我正在尝试实现 Book Hadoop In Action 中给出的一个用例，但我不会编译代码。我是 Java 的新手，所以无法理解错误背后的确切原因。

有趣的是，另一段使用相同类和方法的代码编译成功。

hadoop@hadoopnode1:~/hadoop-0.20.2/playground/src$ javac -classpath /home/hadoop/hadoop-0.20.2/hadoop-0.20.2-core.jar:/home/hadoop/hadoop-0.20.2/lib/commons-cli-1.2.jar:/home/hadoop/hadoop-0.20.2/contrib/datajoin/hadoop-0.20.2-datajoin.jar -d ../classes DataJoin2.java 
DataJoin2.java:49: cannot find symbol
symbol  : constructor TaggedWritable(org.apache.hadoop.io.Text)
location: class DataJoin2.TaggedWritable
            TaggedWritable retv = new TaggedWritable((Text) value);
                                  ^
DataJoin2.java:69: cannot find symbol
symbol  : constructor TaggedWritable(org.apache.hadoop.io.Text)
location: class DataJoin2.TaggedWritable
            TaggedWritable retv = new TaggedWritable(new Text(joinedStr));
                                  ^
DataJoin2.java:113: setMapperClass(java.lang.Class<? extends org.apache.hadoop.mapreduce.Mapper>) in org.apache.hadoop.mapreduce.Job cannot be applied to (java.lang.Class<DataJoin2.MapClass>)
        job.setMapperClass(MapClass.class);
           ^
DataJoin2.java:114: setReducerClass(java.lang.Class<? extends org.apache.hadoop.mapreduce.Reducer>) in org.apache.hadoop.mapreduce.Job cannot be applied to (java.lang.Class<DataJoin2.Reduce>)
        job.setReducerClass(Reduce.class);
           ^
4 errors

----------------代码------------------------

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;


import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapred.KeyValueTextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.util.GenericOptionsParser;

// DataJoin Classes
import org.apache.hadoop.contrib.utils.join.DataJoinMapperBase;
import org.apache.hadoop.contrib.utils.join.TaggedMapOutput;
import org.apache.hadoop.contrib.utils.join.DataJoinReducerBase;

import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;


public class DataJoin2
{
    public static class MapClass extends DataJoinMapperBase
    {
        protected Text generateInputTag(String inputFile)
        {
            String datasource = inputFile.split("-")[0];
            return new Text(datasource);            
        }

        protected Text generateGroupKey(TaggedMapOutput aRecord)
        {
            String line = ((Text) aRecord.getData()).toString();
            String[] tokens = line.split(",");
            String groupKey = tokens[0];
            return new Text(groupKey);
        }

        protected TaggedMapOutput generateTaggedMapOutput(Object value)
        {
            TaggedWritable retv = new TaggedWritable((Text) value);
            retv.setTag(this.inputTag);
            return retv;
        }
    } // End of class MapClass

    public static class Reduce extends DataJoinReducerBase
    {
        protected TaggedMapOutput combine(Object[] tags, Object[] values)
        {
            if (tags.length < 2) return null;
            String joinedStr = "";
            for (int i=0;i<values.length;i++)
            {
                if (i>0) joinedStr += ",";
                TaggedWritable tw = (TaggedWritable) values[i];
                String line = ((Text) tw.getData()).toString();
                String[] tokens = line.split(",",2);
                joinedStr += tokens[1];
            }
            TaggedWritable retv = new TaggedWritable(new Text(joinedStr));
            retv.setTag((Text) tags[0]);
            return retv;
        }
    } // End of class Reduce

    public static class TaggedWritable extends TaggedMapOutput 
    {
        private Writable data;

        public TaggedWritable()
        {
            this.tag = new Text("");
            this.data = data;
        }

        public Writable getData()
        {
            return data;
        }

        public void write(DataOutput out) throws IOException
        {
            this.tag.write(out);
            this.data.write(out);
        }

        public void readFields(DataInput in) throws IOException
        {
            this.tag.readFields(in);
            this.data.readFields(in);
        }       
    } // End of class TaggedWritable

    public static void main(String[] args) throws Exception
    {
        Configuration conf = new Configuration();
        String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
        if (otherArgs.length != 2) {
          System.err.println("Usage: DataJoin2 <in> <out>");
          System.exit(2);
        }
        Job job = new Job(conf, "DataJoin");
        job.setJarByClass(DataJoin2.class);     
        job.setMapperClass(MapClass.class);
        job.setReducerClass(Reduce.class);
        job.setInputFormatClass(TextInputFormat.class);

        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(TaggedWritable.class);

        FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
        FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
        System.exit(job.waitForCompletion(true) ? 0 : 1);               
    }
}

最佳答案

错误消息没有任何歧义。它告诉您您没有为 TaggedWritable 提供构造函数，该构造函数采用 Text 类型的参数。您仅在发布的代码中显示无参数构造函数。

关于hadoop - Hadoop MapReduce 中的数据连接，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/10778372/