Question

我正在尝试使用新API（0.20.2）编写简单的map reduce程序来查找最大的素数。这就是我的Map和reduce类的样子......

public class PrimeNumberMap extends Mapper<LongWritable, Text, IntWritable, IntWritable> {

public void map (LongWritable key, Text Kvalue,Context context) throws IOException,InterruptedException
{
    Integer value = new Integer(Kvalue.toString());
    if(isNumberPrime(value))
    {
            context.write(new IntWritable(value), new IntWritable(new Integer(key.toString())));
    }
}

boolean isNumberPrime(Integer number)
{
    if (number == 1) return false;
     if (number == 2) return true;

     for (int counter =2; counter<(number/2);counter++)
     {
         if(number%counter ==0 )
             return false;
     }
            return true;

}
}
public class PrimeNumberReduce extends Reducer<IntWritable, IntWritable, IntWritable, IntWritable>  {

public void reduce ( IntWritable primeNo, Iterable<IntWritable> Values,Context context) throws IOException ,InterruptedException
{
    int maxValue = Integer.MIN_VALUE;
    for (IntWritable value : Values)
    {
        maxValue=  Math.max(maxValue, value.get());
    }
    //output.collect(primeNo, new IntWritable(maxValue));
    context.write(primeNo, new IntWritable(maxValue));  }

}  

 public static void main(String[] args)  throws IOException, InterruptedException, ClassNotFoundException{

    if (args.length ==0)
    {
        System.err.println(" Usage:\n\tPrimenumber <input Directory> <output Directory>");
        System.exit(-1);
    }
    Job job = new Job();
    job.setJarByClass(Main.class);

    job.setJobName("Prime");
    // Creating job configuration object

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

    job.setMapOutputKeyClass(IntWritable.class);
    job.setMapOutputValueClass(IntWritable.class);

    job.setOutputKeyClass(IntWritable.class);
    job.setOutputValueClass(IntWritable.class);
    String star ="*********************************************";
    System.out.println(star+"\n Prime number computer \n"+star);
    System.out.println(" Application started ... keeping fingers crossed :/ ");
    System.exit(job.waitForCompletion(true)?0:1);

}

}

我仍然收到有关地图密钥不匹配的错误

java.io.IOException：键入map中的键不匹配：expected org.apache.hadoop.io.IntWritable，recieved org.apache.hadoop.io.LongWritable at org.apache.hadoop.mapred.MapTask $ MapOutputBuffer.collect（MapTask.java:1034） at org.apache.hadoop.mapred.MapTask $ NewOutputCollector.write（MapTask.java:595） at org.apache.hadoop.mapreduce.TaskInputOutputContext.write（TaskInputOutputContext.java:80）在org.apache.hadoop.mapreduce.Mapper.map（Mapper.java:124）在org.apache.hadoop.mapreduce.Mapper.run（Mapper.java:144） at org.apache.hadoop.mapred.MapTask.runNewMapper（MapTask.java:668）在org.apache.hadoop.mapred.MapTask.run（MapTask.java:334）在org.apache.hadoop.mapred.Child $ 4.run（Child.java:270） at java.security.AccessController.doPrivileged（Native Method）在javax.security.auth.Subject.doAs（Subject.java:396）在org.apache.hadoop.security.UserGroupInformation.doAs（UserGroupInformation.java:1109）在org.apache.hadoop.mapred.Child.main（Child.java:264） 2012-06-13 14：27：21,116 INFO org.apache.hadoop.mapred.Task：Runnning cleanup for the task

有人可以建议出现什么问题。我试过所有的钩子和骗子。

Answer 1

你没有在主块中配置Mapper或reducer类，所以使用了默认的Mapper - 它被称为身份映射器 - 它输出的每一对都是输出（因此LongWritable作为输出键））：

job.setMapperClass(PrimeNumberMap.class);
job.setReducerClass(PrimeNumberReduce.class);

Answer 2

映射器应定义如下，

public class PrimeNumberMap extends Mapper<**IntWritable**, Text, IntWritable, IntWritable> {

而不是

public class PrimeNumberMap extends Mapper<LongWritable, Text, IntWritable, IntWritable> {

正如在评论中提到的那样，你应该定义mapper和reducer。

job.setMapperClass(PrimeNumberMap.class);
job.setReducerClass(PrimeNumberReduce.class);

请参阅Hadoop权威指南第3版，第2章，第24页

Answer 3

我是hadoop mapreduce计划的新手。

映射时，我使用IntWritable但我减少了IntWritable格式的值，并在上下文写入中使用DoubleWritable之前将结果转换为double。

运行时失败。

我在map中处理covert int的方法是double in double：

Mapper(LongWritable,Text,Text,DoubleWritable)
Reducer(Text,DoubleWritable,Text,DoubleWritable)
job.setOutputValueClass(DoubleWritable.Class)

“键入地图中的键不匹配：期望org.apache.hadoop.io.IntWritable，收到org.apache.hadoop.io.LongWritable” - 每件事看起来都是正确的

3 个答案: