Hadoop中采样是由org.apache.hadoop.mapred.lib.InputSampler类来实现的。
InputSampler类实现了三种采样方法：SplitSampler、RandomSampler和IntervalSampler。
SplitSampler、RandomSampler和IntervalSampler都是InputSampler的静态内部类，它们都实现了InputSampler的内部接口Sampler接口：
public interface Sampler<K,V>{
K[] getSample(InputFormat<K,V> inf,JobConf job) throws IOException;
}
getSample方法根据job的配置信息以及输入格式获得抽样结果，三个采样类各自有不同的实现。
RandomSampler随机地从输入数据中抽取Key，是一个通用的采样器。RandomSampler类有三个属性：freq（一个Key被选中的概率），numSamples（从所有被选中的分区中获得的总共的样本数目），maxSplitsSampled（需要检查扫描的最大分区数目）。
RandomSampler中getSample方法的实现如下：
public K[] getSample(InputFormat<K,V> inf, JobConf job) throws IOException {
InputSplit[] splits = inf.getSplits(job, job.getNumMapTasks());
ArrayList<K> samples = new ArrayList<K>(numSamples);
int splitsToSample = Math.min(maxSplitsSampled, splits.length);
Random r = new Random();
long seed = r.nextLong();
r.setSeed(seed);
LOG.debug("seed: " + seed);
// shuffle splits
for (int i = 0; i < splits.length; ++i) {
InputSplit tmp = splits[i];
int j = r.nextInt(splits.length);
splits[i] = splits[j];
splits[j] = tmp;
}
// our target rate is in terms of the maximum number of sample splits,
// but we accept the possibility of sampling additional splits to hit
// the target sample keyset
for (int i = 0; i < splitsToSample ||
(i < splits.length && samples.size() < numSamples); ++i) {
RecordReader<K,V> reader = inf.getRecordReader(splits[i], job,
Reporter.NULL);
K key = reader.createKey();
V value = reader.createValue();
while (reader.next(key, value)) {
if (r.nextDouble() <= freq) {
if (samples.size() < numSamples) {
samples.add(key);
} else {
// When exceeding the maximum number of samples, replace a
// random element with this one, then adjust the frequency
// to reflect the possibility of existing elements being
// pushed out
int ind = r.nextInt(numSamples);
if (ind != numSamples) {
samples.set(ind, key);
}
freq *= (numSamples - 1) / (double) numSamples;
}
key = reader.createKey();
}
}
reader.close();
}
return (K[])samples.toArray();
}