Advantages of using NullWritable in Hadoop

Question

What are the advantages of using NullWritable for null keys/values over using null texts (i.e. new Text(null)). I see the fol

Answer 1

I change the run method. and success

@Override
public int run(String[] strings) throws Exception {
    Configuration config = HBaseConfiguration.create();  
    //set job name
    Job job = new Job(config, "Import from file ");
    job.setJarByClass(LogRun.class);
    //set map class
    job.setMapperClass(LogMapper.class);

    //set output format and output table name
    //job.setOutputFormatClass(TableOutputFormat.class);
    //job.getConfiguration().set(TableOutputFormat.OUTPUT_TABLE, "crm_data");
    //job.setOutputKeyClass(ImmutableBytesWritable.class);
    //job.setOutputValueClass(Put.class);

    TableMapReduceUtil.initTableReducerJob("crm_data", null, job);
    job.setNumReduceTasks(0);
    TableMapReduceUtil.addDependencyJars(job);

    FileInputFormat.addInputPath(job, new Path(strings[0]));

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

Answer 2

You can always wrap your string in your own Writable class and have a boolean indicating it has blank strings or not:

@Override
public void readFields(DataInput in) throws IOException { 
    ...
    boolean hasWord = in.readBoolean();
    if( hasWord ) {
        word = in.readUTF();
    }
    ...
}

and

@Override
public void write(DataOutput out) throws IOException {
    ...
    boolean hasWord = StringUtils.isNotBlank(word);
    out.writeBoolean(hasWord);
    if(hasWord) {
        out.writeUTF(word);
    }
    ...
}

Answer 3

The key/value types must be given at runtime, so anything writing or reading NullWritables will know ahead of time that it will be dealing with that type; there is no marker or anything in the file. And technically the NullWritables are "read", it's just that "reading" a NullWritable is actually a no-op. You can see for yourself that there's nothing at all written or read:

NullWritable nw = NullWritable.get();
ByteArrayOutputStream out = new ByteArrayOutputStream();
nw.write(new DataOutputStream(out));
System.out.println(Arrays.toString(out.toByteArray())); // prints "[]"

ByteArrayInputStream in = new ByteArrayInputStream(new byte[0]);
nw.readFields(new DataInputStream(in)); // works just fine

And as for your question about new Text(null), again, you can try it out:

Text text = new Text((String)null);
ByteArrayOutputStream out = new ByteArrayOutputStream();
text.write(new DataOutputStream(out)); // throws NullPointerException
System.out.println(Arrays.toString(out.toByteArray()));

Text will not work at all with a null String.