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我试图实现Book Hadoop在Action中给出的一个用例，但我不是要编译代码。我是Java新手，因此无法理解错误背后的确切原因。Hadoop MapReduce中的DataJoins
有趣的是，使用相同的类和方法的另一个编码编译成功。
[email protected]:~/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);
}
}
感谢您指点....当来自Oracle背景的人尝试使用Java时，会发生这种情况 – Sandeep