下面是我编写的Map程序，用于解析so posts xml并在hadoop上创建一个以tab分隔的文件，供其他map reduce作业、hive或pig使用。
制图器

package com.aravind.learning.hadoop.mapred.techtalks;

import java.io.IOException;
import java.io.StringReader;
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

import com.google.common.base.Joiner;

public class StackoverflowDataWranglerMapper extends Mapper<LongWritable, Text, Text, Text>
{
    static enum BadRecordCounters
    {
        NO_CREATION_DATE, UNKNOWN_USER_ID, UNPARSEABLE_RECORD, UNTAGGED_POSTS
    }

    private final Text outputKey = new Text();
    private final Text outputValue = new Text();

    private final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    private DocumentBuilder builder;
    private static final Joiner TAG_JOINER = Joiner.on(",").skipNulls();
    // 2008-07-31T21:42:52.667
    private static final DateFormat DATE_PARSER = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS");
    private static final SimpleDateFormat DATE_BUILDER = new SimpleDateFormat("yyyy-MM-dd");

    @Override
    protected void setup(Context context) throws IOException, InterruptedException
    {
        try
        {
            builder = factory.newDocumentBuilder();
        }
        catch (ParserConfigurationException e)
        {
            new IOException(e);
        }
    }

    @Override
    protected void map(LongWritable inputKey, Text inputValue, Mapper<LongWritable, Text, Text, Text>.Context context)
            throws IOException, InterruptedException
    {
        try
        {
            String entry = inputValue.toString();
            if (entry.contains("<row "))
            {
                Document doc = builder.parse(new InputSource(new StringReader(entry)));
                Element rootElem = doc.getDocumentElement();

                String id = rootElem.getAttribute("Id");
                String postedBy = rootElem.getAttribute("OwnerUserId").trim();
                String viewCount = rootElem.getAttribute("ViewCount");
                String postTypeId = rootElem.getAttribute("PostTypeId");
                String score = rootElem.getAttribute("Score");
                String title = rootElem.getAttribute("Title");
                String tags = rootElem.getAttribute("Tags");
                String answerCount = rootElem.getAttribute("AnswerCount");
                String commentCount = rootElem.getAttribute("CommentCount");
                String favoriteCount = rootElem.getAttribute("FavoriteCount");
                String creationDate = rootElem.getAttribute("CreationDate");

                Date parsedDate = null;
                if (creationDate != null && creationDate.trim().length() > 0)
                {
                    try
                    {
                        parsedDate = DATE_PARSER.parse(creationDate);
                    }
                    catch (ParseException e)
                    {
                        context.getCounter("Bad Record Counters", "Posts missing CreationDate").increment(1);
                    }
                }

                if (postedBy.length() == 0 || postedBy.trim().equals("-1"))
                {
                    context.getCounter("Bad Record Counters", "Posts with either empty UserId or UserId contains '-1'")
                            .increment(1);
                    try
                    {
                        parsedDate = DATE_BUILDER.parse("2100-00-01");
                    }
                    catch (ParseException e)
                    {
                        // ignore
                    }
                }

                tags = tags.trim();
                String tagTokens[] = null;

                if (tags.length() > 1)
                {
                    tagTokens = tags.substring(1, tags.length() - 1).split("><");
                }
                else
                {
                    context.getCounter("Bad Record Counters", "Untagged Posts").increment(1);
                }

                outputKey.clear();
                outputKey.set(id);

                StringBuilder sb = new StringBuilder(postedBy).append("\t").append(parsedDate.getTime()).append("\t")
                        .append(postTypeId).append("\t").append(title).append("\t").append(viewCount).append("\t").append(score)
                        .append("\t");

                if (tagTokens != null)
                {
                    sb.append(TAG_JOINER.join(tagTokens)).append("\t");
                }
                else
                {
                    sb.append("").append("\t");
                }
                sb.append(answerCount).append("\t").append(commentCount).append("\t").append(favoriteCount).toString();

                outputValue.set(sb.toString());

                context.write(outputKey, outputValue);
            }
        }
        catch (SAXException e)
        {
            context.getCounter("Bad Record Counters", "Unparsable records").increment(1);
        }
        finally
        {
            builder.reset();
        }
    }
}

司机

public class StackoverflowDataWranglerDriver extends Configured implements Tool
{
    @Override
    public int run(String[] args) throws Exception
    {
        if (args.length != 2)
        {
            System.err.printf("Usage: %s [generic options] <input> <output>\n", getClass().getSimpleName());
            ToolRunner.printGenericCommandUsage(System.err);
            return -1;
        }

        Job job = Job.getInstance(getConf());

        job.setJobName("Tech Talks - Stackoverflow Forum Posts - Data Wrangler");

        TextInputFormat.addInputPath(job, new Path(args[0]));

        TextOutputFormat.setOutputPath(job, new Path(args[1]));

        job.setInputFormatClass(TextInputFormat.class);
        job.setOutputFormatClass(TextOutputFormat.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(Text.class);

        job.setJarByClass(StackoverflowDataWranglerMapper.class);// required for mr1
        job.setMapperClass(StackoverflowDataWranglerMapper.class);
        job.setNumReduceTasks(0);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);

        return job.waitForCompletion(true) ? 0 : 1;
    }

    public static void main(String args[]) throws Exception
    {
        int exitCode = ToolRunner.run(new Configuration(), new StackoverflowDataWranglerDriver(), args);
        System.exit(exitCode);
    }
}

作业提交命令

hadoop jar ./hadoop-examples-0.0.1-SNAPSHOT.jar com.aravind.learning.hadoop.mapred.techtalks.StackoverflowDataWranglerDriver data/stackoverflow-posts.xml data/so-posts-tsv

经过大量的研究和实验，终于学会了在xml文件中为parsin编写map的方法，这些xml文件的语法和我提供的一样。我改变了我的方法，这是我的新Map程序代码。。。它为我的用例工作。
希望它能帮助一些人节省时间：）

import java.io.IOException;
import java.util.StringTokenizer;

import javax.xml.parsers.ParserConfigurationException;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.xml.sax.SAXException;

public class Map extends Mapper<LongWritable, Text, NullWritable, Text> {
    NullWritable obj;

    @Override
    public void map(LongWritable key, Text value, Context context) throws InterruptedException {
        StringTokenizer tok= new StringTokenizer(value.toString()); 
        String pa=null,ow=null,pi=null,v;
        while (tok.hasMoreTokens()) {
            String[] arr;
            String val = (String) tok.nextToken();
            if(val.contains("PostTypeId")){
                arr= val.split("[\"]");
                pi=arr[arr.length-1];
                if(pi.equals("2")){
                    continue;
                }
                else break;
            }
            if(val.contains("ParentId")){
                arr= val.split("[\"]");
                pa=arr[arr.length-1];
            } 
            else if(val.contains("OwnerUserId") ){
                arr= val.split("[\"]");
                ow=arr[arr.length-1];
                try {
                    if(pa!=null && ow != null){
                        v=String.format("{0},{1}", ow,pa);
                        context.write(obj,new Text(v));

                    }
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }

    }

}