MAPREDUCE-6827. Fixed bug: the second foreach-loop was not executed

[javeme]

Failed to traverse Iterable values with foreach at the second time in reduce() method, because the second foreach-loop was not executed.

JIRA MAPREDUCE-6827

The following code is a reduce() method (of WordCount):

public static class WcReducer extends Reducer<Text, IntWritable, Text, IntWritable> {

	@Override
	protected void reduce(Text key, Iterable<IntWritable> values, Context context)
			throws IOException, InterruptedException {

		// print some logs
		List<String> vals = new LinkedList<>();
		for(IntWritable i : values) {
			vals.add(i.toString());
		}
		System.out.println(String.format(">>>> reduce(%s, [%s])",
				key, String.join(", ", vals)));

		// sum of values
		int sum = 0;
		for(IntWritable i : values) {
			sum += i.get();
		}
		System.out.println(String.format(">>>> reduced(%s, %s)",
				key, sum));

		context.write(key, new IntWritable(sum));
	}
}

After running it, we got the result that the value of the variable sum is always 0！

After debugging, it was found that the second foreach-loop was not executed, and the root cause was the returned value of Iterable.iterator(), it returned the same instance in the two calls called by foreach-loop. In general, Iterable.iterator() should return a new instance in each call, such as ArrayList.iterator(). This patch fixed the bug.

Signed-off-by: Javeme [email protected]

[javeme]

NOTE: The following is a test about foreach with int[]/ArrayList, and the test results are expected(the second for-loop is also executed correctly):

import java.util.ArrayList;

public class TestForeach {

	public static void main(String[] args) {
		
		// test foreach twice with int[]
		int list1[] = new int[]{1, 2};
		
		System.out.println("==== int[] 1");
		for(int i : list1) {
			System.out.println(i);
		}
		
		System.out.println("===int[] 2");
		for(int i : list1) {
			System.out.println(i);
		}
		
		// test foreach twice with ArrayList
		ArrayList<String> list = new ArrayList<String>();
		list.add("1");
		list.add("2");
		Iterable<String> list2 = list;

		System.out.println();
		System.out.println("===ArrayList 1");
		for(String i : list2) {
			System.out.println(i);
		}
		
		System.out.println("===ArrayList 2");
		for(String i : list2) {
			System.out.println(i);
		}
	}

}

[javeme]

According to Daniel Templeton, we think it is expected.