java - Hadoop:Reduce 没有产生所需的输出，它与 map 输出相同

Question

这是我的 map

 public static class MapClass extends Mapper<LongWritable, Text, Text, Text> {

        public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException{
            String[] fields = value.toString().split(",", -20);
            String country = fields[4];
            String numClaims = fields[8];
            if (numClaims.length() > 0 && !numClaims.startsWith("\"")) {
                context.write(new Text(country), new Text(numClaims + ",1"));
            }
        }
    }

这是我的Reduce

public void reduce(Text key, Iterator<Text> values, Context context) throws IOException, InterruptedException {
            double sum = 0.0;
            int count = 0;

            while (values.hasNext()) {
                String[] fields = values.next().toString().split(",");
                sum += Double.parseDouble(fields[0]);
                count += Integer.parseInt(fields[1]);
            }

            context.write(new Text(key), new DoubleWritable(sum/count));
        }

这是它的配置方式

Job job = new Job(getConf());

            job.setJarByClass(AverageByAttributeUsingCombiner.class);
            job.setJobName("AverageByAttributeUsingCombiner");

            job.setOutputKeyClass(Text.class);
            job.setOutputValueClass(Text.class);

            job.setMapperClass(MapClass.class);
    //        job.setCombinerClass(Combinber.class);
            job.setReducerClass(Reduce.class);

            job.setInputFormatClass(TextInputFormat.class);
            job.setOutputFormatClass(TextOutputFormat.class);

            FileInputFormat.setInputPaths(job, new Path(args[0]));
            FileOutputFormat.setOutputPath(job, new Path(args[1]));

    //        job.setNumReduceTasks(0); // to not run the reducer
            boolean success = job.waitForCompletion(true);
            return success ? 0 : 1;

输入的形式是

   "PATENT","GYEAR","GDATE","APPYEAR","COUNTRY","POSTATE","ASSIGNEE","ASSCODE","CLAIMS","NCLASS","CAT","SUBCAT","CMADE","CRECEIVE","RATIOCIT","GENERAL","ORIGINAL","FWDAPLAG","BCKGTLAG","SELFCTUB","SELFCTLB","SECDUPBD│                                                                                                                                                                                                                
    ","SECDLWBD"                                                                                                                                                                                                         │                                                                                                                                                                                                                
    3070801,1963,1096,,"BE","",,1,,269,6,69,,1,,0,,,,,,,                                                                                                                                                                 │                                                                                                                                                                                                                
    3070802,1963,1096,,"US","TX",,1,,2,6,63,,0,,,,,,,,,                                                                                                                                                                  │                                                                                                                                                                                                                
    3070803,1963,1096,,"US","IL",,1,,2,6,63,,9,,0.3704,,,,,,,                                                                                                                                                            │                                                                                                                                                                                                                
    3070804,1963,1096,,"US","OH",,1,,2,6,63,,3,,0.6667,,,,,,,        

整个map reduce的输出看起来像

“AR”5,1 │
"AR"9,1 │
"AR"2,1 │
"AR"15,1 │
"AR"13,1 │
"AR"1,1 │
"AR"34,1 │
"AR"12,1 │
"AR"8,1 │
"AR"7,1 │
"AR"23,1 │
"AR"3,1 │
"AR"4,1 │
“AR”4,1

我该如何调试和解决这个问题？我正在学习hadoop

Answer 1

如前所述，问题在于您没有覆盖默认抽象 Reducer 类的默认 reduce 方法。

更具体地说，到目前为止的(一个/那个)问题是您的 reduce 方法签名是:

 public void reduce(Text key, **Iterator**<Text> values, Context context) 
             throws IOException, InterruptedException

相反，它应该是:

 public void reduce(Text key, **Iterable**<Text> values, Context context) 
             throws IOException, InterruptedException

旧的 API 版本是正确的，您实现 Reducer 接口(interface) reduce() 方法并且它可以工作。

对这种情况的一个很好的验证是使用 @Override，因为它强制编译时检查签名不匹配。