1、map階段
通過map方法将資料構造成資料小于K的TreeMap,在每次map後判斷TreeMap的大小和K的大小,當TreeMap的資料量大于K時,取出最小的數。在map結束後會執行cleanup方法,該方法将map中的前K個資料傳入reduce任務中。
2、reduce階段
在 reduce方法中,依次将map方法中傳入K個資料放入 TreeMap中,進而将K個數
據利用紅黑樹的 firstKey方法按從大到小者利用紅黑樹的 lastKey方法按從小到大的順序
排列。進而求出前K個數。
3、代碼部分: 從1000w資料中找到最大的100個數。
public class TopKAapp{
private static final String INPUT PATH ="hdfs:/xxx/topk_input";
private static final String OUT PATH="hdfs://xxx/topk out";
public static void main (String[] args) throws Exception{
Configuration conf new ConfigurationO;
final FileSystem fileSystem FileSystem.get(new URI(INPUT_PATH), conf);
final Path outPath new Path (OUT PATH);
if (fileSystem.exists (outPath)){
fileSystem.delete(outPath, true);
}
final Job job= new Job(conf, TopKAapp.class.getSimpleNameO);
FilelnputFormat.setInputPaths(job, INPUT PATH);
job.setMapperClass(MyMapper.class);
job.setPartitionerClass(HashPartitioner.class);
job.setNumReduceTasks(1);
job.setReducerClass(MyReducer.class);
job.setOutputKeyClass(NullWritable.class);
job. setOutput ValueClass(Long Writable.class);
FileOutputFormat.setOutputPath(job, new Path(OUT_PATH));
job.setOutputFormatClass(TextOutputFormat.class);
job.waitForCompletion(true);
}
static class MyMapper extends Mapper<Long Writable, Text, Null Writable, Long Writable>{
public static final int K=100;
private TreeMap<Long, Long> tree= new TreeMap<Long, Long>O;
public void map(Long Writable key, Text text, Context context) throws IOException, InterruptedException{
long temp Long.parseLong(text.toStringO);
tree.put(temp, temp);
if (tree.size() > K)
tree.remove(tree.firstKeyO);
}
@Override
protected void cleanup(Context context) throws IOException, InterruptedException
for (Long text tree. values)){
context. write(Null Writable.get), new Long Writable(text));
}
}
}
static class MyReducer extends Reducer<NullWritable, Long Writable, NullWritable,
Long Writable>{
public static final int K=100;
private TreeMap<Long, Long> tree new TreeMap<Long, Long>);
@Override
protected void cleanup(Context context) throws IOException, InterruptedException{
for (Long val tree.descendingKeySet()){
context.write(Null Writable.get(), new Long Writable(val));
}
}
@Override
protected void reduce(Null Writable key, Iterable<Long Writable> values, Context context) throws IOException, InterruptedException{
for (Long Writable value values){
tree.put(value.get), value.getO);
if(tree.size()>K)
tree.remove(tree.firstKey));
}
}
}
}