当我运行nutch命令：~/nutch/runtime/deploy$bin/nutch crawl urls-dir/user/dlequoc/urls-depth 2-topn 5时，出现以下异常：

13/08/26 16:30:15 info mapred.jobclient:map 100%reduce 0%13/08/26 16:30:29 info mapred.jobclient:task id:attempt\u 201308261546\u 0004\u r\u000000\u 0，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:32 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000001 \u 0，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:32 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000005 \u 0，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:32 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000004 \u 0，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:32 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000002 \u 0，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:32 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000003 \u 0，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:44 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000001 \u 1，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:47 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000000 \u 1，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:47 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000005 \u 1，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:47 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000002 \u 1，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:47 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000004 \u 1，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:47 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u000003 \u 1，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)在org.apache.hadoop.security.usergroupinformation.doas（usergroupinformation。java:1121)在org.apache.hadoop.mapred.child.main（child。java:249)13/08/26 16:30:59 info mapred.jobclient:任务id:尝试\u 201308261546 \u 0004 \u r\u0000002，状态：位于org.apache.avro.util.utf8的java.lang.nullpointerexception失败（utf8）。java:37)在org.apache.nutch.crawl.generatorreducer.setup（generatorreducer。java:100)在org.apache.hadoop.mapreduce.reducer.run（reducer。java:174)在org.apache.hadoop.mapred.reducetask.runnewreducer（reducetask。java:649)在org.apache.hadoop.mapred.reducetask.run（reducetask。java:417)在org.apache.hadoop.mapred.child$4.run（child。java:255)位于javax.security.auth.subject.doas（subject）的java.security.accesscontroller.doprivileged（本机方法）。java:396)访问org.apache.hadoop.security.u