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Accessing a mapper's counter from a reducer
static enum TestCounters { TEST }

@Override
protected void map(Text key, Text value, Context context)
                throws IOException, InterruptedException {
    context.getCounter(TestCounters.TEST).increment(1);
    context.write(key, value);
}

@Override
protected void reduce(Text key, Iterable<Text> values, Context context)
                    throws IOException, InterruptedException {
    Counter counter = context.getCounter(CounterMapper.TestCounters.TEST);
    long counterValue = counter.getValue();
    context.write(key, new LongWritable(counterValue));
}

// in the Reducer class...
private long mapperCounter;

@Override
public void configure(JobConf conf) {
    JobClient client = new JobClient(conf);
    RunningJob parentJob =
        client.getJob(JobID.forName( conf.get("mapred.job.id") ));
    mapperCounter = parentJob.getCounters().getCounter(MAP_COUNTER_NAME);
}