Hadoop:找不到类

Question

我刚刚开始学习 hadoop 并遵循“Hadoop - The Definitive Guide”。

我测试了第一种编写 Map 和 Reduce 类的方法，其中 Mapper 和 Reducer 是接口(interface)。代码运行良好。
然后我开始编写代码，其中 Map 和 Reduce 是具有 Context 类的抽象类。
顺便说一句，我正在使用 hadoop 1.2.1
我看到以下错误

MaxTemperatureReducer.java:5: error: cannot find symbol
public class MaxTemperatureReducer extends Reducer<Text, IntWritable, Text, IntWritable> 
                                       ^
  symbol: class Reducer
MaxTemperatureReducer.java:7: error: cannot find symbol
    public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException,InterruptedException 
                                                           ^
  symbol:   class Context
  location: class MaxTemperatureReducer
MaxTemperature.java:5: error: cannot find symbol
import org.apache.hadoop.mapreduce.FileInputFormat;
                              ^
  symbol:   class FileInputFormat
  location: package org.apache.hadoop.mapreduce
MaxTemperature.java:6: error: cannot find symbol
import org.apache.hadoop.mapreduce.FileOutputFormat;
                              ^
  symbol:   class FileOutputFormat
  location: package org.apache.hadoop.mapreduce
MaxTemperature.java:7: error: cannot find symbol
import org.apache.hadoop.mapreduce.JobClient;

我的 Mapper 类看起来像

import java.io.IOException;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
//import org.apache.hadoop.mapreduce.MapReduceBase;
import org.apache.hadoop.mapreduce.Mapper;
public class MaxTemperatureMapper extends Mapper<LongWritable, Text, Text, IntWritable>
{
    private static final int MISSING = 9999;
    public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException
    {
    String line = value.toString();
    String year = line.substring(0, 4);
    int airTemperature;
    if (line.charAt(4) == '+')
    {
        airTemperature = Integer.parseInt(line.substring(5, 10));
    }
    else
    {
        System.out.println( line );
        airTemperature = Integer.parseInt(line.substring(4, 9));
    }
    System.out.println( "Mapper: " + year + ", " + airTemperature );
    context.write(new Text(year), new IntWritable(airTemperature));
    }
}

有人可以帮忙吗？

Answer 1

看来问题出在您的 Driver 类中的导入语句中，您试图对 FileInputFormat 使用错误的导入语句和 FileOutputFormat .使用这些导入语句而不是 FileInputFormat & FileOutputFormat在您的驱动程序(MaxTemperature)类中:

import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

使用这个 example作为引用，以确保您的所有导入语句都是有效的。