INFO mapreduce.Job：任务ID：attempt_ *，状态：失败

Question

我正在尝试实现共同的朋友代码。在进入reducer后成功运行此代码的映射器后，它会抛出错误而无法产生所需的输出。

我有以下驱动程序类：

public class MutualFriends {
public static void main(String args[]) throws Exception {
        // Standard Job setup procedure.
        Configuration conf = new Configuration();

    long milliseconds = 1000*60*60;
    conf.setLong("mapreduce.task.timeout",milliseconds);

        Job job = new Job(conf, "Mutual Friends");
        job.setJarByClass(MutualFriends.class);
        job.setMapperClass(FriendsMapper.class);
        //job.setReducerClass(FriendsReducer.class);
    //job.setCombinerClass(FriendsReducer.class);
    job.setReducerClass(FriendsReducer.class);        
    job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        System.exit(job.waitForCompletion(true) ? 0 : 1);
    }

映射

 public static class FriendsMapper
            extends Mapper<Object, Text, Text, Text> {
        private Text m_id = new Text();
        private Text m_others = new Text();

        public void map(Object key, Text value, Context context)
                throws IOException, InterruptedException {
            // In our case, the key is null and the value is one line of our input file.
            // Split by space to separate the user and its friends list.
            String line = value.toString();
            String[] split = line.split(" ");
            String subject = split[0];
            String[] friends = Arrays.copyOfRange(split, 1, split.length);

            // For each friend in the list, output the (UserFriend, ListOfFriends) pair
            for(String friend : friends) {
                String others = line.replace(subject, "").replace(" ", "");
                String id = subject.compareTo(friend) < 0 ? subject+friend : friend+subject;
                m_id.set(id);
                m_others.set(others);
                context.write(m_id, m_others);
            }
        }
    }

减速机：

public static class FriendsReducer
            extends Reducer<Text, Text, Text, Text> {
        private Text m_result = new Text();

        // Calculates intersection of two given Strings, i.e. friends lists
        public String intersection(String s1, String s2) {
            HashSet<Character> h1 = new HashSet<Character>();
            HashSet<Character> h2 = new HashSet<Character>();

            for(int i = 0; i < s1.length(); i++) {
                h1.add(s1.charAt(i));
            }
            for(int j = 0; j < s2.length(); j++) {
                h2.add(s2.charAt(j));
            }

            h1.retainAll(h2);
            Character[] res = h1.toArray(new Character[0]);
            String intersect = new String("");
            for (int i = 0; i < res.length; i++) {
                intersect += res[i];
            }

            char[] letters = intersect.toCharArray();
            Arrays.sort(letters);
            String sortedIntersect = new String(letters);
            return sortedIntersect;
        }

        public void reduce(Text key, Iterable<Text> values, Context context)
                throws IOException, InterruptedException {
            // Prepare a 2-String-Array to hold the values, i.e. the friends lists of
            // our current friends pair.
            String[] combined = new String[2];
            int cur = 0;
            for(Text value : values) {
                combined[cur++] = value.toString();
            }

            // Calculate the intersection of these lists and write result in the form (UserAUserB, MutualFriends).
            m_result.set(intersection(combined[0], combined[1]));
            context.write(key, m_result);
        }
    }

错误：

15/09/22 16:27:29 INFO mapreduce.Job: Task Id : attempt_1442915698147_0006_r_000000_0, Status : FAILED
Error: java.lang.NullPointerException
    at MutualFriends$FriendsReducer.intersection(MutualFriends.java:54)
    at MutualFriends$FriendsReducer.reduce(MutualFriends.java:82)
    at MutualFriends$FriendsReducer.reduce(MutualFriends.java:42)
    at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:171)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:627)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:389)
    at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:163)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:415)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1628)
    at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)

目前M使用： Ubuntu 14-04 Hadoop 2.6.0