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

 1 14/10/06 03:08:51 INFO mapred.JobClient: Task Id : attempt_201410021756_0043_m_000000_0, Status : FAILED

 2 java.io.IOException: Type mismatch in value from map: expected org.apache.hadoop.io.IntWritable, recieved org.apache.hadoop.io.Text

 3     at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:1024)

 4     at org.apache.hadoop.mapred.MapTask$NewOutputCollector.write(MapTask.java:690)

 5     at org.apache.hadoop.mapreduce.TaskInputOutputContext.write(TaskInputOutputContext.java:80)

 6     at main.ch5.ReSort$Map.map(ReSort.java:51)

 7     at main.ch5.ReSort$Map.map(ReSort.java:43)

 8     at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:145)

 9     at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:764)

10     at org.apache.hadoop.mapred.MapTask.run(MapTask.java:364)

11     at org.apache.hadoop.mapred.Child$4.run(Child.java:255)

12     at java.security.AccessController.doPrivileged(Native Method)

13     at javax.security.auth.Subject.doAs(Subject.java:396)

14     at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1190)

15     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);