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我不明白Hadoop中的清理方法究竟是幹什麼的，它是如何工作的？我有以下Map-Reduce代碼來計算一堆數字的最大值，最小值和平均值。清理（上下文）方法有什麼作用？
public class Statistics
{
public static class Map extends Mapper<LongWritable, Text, Text, Text>
{
public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException
{
/* code to calculate min, max, and mean from among a bunch of numbers */
}
public void cleanup(Context context) throws IOException, InterruptedException
{
Text key_min = new Text();
key_min.set("min");
Text value_min = new Text();
value_min.set(String.valueOf(min));
context.write(key_min,value_min);
Text key_max = new Text();
key_max.set("max");
Text value_max = new Text();
value_max.set(String.valueOf(max));
context.write(key_max,value_max);
Text key_avg = new Text();
key_avg.set("avg");
Text value_avg = new Text();
value_avg.set(String.valueOf(linear_sum)+","+count);
context.write(key_avg,value_avg);
Text key_stddev = new Text();
key_stddev.set("stddev");
Text value_stddev = new Text();
value_stddev.set(String.valueOf(linear_sum)+","+count+","+String.valueOf(quadratic_sum));
context.write(key_stddev,value_stddev);
}
}
public static class Reduce extends Reducer<Text,Text,Text,Text>
{
public void reduce(Text key, Iterable<Text> values,Context context) throws IOException, InterruptedException
{
/* code to further find min, max and mean from among the outputs of different mappers */
}
}
public static void main(String[] args) throws Exception
{
/* driver program */
}
}
那麼cleanup(Context context)方法究竟在做什麼呢？我假設它從一羣映射器收集輸出（鍵，值）對並將其傳遞給reducer。在其他網站上，我讀過在MapReduce中運行的命令是：setup - > map - > cleanup，然後設置 - > reduce - > cleanup。爲什麼這個程序不使用設置方法？