MR案例：输出/输入SequenceFile

SequenceFile文件是Hadoop用来存储二进制形式的key-value对而设计的一种平面文件(Flat File)。在SequenceFile文件中，每一个key-value对被看做是一条记录(Record)，基于Record的压缩策略，SequenceFile文件支持三种压缩类型：

NONE: 对records不进行压缩; (组合1)

RECORD: 仅压缩每一个record中的value值(不包括key); (组合2)

BLOCK: 将一个block中的所有records(包括key)压缩在一起;(组合3)

package test0820;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.SequenceFile.CompressionType;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.VLongWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.SequenceFileOutputFormat;

public class Test0829 {

    public static void main(String[] args) throws Exception {        
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf);
        job.setJarByClass(Test0829.class);

        job.setMapperClass(WCMapper.class);
        job.setReducerClass(WCReducer.class);

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

        // 设置输出类
        job.setOutputFormatClass(SequenceFileOutputFormat.class);

        /**
         * 设置sequecnfile的格式，对于sequencefile的输出格式，有多种组合方式,
         * 从下面的模式中选择一种，并将其余的注释掉
         */

        // 组合方式1：不压缩模式
        SequenceFileOutputFormat.setOutputCompressionType(job, CompressionType.NONE);

        //组合方式2：record压缩模式，并指定采用的压缩方式 ：默认、gzip压缩等
        //        SequenceFileOutputFormat.setOutputCompressionType(job,
        //                CompressionType.RECORD);
        //        SequenceFileOutputFormat.setOutputCompressorClass(job,
        //                DefaultCodec.class);


        //组合方式3：block压缩模式，并指定采用的压缩方式 ：默认、gzip压缩等
        //        SequenceFileOutputFormat.setOutputCompressionType(job,
        //                CompressionType.BLOCK);
        //        SequenceFileOutputFormat.setOutputCompressorClass(job,
        //                DefaultCodec.class);

        FileInputFormat.addInputPaths(job, args[0]);
        SequenceFileOutputFormat.setOutputPath(job, new Path(args[1]));

        job.waitForCompletion(true);
    }
    //map
    public static class WCMapper extends
    Mapper<LongWritable, Text, Text, VLongWritable> {
        public void map(LongWritable key, Text value, Context context)
                throws IOException, InterruptedException {
            String[] split = value.toString().split(":",2);
            if(split.length!=1){
                String[] splited = split[1].split(",");
                for(String s : splited){
                    context.write(new Text(s), new VLongWritable(1L));
                }            
            }
        }
    }
    //reduce
    public static class WCReducer extends Reducer<Text, VLongWritable, Text, VLongWritable>{
        @Override
        protected void reduce(Text key, Iterable<VLongWritable> v2s, Context context)
                throws IOException, InterruptedException {

            long sum=0;

            for(VLongWritable vl : v2s){
                sum += vl.get();                 
            }
            context.write(key, new VLongWritable(sum));
        }
    }
}

MR输入SequenceFile

当输入文件格式是SequenceFile的时候，要使用SequenceFileInputformat类。由于SequenceFile都是以key和value的二进制形式存放的(注意hadoop类型的二进制的解释方式和原始二进制不一样，会多一些维护信息)，所以在读取SequenceFile文件时必须预先知道key和value对应的hadoop类型。

对于上面代码产生的SequenceFile结果文件，以SequenceFileInputformat类进行读取。其中key为Text类型，value为VLongWritable类型。

package test0820;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.VLongWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.SequenceFileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;


public class SFInput02 {
    public static void main(String[] args) throws Exception {
        Job job = Job.getInstance(new Configuration());
        job.setJarByClass(SFinput.class);

        job.setMapperClass(SFMapper.class);
        job.setReducerClass(SFReducer.class);

        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(VLongWritable.class);

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

        job.setInputFormatClass(SequenceFileInputFormat.class);

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

        job.waitForCompletion(true);
    }
    public static class SFMapper extends Mapper<Text, VLongWritable,Text, VLongWritable> {
        public void map(Text key, VLongWritable value, Context context)
                throws IOException, InterruptedException {
            context.write(key, value);
        }

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

如若不清楚SequenceFile文件中key和value的类型，可以使用SequenceFileAsTextInputFormat类。它将SequenceFile的key和value都转化成Text对象传入map中。  

package test0820;

import java.io.IOException;

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.Reducer;
import org.apache.hadoop.mapreduce.lib.input.SequenceFileAsTextInputFormat;
import org.apache.hadoop.mapreduce.lib.input.SequenceFileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;


public class SFinput {
    public static void main(String[] args) throws Exception {
        Job job = Job.getInstance(new Configuration());
        job.setJarByClass(SFinput.class);

        job.setMapperClass(SFMapper.class);
        job.setReducerClass(SFReducer.class);

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

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

        job.setInputFormatClass(SequenceFileAsTextInputFormat.class);

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

        job.waitForCompletion(true);
    }
    public static class SFMapper extends Mapper<Text, Text,Text, Text> {
        public void map(Text key, Text value, Context context)
                throws IOException, InterruptedException {
            context.write(key, value);
        }

    }
    //reduce
    public static class SFReducer 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);
            }
        }
    }
}

最后还有一种sequencefileAsBinaryInputFormat 类，它将SequenceFile中的key和value都以原始二进制的形式封装在byteswritable对象中传给map，如何对二进制数据进行解释是map函数编写者的工作。