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在Map阶段，使用job.setInputFormatClass定义的InputFormat将输入的数据集分割成小数据块splites，同时InputFormat提供一个RecordReader的实现。本实验中使用的是TextInputFormat，他提供的RecordReader会将文本的字节偏移量作为key，这一行的文本作为value。这就是自定义Map的输入是<LongWritable, Text>的原因。然后调用自定义Map的map方法，将一个个<LongWritable, Text>键值对输入给Map的map方法。注意输出应该符合自定义Map中定义的输出<IntPair, IntWritable>。最终生成一个List<IntPair, IntWritable>。在map阶段的最后，会先调用job.setPartitionerClass对这个List进行分区，每个分区映射到一个reducer。每个分区内又调用job.setSortComparatorClass设置的key比较函数类排序。可以看到，这本身就是一个二次排序。 如果没有通过job.setSortComparatorClass设置key比较函数类，则可以使用key实现的compareTo方法进行排序。 在本实验中，就使用了IntPair实现的compareTo方法。
在Reduce阶段，reducer接收到所有映射到这个reducer的map输出后，也是会调用job.setSortComparatorClass设置的key比较函数类对所有数据对排序。然后开始构造一个key对应的value迭代器。这时就要用到分组，使用job.setGroupingComparatorClass设置的分组函数类。只要这个比较器比较的两个key相同，他们就属于同一个组，它们的value放在一个value迭代器，而这个迭代器的key使用属于同一个组的所有key的第一个key。最后就是进入Reducer的reduce方法，reduce方法的输入是所有的（key和它的value迭代器）。同样注意输入与输出的类型必须与自定义的Reducer中声明的一致。
二次排序：在mapreduce中，所有的key是需要被比较和排序的，并且是二次，先根据partitioner，再根据大小。而本例中也是要比较两次。先按照第一字段排序，然后在第一字段相同时按照第二字段排序。根据这一点，我们可以构造一个复合类IntPair，他有两个字段，先利用分区对第一字段排序，再利用分区内的比较对第二字段排序。Java代码主要分为四部分：自定义key，自定义分区函数类，map部分，reduce部分。
public static class IntPair implements WritableComparable<IntPair>
{
int first; //第一个成员变量
int second; //第二个成员变量
public void set(int left, int right)
{
first = left;
second = right;
}
public int getFirst()
{
return first;
}
public int getSecond()
{
return second;
}
@Override
//反序列化，从流中的二进制转换成IntPair
public void readFields(DataInput in) throws IOException
{
// TODO Auto-generated method stub
first = in.readInt();
second = in.readInt();
}
@Override
//序列化，将IntPair转化成使用流传送的二进制
public void write(DataOutput out) throws IOException
{
// TODO Auto-generated method stub
out.writeInt(first);
out.writeInt(second);
}
@Override
//key的比较
public int compareTo(IntPair o)
{
// TODO Auto-generated method stub
if (first != o.first)
{
return first < o.first ? 1 : -1;
}
else if (second != o.second)
{
return second < o.second ? -1 : 1;
}
else
{
return 0;
}
}
@Override
public int hashCode()
{
return first * 157 + second;
}
@Override
public boolean equals(Object right)
{
if (right == null)
return false;
if (this == right)
return true;
if (right instanceof IntPair)
{
IntPair r = (IntPair) right;
return r.first == first && r.second == second;
}
else
{
return false;
}
}
}
所有自定义的key应该实现接口WritableComparable，因为是可序列的并且可比较的，并重载方法。该类中包含以下几种方法：
1.反序列化，从流中的二进制转换成IntPair 方法为public void readFields(DataInput in) throws IOException
2.序列化，将IntPair转化成使用流传送的二进制 方法为public void write(DataOutput out)
3. key的比较 public int compareTo(IntPair o) 另外新定义的类应该重写的两个方法 public int hashCode() 和public boolean equals(Object right) 。
public static class FirstPartitioner extends Partitioner<IntPair, IntWritable>
{
@Override
public int getPartition(IntPair key, IntWritable value,int numPartitions)
{
return Math.abs(key.getFirst() * 127) % numPartitions;
}
}
对key进行分区，根据自定义key中first乘以127取绝对值再对numPartions取余来进行分区。这主要是为实现第一次排序。
public static class GroupingComparator extends WritableComparator
{
protected GroupingComparator()
{
super(IntPair.class, true);
}
@Override
//Compare two WritableComparables.
public int compare(WritableComparable w1, WritableComparable w2)
{
IntPair ip1 = (IntPair) w1;
IntPair ip2 = (IntPair) w2;
int l = ip1.getFirst();
int r = ip2.getFirst();
return l == r ? 0 : (l < r ? -1 : 1);
}
}
分组函数类。在reduce阶段，构造一个key对应的value迭代器的时候，只要first相同就属于同一个组，放在一个value迭代器。这是一个比较器，需要继承WritableComparator。
public static class Map extends Mapper<LongWritable, Text, IntPair, IntWritable>
{
//自定义map
private final IntPair intkey = new IntPair();
private final IntWritable intvalue = new IntWritable();
public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException
{
String line = value.toString();
StringTokenizer tokenizer = new StringTokenizer(line);
int left = 0;
int right = 0;
if (tokenizer.hasMoreTokens())
{
left = Integer.parseInt(tokenizer.nextToken());
if (tokenizer.hasMoreTokens())
right = Integer.parseInt(tokenizer.nextToken());
intkey.set(right, left);
intvalue.set(left);
context.write(intkey, intvalue);
}
}
}
public static class Reduce extends Reducer<IntPair, IntWritable, Text, IntWritable>
{
private final Text left = new Text();
private static final Text SEPARATOR = new Text("------------------------------------------------");
public void reduce(IntPair key, Iterable<IntWritable> values,Context context) throws IOException, InterruptedException
{
context.write(SEPARATOR, null);
left.set(Integer.toString(key.getFirst()));
System.out.println(left);
for (IntWritable val : values)
{
context.write(left, val);
//System.out.println(val);
}
}
}
package mapreduce;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.util.StringTokenizer;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Partitioner;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
public class SecondarySort
{
public static class IntPair implements WritableComparable<IntPair>
{
int first;
int second;
public void set(int left, int right)
{
first = left;
second = right;
}
public int getFirst()
{
return first;
}
public int getSecond()
{
return second;
}
@Override
public void readFields(DataInput in) throws IOException
{
// TODO Auto-generated method stub
first = in.readInt();
second = in.readInt();
}
@Override
public void write(DataOutput out) throws IOException
{
// TODO Auto-generated method stub
out.writeInt(first);
out.writeInt(second);
}
@Override
public int compareTo(IntPair o)
{
// TODO Auto-generated method stub
if (first != o.first)
{
return first < o.first ? 1 : -1;
}
else if (second != o.second)
{
return second < o.second ? -1 : 1;
}
else
{
return 0;
}
}
@Override
public int hashCode()
{
return first * 157 + second;
}
@Override
public boolean equals(Object right)
{
if (right == null)
return false;
if (this == right)
return true;
if (right instanceof IntPair)
{
IntPair r = (IntPair) right;
return r.first == first && r.second == second;
}
else
{
return false;
}
}
}
public static class FirstPartitioner extends Partitioner<IntPair, IntWritable>
{
@Override
public int getPartition(IntPair key, IntWritable value,int numPartitions)
{
return Math.abs(key.getFirst() * 127) % numPartitions;
}
}
public static class GroupingComparator extends WritableComparator
{
protected GroupingComparator()
{
super(IntPair.class, true);
}
@Override
//Compare two WritableComparables.
public int compare(WritableComparable w1, WritableComparable w2)
{
IntPair ip1 = (IntPair) w1;
IntPair ip2 = (IntPair) w2;
int l = ip1.getFirst();
int r = ip2.getFirst();
return l == r ? 0 : (l < r ? -1 : 1);
}
}
public static class Map extends Mapper<LongWritable, Text, IntPair, IntWritable>
{
private final IntPair intkey = new IntPair();
private final IntWritable intvalue = new IntWritable();
public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException
{
String line = value.toString();
StringTokenizer tokenizer = new StringTokenizer(line);
int left = 0;
int right = 0;
if (tokenizer.hasMoreTokens())
{
left = Integer.parseInt(tokenizer.nextToken());
if (tokenizer.hasMoreTokens())
right = Integer.parseInt(tokenizer.nextToken());
intkey.set(right, left);
intvalue.set(left);
context.write(intkey, intvalue);
}
}
}
public static class Reduce extends Reducer<IntPair, IntWritable, Text, IntWritable>
{
private final Text left = new Text();
private static final Text SEPARATOR = new Text("------------------------------------------------");
public void reduce(IntPair key, Iterable<IntWritable> values,Context context) throws IOException, InterruptedException
{
context.write(SEPARATOR, null);
left.set(Integer.toString(key.getFirst()));
System.out.println(left);
for (IntWritable val : values)
{
context.write(left, val);
//System.out.println(val);
}
}
}
public static void main(String[] args) throws IOException, InterruptedException, ClassNotFoundException
{
Configuration conf = new Configuration();
Job job = new Job(conf, "secondarysort");
job.setJarByClass(SecondarySort.class);
job.setMapperClass(Map.class);
job.setReducerClass(Reduce.class);
job.setPartitionerClass(FirstPartitioner.class);
job.setGroupingComparatorClass(GroupingComparator.class);
job.setMapOutputKeyClass(IntPair.class);
job.setMapOutputValueClass(IntWritable.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
job.setInputFormatClass(TextInputFormat.class);
job.setOutputFormatClass(TextOutputFormat.class);
String[] otherArgs=new String[2];
otherArgs[0]="hdfs://localhost:9000/mymapreduce8/in/goods_visit2";
otherArgs[1]="hdfs://localhost:9000/mymapreduce8/out";
FileInputFormat.setInputPaths(job, new Path(otherArgs[0]));
FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
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