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
1 package yjmyzz.mr.job.link;
2
3 import org.apache.hadoop.conf.Configuration;
4 import org.apache.hadoop.fs.Path;
5 import org.apache.hadoop.io.DoubleWritable;
6 import org.apache.hadoop.io.LongWritable;
7 import org.apache.hadoop.io.Text;
8 import org.apache.hadoop.mapreduce.Job;
9 import org.apache.hadoop.mapreduce.Mapper;
10 import org.apache.hadoop.mapreduce.Reducer;
11 import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
12 import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
13 import yjmyzz.util.HDFSUtil;
14
15 import java.io.IOException;
16
17
18 public class Avg2 {
19
20 private static final Text TEXT_SUM = new Text("SUM");
21 private static final Text TEXT_COUNT = new Text("COUNT");
22 private static final Text TEXT_AVG = new Text("AVG");
23
24 //计算Sum
25 public static class SumMapper
26 extends Mapper<LongWritable, Text, Text, LongWritable> {
27
28 public long sum = 0;
29
30 public void map(LongWritable key, Text value, Context context)
31 throws IOException, InterruptedException {
32 sum += Long.parseLong(value.toString());
33 }
34
35 protected void cleanup(Context context) throws IOException, InterruptedException {
36 context.write(TEXT_SUM, new LongWritable(sum));
37 }
38
39 }
40
41 public static class SumReducer extends Reducer<Text, LongWritable, Text, LongWritable> {
42
43 public long sum = 0;
44
45 public void reduce(Text key, Iterable<LongWritable> values, Context context)
46 throws IOException, InterruptedException {
47 for (LongWritable v : values) {
48 sum += v.get();
49 }
50 context.write(TEXT_SUM, new LongWritable(sum));
51 }
52
53 }
54
55 //计算Count
56 public static class CountMapper
57 extends Mapper<LongWritable, Text, Text, LongWritable> {
58
59 public long count = 0;
60
61 public void map(LongWritable key, Text value, Context context)
62 throws IOException, InterruptedException {
63 count += 1;
64 }
65
66 protected void cleanup(Context context) throws IOException, InterruptedException {
67 context.write(TEXT_COUNT, new LongWritable(count));
68 }
69
70 }
71
72 public static class CountReducer extends Reducer<Text, LongWritable, Text, LongWritable> {
73
74 public long count = 0;
75
76 public void reduce(Text key, Iterable<LongWritable> values, Context context)
77 throws IOException, InterruptedException {
78 for (LongWritable v : values) {
79 count += v.get();
80 }
81 context.write(TEXT_COUNT, new LongWritable(count));
82 }
83
84 }
85
86 //计算Avg
87 public static class AvgMapper
88 extends Mapper<LongWritable, Text, LongWritable, LongWritable> {
89
90 public long count = 0;
91 public long sum = 0;
92
93 public void map(LongWritable key, Text value, Context context)
94 throws IOException, InterruptedException {
95 String[] v = value.toString().split("\t");
96 if (v[0].equals("COUNT")) {
97 count = Long.parseLong(v[1]);
98 } else if (v[0].equals("SUM")) {
99 sum = Long.parseLong(v[1]);
100 }
101 }
102
103 protected void cleanup(Context context) throws IOException, InterruptedException {
104 context.write(new LongWritable(sum), new LongWritable(count));
105 }
106
107 }
108
109
110 public static class AvgReducer extends Reducer<LongWritable, LongWritable, Text, DoubleWritable> {
111
112 public long sum = 0;
113 public long count = 0;
114
115 public void reduce(LongWritable key, Iterable<LongWritable> values, Context context)
116 throws IOException, InterruptedException {
117 sum += key.get();
118 for (LongWritable v : values) {
119 count += v.get();
120 }
121 }
122
123 protected void cleanup(Context context) throws IOException, InterruptedException {
124 context.write(TEXT_AVG, new DoubleWritable(new Double(sum) / count));
125 }
126
127 }
128
129
130 public static void main(String[] args) throws Exception {
131
132 Configuration conf = new Configuration();
133
134 String inputPath = "/input/duplicate.txt";
135 String maxOutputPath = "/output/max/";
136 String countOutputPath = "/output/count/";
137 String avgOutputPath = "/output/avg/";
138
139 //删除输出目录(可选,省得多次运行时,总是报OUTPUT目录已存在)
140 HDFSUtil.deleteFile(conf, maxOutputPath);
141 HDFSUtil.deleteFile(conf, countOutputPath);
142 HDFSUtil.deleteFile(conf, avgOutputPath);
143
144 Job job1 = Job.getInstance(conf, "Sum");
145 job1.setJarByClass(Avg2.class);
146 job1.setMapperClass(SumMapper.class);
147 job1.setCombinerClass(SumReducer.class);
148 job1.setReducerClass(SumReducer.class);
149 job1.setOutputKeyClass(Text.class);
150 job1.setOutputValueClass(LongWritable.class);
151 FileInputFormat.addInputPath(job1, new Path(inputPath));
152 FileOutputFormat.setOutputPath(job1, new Path(maxOutputPath));
153
154
155 Job job2 = Job.getInstance(conf, "Count");
156 job2.setJarByClass(Avg2.class);
157 job2.setMapperClass(CountMapper.class);
158 job2.setCombinerClass(CountReducer.class);
159 job2.setReducerClass(CountReducer.class);
160 job2.setOutputKeyClass(Text.class);
161 job2.setOutputValueClass(LongWritable.class);
162 FileInputFormat.addInputPath(job2, new Path(inputPath));
163 FileOutputFormat.setOutputPath(job2, new Path(countOutputPath));
164
165
166 Job job3 = Job.getInstance(conf, "Average");
167 job3.setJarByClass(Avg2.class);
168 job3.setMapperClass(AvgMapper.class);
169 job3.setReducerClass(AvgReducer.class);
170 job3.setMapOutputKeyClass(LongWritable.class);
171 job3.setMapOutputValueClass(LongWritable.class);
172 job3.setOutputKeyClass(Text.class);
173 job3.setOutputValueClass(DoubleWritable.class);
174
175 //将job1及job2的输出为做job3的输入
176 FileInputFormat.addInputPath(job3, new Path(maxOutputPath));
177 FileInputFormat.addInputPath(job3, new Path(countOutputPath));
178 FileOutputFormat.setOutputPath(job3, new Path(avgOutputPath));
179
180 //提交job1及job2,并等待完成
181 if (job1.waitForCompletion(true) && job2.waitForCompletion(true)) {
182 System.exit(job3.waitForCompletion(true) ? 0 : 1);
183 }
184
185 }
186
187
188 }输入文本在上一篇可以找到，上面这段代码的主要思路：
1. Sum和Count均采用相同的输入/input/duplicate.txt，然后将各自的处理结果分别输出到/output/max/及/output/count/下
2. Avg从/output/max及/output/count获取结果做为输入，然后根据Key值不同，拿到sum和count的值，最终计算并输出到/output/avg/下
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