Hadoop：java.lang.ClassCastException：org.apache.hadoop.io.LongWritable 无法转换为 org.apache.hadoop.io.Text

我的程序看起来像

public class TopKRecord extends Configured implements Tool {

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

        public void map(Text key, Text value, Context context) throws IOException, InterruptedException {
            // your map code goes here
            String[] fields = value.toString().split(",");
            String year = fields[1];
            String claims = fields[8];

            if (claims.length() > 0 && (!claims.startsWith("\""))) {
                context.write(new Text(year.toString()), new Text(claims.toString()));
            }
        }
    }
   public int run(String args[]) throws Exception {
        Job job = new Job();
        job.setJarByClass(TopKRecord.class);

        job.setMapperClass(MapClass.class);

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

        job.setJobName("TopKRecord");
        job.setMapOutputValueClass(Text.class);
        job.setNumReduceTasks(0);
        boolean success = job.waitForCompletion(true);
        return success ? 0 : 1;
    }

    public static void main(String args[]) throws Exception {
        int ret = ToolRunner.run(new TopKRecord(), args);
        System.exit(ret);
    }
}

数据看起来像

"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,,,,,,,

运行该程序时，我在控制台上看到以下内容

12/08/02 12:43:34 INFO mapred.JobClient: Task Id : attempt_201208021025_0007_m_000000_0, Status : FAILED
java.lang.ClassCastException: org.apache.hadoop.io.LongWritable cannot be cast to org.apache.hadoop.io.Text
    at com.hadoop.programs.TopKRecord$MapClass.map(TopKRecord.java:26)
    at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:144)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:764)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:370)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1121)
    at org.apache.hadoop.mapred.Child.main(Child.java:249)

我相信类类型已正确映射，类映射器 http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapreduce/Mapper.html,

请让我知道我在这里做错了什么？

当您使用 M/R 程序读取文件时，映射器的输入键应该是文件中该行的索引，而输入值将为整行。

所以这里发生的事情是你试图将行索引作为Text对象是错误的，你需要一个LongWritable相反，Hadoop 不会抱怨类型。

试试这个：

public class TopKRecord extends Configured implements Tool {

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

        public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
            // your map code goes here
            String[] fields = value.toString().split(",");
            String year = fields[1];
            String claims = fields[8];

            if (claims.length() > 0 && (!claims.startsWith("\""))) {
                context.write(new Text(year.toString()), new Text(claims.toString()));
            }
        }
    }

    ...
}

另外，您可能需要重新考虑代码中的一件事，即您正在创建 2Text您正在处理的每条记录的对象。您应该只在开始时创建这两个对象，然后在映射器中使用set方法。如果您要处理大量数据，这将为您节省大量时间。