使用Cassandra在Hadoop上运行Nutch时出现空指针异常

我在Hadoop集群上运行Nutch，爬网的数据存储在Cassandra集群中。运行Nutch作业时，出现以下错误：

java.lang.NullPointerException
    at org.apache.avro.util.Utf8.<init>(Utf8.java:38)
    at org.apache.nutch.crawl.GeneratorReducer.setup(GeneratorReducer.java:100)
    at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:174)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:649)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:418)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1190)
    at org.apache.hadoop.mapred.Child.main(Child.java:249)

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您有一个与安全权限相关的异常。例如，检查文件夹的权限。我刚刚发现我正在运行“爬网”，这是不推荐使用的。运行“爬虫”修复了它！

$HADOOP_HOME/bin/hadoop jar /nutch/apache-nutch-2.2.1.job org.apache.nutch.crawl.Crawl urls -dir crawl -depth 3 -topN 5