java.io.IOException:Type mismatch in key from 地图:expected org.apache.hadoop.io

java.io.IOException:Type mismatch in key from map:expected org.apache.hadoop.io.

 

                job.setMapOutputKeyClass(IntWritable.class);
                job.setMapOutputValueClass(Text.class);
                job.setOutputKeyClass(Text.class);
                job.setOutputValueClass(IntWritable.class);

if (key.getClass() != keyClass) {
        throw new IOException("Type mismatch in key from map: expected "
                              + keyClass.getName() + ", recieved "
                              + key.getClass().getName());
      }
      if (value.getClass() != valClass) {
        throw new IOException("Type mismatch in value from map: expected "
                              + valClass.getName() + ", recieved "
                              + value.getClass().getName());
      }

在MapOutputBuffer.collet()会检查实际传过来的Key Value 类型，和MapOutputBuffer初始化指定的KeyClass和ValueClass是否一样。 

keyClass = (Class<K>)job.getMapOutputKeyClass();
valClass = (Class<V>)job.getMapOutputValueClass();

public Class<?> getMapOutputKeyClass() {
    Class<?> retv = getClass("mapred.mapoutput.key.class", null, Object.class);
    if (retv == null) {
      retv = getOutputKeyClass();
    }
    return retv;
  }

public Class<?> getOutputKeyClass() {
    return getClass("mapred.output.key.class",
                    LongWritable.class, Object.class);  没用指定即采用默认的LongWritable
  }

MapOutputBuffer.collet()

public synchronized void collect(K key, V value)
        throws IOException {...}

上述在程序中直接修改是一种方法，还有一种直接在命令行中指定key或者value类型

-D  mapred.mapoutput.key.class=org.apache.hadoop.io.LongWritable或者

-jobconf mapred.mapoutput.key.class=org.apache.hadoop.io.LongWritable