Custom WritableCompare displays object reference as output

问题

I am new to Hadoop and Java, and I feel there is something obvious I am just missing. I am using Hadoop 1.0.3 if that means anything.

My goal for using hadoop is to take a bunch of files and parse them one file at a time (as opposed to line by line). Each file will produce multiple key-values, but context to the other lines is important. The key and value are multi-value/composite, so I have implemented WritableCompare for the key and Writable for the value. Because the processing of each file take a bit of CPU, I want to save the output of the mapper, then run multiple reducers later on.

For the composite keys, I followed [http://stackoverflow.com/questions/12427090/hadoop-composite-key][1]

The problem is, the output is just Java object references as opposed to the composite key and value. Example: LinkKeyWritable@bd2f9730 LinkValueWritable@8752408c

I am not sure if the problem is related to not reducing the data at all or

Here is my main class:

public static void main(String[] args) throws Exception {
  JobConf conf = new JobConf(Parser.class);
  conf.setJobName("raw_parser");

  conf.setOutputKeyClass(LinkKeyWritable.class);
  conf.setOutputValueClass(LinkValueWritable.class);

  conf.setMapperClass(RawMap.class);
  conf.setNumMapTasks(0);

  conf.setInputFormat(PerFileInputFormat.class);
  conf.setOutputFormat(TextOutputFormat.class);

  PerFileInputFormat.setInputPaths(conf, new Path(args[0]));
  FileOutputFormat.setOutputPath(conf, new Path(args[1]));

  JobClient.runJob(conf);
}

And my Mapper class:

public class RawMap extends MapReduceBase implements Mapper {

    public void map(NullWritable key, Text value,
            OutputCollector<LinkKeyWritable, LinkValueWritable> output,
            Reporter reporter) throws IOException {
        String json = value.toString();
        SerpyReader reader = new SerpyReader(json);
        GoogleParser parser = new GoogleParser(reader);
        for (String page : reader.getPages()) {
            String content = reader.readPageContent(page);
            parser.addPage(content);
        }
        for (Link link : parser.getLinks()) {
            LinkKeyWritable linkKey = new LinkKeyWritable(link);
            LinkValueWritable linkValue = new LinkValueWritable(link);
            output.collect(linkKey, linkValue);
        }
    }
}

Link is basically a struct of various information that get's split between LinkKeyWritable and LinkValueWritable

LinkKeyWritable:

public class LinkKeyWritable implements WritableComparable<LinkKeyWritable>{
    protected Link link;

    public LinkKeyWritable() {
        super();
        link = new Link();
    }

    public LinkKeyWritable(Link link) {
        super();
        this.link = link;
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        link.batchDay = in.readLong();
        link.source = in.readUTF();
        link.domain = in.readUTF();
        link.path = in.readUTF();
    }

    @Override
    public void write(DataOutput out) throws IOException {
        out.writeLong(link.batchDay);
        out.writeUTF(link.source);
        out.writeUTF(link.domain);
        out.writeUTF(link.path);
    }

    @Override
    public int compareTo(LinkKeyWritable o) {
        return ComparisonChain.start().
                compare(link.batchDay, o.link.batchDay).
                compare(link.domain, o.link.domain).
                compare(link.path, o.link.path).
                result();
    }

    @Override
    public int hashCode() {
        return Objects.hashCode(link.batchDay, link.source, link.domain, link.path);
    }

    @Override
    public boolean equals(final Object obj){
        if(obj instanceof LinkKeyWritable) {
            final LinkKeyWritable o = (LinkKeyWritable)obj;
            return Objects.equal(link.batchDay, o.link.batchDay)
                    && Objects.equal(link.source, o.link.source)
                    && Objects.equal(link.domain, o.link.domain)
                    && Objects.equal(link.path, o.link.path);
        }
        return false;
    }
}

LinkValueWritable:

public class LinkValueWritable implements Writable{
    protected Link link;

    public LinkValueWritable() {
        link = new Link();
    }

    public LinkValueWritable(Link link) {
        this.link = new Link();
        this.link.type = link.type;
        this.link.description = link.description;
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        link.type = in.readUTF();
        link.description = in.readUTF();
    }

    @Override
    public void write(DataOutput out) throws IOException {
        out.writeUTF(link.type);
        out.writeUTF(link.description);
    }

    @Override
    public int hashCode() {
        return Objects.hashCode(link.type, link.description);
    }

    @Override
    public boolean equals(final Object obj){
        if(obj instanceof LinkKeyWritable) {
            final LinkKeyWritable o = (LinkKeyWritable)obj;
            return Objects.equal(link.type, o.link.type)
                    && Objects.equal(link.description, o.link.description);
        }
        return false;
    }
}

回答1:

I think the answer is in the implementation of the TextOutputFormat. Specifically, the LineRecordWriter's writeObject method:

/**
 * Write the object to the byte stream, handling Text as a special
 * case.
 * @param o the object to print
 * @throws IOException if the write throws, we pass it on
 */
private void writeObject(Object o) throws IOException {
  if (o instanceof Text) {
    Text to = (Text) o;
    out.write(to.getBytes(), 0, to.getLength());
  } else {
    out.write(o.toString().getBytes(utf8));
  }
}

As you can see, if your key or value is not a Text object, it calls the toString method on it and writes that out. Since you've left toString unimplemented in your key and value, it's using the Object class's implementation, which is writing out the reference.

I'd say that you should try writing an appropriate toString function or using a different OutputFormat.

回答2:

It looks like you have a list of objects just like you wanted. You need to implement toString() on your writable if you want a human-readable version printed out instead of an ugly java reference.

来源：https://stackoverflow.com/questions/12937581/custom-writablecompare-displays-object-reference-as-output

标签

Hadoop

MapReduce