Hadoop运行错误 - Output directory hdfs://master:9000/output already exists

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本文介绍在运行Hadoop WordCount作业时遇到“FileAlreadyExistsException”错误的原因及解决办法，通过删除已存在的输出目录，成功解决该问题。

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运行hadoop出现以下问题：

[its312@master bin]$ hadoop jar /usr/local/hadoop/share/hadoop/mapreduce/hadoop-mapreduce-examples-3.1.0.jar wordcount /input /output
2018-11-06 21:31:12,192 INFO client.RMProxy: Connecting to ResourceManager at /0.0.0.0:8032
org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory hdfs://master:9000/output already exists
	at org.apache.hadoop.mapreduce.lib.output.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:164)
	at org.apache.hadoop.mapreduce.JobSubmitter.checkSpecs(JobSubmitter.java:280)
	at org.apache.hadoop.mapreduce.JobSubmitter.submitJobInternal(JobSubmitter.java:146)
	at org.apache.hadoop.mapreduce.Job$11.run(Job.java:1570)
	at org.apache.hadoop.mapreduce.Job$11.run(Job.java:1567)
	at java.security.AccessController.doPrivileged(Native Method)
	at javax.security.auth.Subject.doAs(Subject.java:422)
	at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1682)
	at org.apache.hadoop.mapreduce.Job.submit(Job.java:1567)
	at org.apache.hadoop.mapreduce.Job.waitForCompletion(Job.java:1588)
	at org.apache.hadoop.examples.WordCount.main(WordCount.java:87)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.apache.hadoop.util.ProgramDriver$ProgramDescription.invoke(ProgramDriver.java:71)
	at org.apache.hadoop.util.ProgramDriver.run(ProgramDriver.java:144)
	at org.apache.hadoop.examples.ExampleDriver.main(ExampleDriver.java:74)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.apache.hadoop.util.RunJar.run(RunJar.java:308)
	at org.apache.hadoop.util.RunJar.main(RunJar.java:222)

问题分析：

hdfs的output目录已经存在。

解决方案：

删除相应文件或文件夹：

/usr/loca/hadoop/bin/hadoop fs -rm -r output