However, the 
Namenode crashs. From the log, we find some NPE in 
BlockPlacementPolicyDefault.chooseRandom. Because *StringBuilder builder* will 
be used 4 times in BlockPlacementPolicyDefault.chooseRandom method. While the 
*builder* only initializes in the first time of this method. If we change the 
logger of BlockPlacementPolicyDefault to debug after the part, the *builder* in 
remaining part is *NULL* and cause *NPE*



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