一、错误描述
执行MR出现如下错误，如执行命令是：
hadoop jar /opt/cloudera/parcels/CDH-5.0.1-1.cdh5.0.1.p0.47/lib/hadoop-mapreduce/hadoop-streaming-2.3.0-cdh5.0.1.jar -input /test2 -output /test12 -mapper org.apache.hadoop.mapred.lib.TokenCountMapper -reducer org.apache.hadoop.mapred.lib.LongSumReducer -jobconf mapred.job.name='java mr' -numReduceTasks 1
错误如下：
14/06/09 15:09:50 INFO mapreduce.Job: Task Id : attempt_1401948435703_0028_m_000000_0, Status : FAILED
Error: java.io.IOException: Type mismatch in key from map: expected org.apache.hadoop.io.LongWritable, received org.apache.hadoop.io.Text
at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:1049)
at org.apache.hadoop.mapred.MapTask$OldOutputCollector.collect(MapTask.java:587)
at org.apache.hadoop.mapred.lib.TokenCountMapper.map(TokenCountMapper.java:54)
at org.apache.hadoop.mapred.lib.TokenCountMapper.map(TokenCountMapper.java:38)
at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:54)
at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:430)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:342)
at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:168)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1548)
at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:163)
二、原因及解决方案
1、map和reduce中的输入输出格式不对。
2、新旧api混用。你的map()方法 没有按新api写， 结果系统不认为它是一个重载，而是一个新方法，不会被调用。
解决办法：在的map(), reduce()前面加上@Override，并按照新的API来写map(), reduce()。