复杂的MapReduce处理中，往往需要将复杂的处理过程，分解成多个简单的Job来执行，第1个Job的输出做为第2个Job的输入，相互之间有一定依赖关系。以上一篇中的求平均数为例，可以分解成三个步骤：

1. 求Sum

2. 求Count

3. 计算平均数

每1个步骤看成一个Job，其中Job3必须等待Job1、Job2完成，并将Job1、Job2的输出结果做为输入，下面的代码演示了如何将这3个Job串起来

代码：

package cn.toto.bigdata.mr.wc;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.DoubleWritable;
import org.apache.hadoop.io.LongWritable;
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.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

public class Avg2 {

	private static final Text TEXT_SUM = new Text("SUM");
	private static final Text TEXT_COUNT = new Text("COUNT");
	private static final Text TEXT_AVG = new Text("AVG");
	
	public static class SumMapper extends Mapper<LongWritable, Text, Text, LongWritable> {
		public long sum = 0;
		
		@Override
		protected void map(LongWritable key, Text value, Context context)
				throws IOException, InterruptedException {
			sum += value.toString().length();
		}
		
		@Override
		protected void cleanup(Mapper<LongWritable, Text, Text, LongWritable>.Context context)
				throws IOException, InterruptedException {
			context.write(TEXT_SUM, new LongWritable(sum));
		}
	}
	
	public static class SumReducer extends Reducer<Text, LongWritable, Text, LongWritable> {
		public long sum = 0;
		
		@Override
		protected void reduce(Text key, Iterable<LongWritable> values,Context context) 
				throws IOException, InterruptedException {
			for (LongWritable v : values) {
				sum += v.get();
			}
			context.write(TEXT_SUM, new LongWritable(sum));
		}
	}
	
	//计算Count
	public static class CountMapper extends Mapper<LongWritable, Text, Text, LongWritable> {
		public long count = 0;
		
		@Override
		protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, Text, LongWritable>.Context context)
				throws IOException, InterruptedException {
			count += 1;
		}
		
		@Override
		protected void cleanup(Context context)
				throws IOException, InterruptedException {
			context.write(TEXT_COUNT, new LongWritable(count));
		}
	}
	
	public static class CountReducer extends Reducer<Text, LongWritable, Text, LongWritable> {
		public long count = 0;
		
		@Override
		public void reduce(Text key, Iterable<LongWritable> values, Context context)
				throws IOException, InterruptedException {
			for (LongWritable v : values) {
				count += v.get();
			}
			context.write(TEXT_COUNT, new LongWritable(count));
		}
	}
	
	//计算Avg 
	public static class AvgMapper extends Mapper<LongWritable, Text, LongWritable, LongWritable> {
		public long count = 0;
		public long sum = 0;
		
		@Override
		protected void map(LongWritable key, Text value,Context context)
				throws IOException, InterruptedException {
			String[] v = value.toString().split("\t");
			if (v[0].equals("COUNT")) {
				count = Long.parseLong(v[1]);
			} else if (v[0].equals("SUM")) {
				sum = Long.parseLong(v[1]);
			}
		}
		
		@Override
		protected void cleanup(Context context) throws IOException, InterruptedException {
			context.write(new LongWritable(sum), new LongWritable(count));
		}
	}
	
	public static class AvgReducer extends Reducer<LongWritable, LongWritable, Text, DoubleWritable> {
		public long sum = 0;
		public long count = 0;
		
		@Override
		protected void reduce(LongWritable key, Iterable<LongWritable> values,Context context)
				throws IOException, InterruptedException {
			sum += key.get();
			for(LongWritable v : values) {
				count += v.get();
			}
		}
		
		@Override
		protected void cleanup(Reducer<LongWritable, LongWritable, Text, DoubleWritable>.Context context)
				throws IOException, InterruptedException {
			context.write(TEXT_AVG, new DoubleWritable(new Double(sum) / count));
		}
	}
	
	public static void main(String[] args) throws Exception {
		Configuration conf = new Configuration();
		
		String inputPath = "E:/wordcount/input/a.txt";
		String maxOutputPath = "E:/wordcount/output/max/";
		String countOutputPath = "E:/wordcount/output/count/";
		String avgOutputPath = "E:/wordcount/output/avg/";
		
		Job job1 = Job.getInstance(conf, "Sum");
		job1.setJarByClass(Avg2.class);
		job1.setMapperClass(SumMapper.class);
		job1.setCombinerClass(SumReducer.class);
		job1.setReducerClass(SumReducer.class);
		job1.setOutputKeyClass(Text.class);
		job1.setOutputValueClass(LongWritable.class);
		FileInputFormat.addInputPath(job1, new Path(inputPath));
		FileOutputFormat.setOutputPath(job1, new Path(maxOutputPath));
		
		Job job2 = Job.getInstance(conf, "Count");
		job2.setJarByClass(Avg2.class);
		job2.setMapperClass(CountMapper.class);
		job2.setCombinerClass(CountReducer.class);
		job2.setReducerClass(CountReducer.class);
		job2.setOutputKeyClass(Text.class);
		job2.setOutputValueClass(LongWritable.class);
		FileInputFormat.addInputPath(job2, new Path(inputPath));
		FileOutputFormat.setOutputPath(job2, new Path(countOutputPath));
		
		Job job3 = Job.getInstance(conf, "Average");
		job3.setJarByClass(Avg2.class);
		job3.setMapperClass(AvgMapper.class);
		job3.setReducerClass(AvgReducer.class);
		job3.setMapOutputKeyClass(LongWritable.class);
		job3.setMapOutputValueClass(LongWritable.class);
		job3.setOutputKeyClass(Text.class);
		job3.setOutputValueClass(DoubleWritable.class);
		
		//将job1及job2的输出为做job3的输入
		FileInputFormat.addInputPath(job3, new Path(maxOutputPath));
		FileInputFormat.addInputPath(job3, new Path(countOutputPath));
		FileOutputFormat.setOutputPath(job3, new Path(avgOutputPath));
		 
		//提交job1及job2,并等待完成
		if (job1.waitForCompletion(true) && job2.waitForCompletion(true)) {
			System.exit(job3.waitForCompletion(true) ? 0 : 1);
		}
	}

}

运行准备：

准备数据文件：

E:/wordcount/input/a.txt

数据文件的内容如下：

运行后：E:\wordcount\output\count\part-r-00000的值如下：

运行后：

E:\wordcount\output\max\part-r-00000的内容如下：

最终的平均值是：E:\wordcount\output\avg\part-r-00000