定制Partitioner

用户可以继承Partitioner基类，也可以继承默认的HashPartitioner类，覆写其中的getPartition()方法实现自己的分区。

需求：本例是对上一个实例的改写，需求不变

package country;

import java.io.IOException;
import java.io.UnsupportedEncodingException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
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.KeyValueLineRecordReader;
import org.apache.hadoop.mapreduce.lib.input.KeyValueTextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

public class Multiples {

    public static void main(String[] args) throws Exception {
        /**
         * 【严重注意】
         * 有分区的例子，必须达成java包在集群上运行
         * 这是因为，eclipse其实是在本地模式运行。所以只能有一个reduce
         */
        //本地模式，使用eclipse测试用的环境变量配置！
        //System.setProperty("hadoop.home.dir", "F:\\JAVA\\hadoop-2.2.0");

        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        job.setJarByClass(Multiples.class);        

        /**
         * 使用KeyValueTextInputFormat作为输入类型
         */
        job.setInputFormatClass(KeyValueTextInputFormat.class);

        /**
         * 指定key和Value的分隔符【默认也是\t】
         */
        conf.set(KeyValueLineRecordReader.KEY_VALUE_SEPERATOR, "\t"); 

        job.setMapperClass(MyMapper.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(Text.class);

        //指定自定义的分区类
        job.setPartitionerClass(MyPartitioner.class);

        job.setReducerClass(MyReducer.class);
        job.setNumReduceTasks(3);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);        

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

        job.waitForCompletion(true);
    }

    /**
     * map阶段
     */
    public static class MyMapper extends Mapper<Text, Text, Text, Text>{
        @Override
        protected void map(Text key, Text value, Context context)
                throws IOException, InterruptedException {

            context.write(key, value);
        }
    }
    /**
     * 分区函数
     */
    public static class MyPartitioner extends Partitioner<Text, Text>{
        @Override
        public int getPartition(Text key, Text value, int numPartitions) {

            //生成以utf-8方式编码的汉字
            String line = null;
            try {
                line = new String(key.getBytes(),0,key.getLength(),"utf-8");
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }

            if(line.equals("中国")){
                return 0;
            }else if (line.equals("美国")) {
                return 1;
            }else
                return 2;
        }
    }

    /**
     * reduce阶段
     */
    public static class MyReducer extends Reducer<Text, Text, Text, Text>{
        @Override
        protected void reduce(Text key, Iterable<Text> v2s, Context context)
                throws IOException, InterruptedException {
            for(Text text : v2s){
                context.write(key, text);
            }
        }
    }
}