Hadoop on Mac with IntelliJ IDEA - 9 解决Type mismatch in value from map问题

修改陆喜恒. Hadoop实战（第2版）5.3排序的代码时遇到IO异常。

环境：Mac OS X 10.9.5, IntelliJ IDEA 13.1.5, Hadoop 1.2.1

异常具体信息如下

 14/10/06 03:08:51 INFO mapred.JobClient: Task Id : attempt_201410021756_0043_m_000000_0, Status : FAILED
 java.io.IOException: Type mismatch in value from map: expected org.apache.hadoop.io.IntWritable, recieved org.apache.hadoop.io.Text
     at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:1024)
     at org.apache.hadoop.mapred.MapTask$NewOutputCollector.write(MapTask.java:690)
     at org.apache.hadoop.mapreduce.TaskInputOutputContext.write(TaskInputOutputContext.java:80)
     at main.ch5.ReSort$Map.map(ReSort.java:51)
     at main.ch5.ReSort$Map.map(ReSort.java:43)
     at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:145)
     at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:764)
     at org.apache.hadoop.mapred.MapTask.run(MapTask.java:364)
     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)

相关代码如下

public static class Map extends Mapper<LongWritable, Text, IntWritable, Text> {
        // ...
}

public static class Reduce extends Reducer<IntWritable, Text, IntWritable, IntWritable> {
        // ...
}

public static void main(String[] args){
        // ...
        job.setOutputFormatClass(TextOutputFormat.class);
        job.setOutputKeyClass(IntWritable.class);
        job.setOutputValueClass(IntWritable.class);
        // ...
}

Map的输出与Reduce的输入类型相符。但是，根据错误信息，Map的value值预期为IntWritable，接受到的却是Text，两者类型不匹配。另外，错误提示与代码定义也不同，因为定义声明了map阶段输出值为<IntWritable, Text>。

造成这个问题的原因等以后阅读源码时再分析，先处理问题。配置作业的MapOutputKeyClass和MapOutputValueClass参数，将其设置成Map输出类型即可。

job.setMapOutputKeyClass(/*K2*/IntWritable.class);
job.setMapOutputValueClass(/*V2*/Text.class);