二、任务分析

1. 这是一个两个数据集关联的任务。关联有map端join ，和reduce端join。 通常在有一个数据集比较小，可在全部放到内存中时，选择map端join；
2. 当两个数据集都很大,不能把一个数据集合部放到内存中时，使用reduce端join，这时也可以使用Bloom过滤器来增加效率。
3. 本次任务我们使用reduce端join实现。
4. 涉及到的内容有：自定义KEY，自定义GroupingComparator,配置多个Mapper类，配置多个输入路径

5. 三、实现思路

   1. 分别编写两个Mapper：
   2. StudentMapper 用来读取和处理student.txt中的内容。
   3. ScoreMapper 用来读取和处理student_scor.txt中的内容。
   4. 这两个mapper的输入都要进行到JoinReducer的reduce函数中处理。
   5. 两个mapper中输出的相同的学生信息进入到同一次reduce调用中，在其中进行关联，同时又要能标记每一条记录是来自学生信息，还是成绩信息。
   6. 还有有可能同一个学号的数据量有很大，不能在reduce中先存下来再处理，所以把学生信息的value放到前面，成绩信息放到后面。
   7. 就可以在一开始遍历values时就可以取到学生姓名，然后把姓名保存起来，后面遍历成绩信息时直接使用姓名值。
   8. 综上分析以如下方法完成此任务：
   9. 1、定义一个自定义KEY,StuentCustomKey, 属性有学生ID，和信息来源（标记此记录来源于学生信息表，还是成绩表），实现comparaTo方法，确保按学号排升序，按source排降序 （student要排在score前面）
   10. 2、StudentMapper 用来读取和处理student.txt中的内容，输出StuentCustomKey、Text。其中设置StuentCustomKey的source值为“student”
   11. 3、ScoreMapper 用来读取和处理student_scor.txt中的内容,输出StuentCustomKey、Text。其中设置StuentCustomKey的source值为“score”
   12. 4、为了保证相同学号的信息进入同一次reduce调用，要实现一个自定义GroupingComparator，实现中使用sid进行比较。（注意自定义GroupingComparator的构造）

   13. 三、实现代码

       
       
   14. 自定义KEY
       

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       1. package join;
          
       2. 
          
       3. import java.io.DataInput;
          
       4. import java.io.DataOutput;
          
       5. import java.io.IOException;
          
       6. 
          
       7. import org.apache.hadoop.io.WritableComparable;
          
       8. 
          
       9. public class StudentCustomKey implements WritableComparable<StudentCustomKey>{
          
       10.         private String sid;
           
       11.         private String source;
           
       12. 
           
       13.         @Override
           
       14.         public void write(DataOutput out) throws IOException {
           
       15.                 out.writeUTF(sid);
           
       16.                 out.writeUTF(source);
           
       17.         }
           
       18. 
           
       19.         @Override
           
       20.         public void readFields(DataInput in) throws IOException {
           
       21.                 this.sid = in.readUTF();
           
       22.                 this.source = in.readUTF();
           
       23.         }
           
       24. 
           
       25.         @Override
           
       26.         public int compareTo(StudentCustomKey key) {
           
       27.                 int r= this.sid.compareTo(key.sid);
           
       28.                 if(r==0)
           
       29.                         r= -this.source.compareTo(key.source);
           
       30.                 return r;
           
       31.         }
           
       32. 
           
       33.         public String getSid() {
           
       34.                 return sid;
           
       35.         }
           
       36. 
           
       37.         public void setSid(String sid) {
           
       38.                 this.sid = sid;
           
       39.         }
           
       40. 
           
       41.         public String getSource() {
           
       42.                 return source;
           
       43.         }
           
       44. 
           
       45.         public void setSource(String source) {
           
       46.                 this.source = source;
           
       47.         }
           
       48. 
           
       49.         @Override
           
       50.         public String toString(){
           
       51.                 return this.sid+","+this.source;
           
       52.         }
           
       53. 
           
       54. }

       
       自定义GroupingComparator
       
       

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       1. package join;
          
       2. 
          
       3. import org.apache.hadoop.io.WritableComparable;
          
       4. import org.apache.hadoop.io.WritableComparator;
          
       5. 
          
       6. public class GroupComparator extends WritableComparator {
          
       7. 
          
       8.         public GroupComparator(){
          
       9.                 super(StudentCustomKey.class,true); //mark
          
       10.         }
           
       11. 
           
       12.         @Override
           
       13.         public int compare(WritableComparable a, WritableComparable b) {
           
       14.                 StudentCustomKey key1 = (StudentCustomKey)a;
           
       15.                 StudentCustomKey key2 = (StudentCustomKey)b;
           
       16.                 return key1.getSid().compareTo(key2.getSid());
           
       17.         }
           
       18. 
           
       19. }

       
       Job类，包括两个Mapper 和一个Reducer。
       
       

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       1. package join;
          
       2. 
          
       3. import java.io.IOException;
          
       4. 
          
       5. import org.apache.commons.lang.StringUtils;
          
       6. import org.apache.hadoop.conf.Configuration;
          
       7. import org.apache.hadoop.conf.Configured;
          
       8. import org.apache.hadoop.fs.FileSystem;
          
       9. import org.apache.hadoop.fs.Path;
          
       10. import org.apache.hadoop.io.LongWritable;
           
       11. import org.apache.hadoop.io.NullWritable;
           
       12. import org.apache.hadoop.io.Text;
           
       13. import org.apache.hadoop.mapreduce.Job;
           
       14. import org.apache.hadoop.mapreduce.Mapper;
           
       15. import org.apache.hadoop.mapreduce.Mapper.Context;
           
       16. import org.apache.hadoop.mapreduce.Reducer;
           
       17. import org.apache.hadoop.mapreduce.lib.input.MultipleInputs;
           
       18. import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
           
       19. import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
           
       20. import org.apache.hadoop.util.Tool;
           
       21. import org.apache.hadoop.util.ToolRunner;
           
       22. import org.apache.log4j.Logger;
           
       23. 
           
       24. public class ReduceJoinJob extends Configured implements Tool {
           
       25. 
           
       26.         public static class StudentMapper extends Mapper<LongWritable,Text,StudentCustomKey,Text>{
           
       27.                 private StudentCustomKey newKey = new StudentCustomKey();
           
       28.                 @Override
           
       29.                 protected void map(LongWritable key, Text value, Context context)
           
       30.                                 throws IOException, InterruptedException {
           
       31.                         String [] words = StringUtils.split(value.toString(),',');
           
       32.                         newKey.setSid(words[0]);
           
       33.                         newKey.setSource("student");
           
       34.                         context.write(newKey, value);
           
       35.                 }
           
       36. 
           
       37.         }
           
       38. 
           
       39.         public static class ScoreMapper extends Mapper<LongWritable,Text,StudentCustomKey,Text>{
           
       40.                 private StudentCustomKey newKey = new StudentCustomKey();
           
       41.                 @Override
           
       42.                 protected void map(LongWritable key, Text value, Context context)
           
       43.                                 throws IOException, InterruptedException {
           
       44.                         String [] words = StringUtils.split(value.toString(),',');
           
       45.                         newKey.setSid(words[0]);
           
       46.                         newKey.setSource("score");
           
       47.                         context.write(newKey, value);
           
       48.                 }
           
       49.         }
           
       50. 
           
       51.         public static class JoinReducer extends Reducer<StudentCustomKey,Text,NullWritable,Text>{
           
       52.                 Logger log = Logger.getLogger(getClass());
           
       53.                 private Text newValue = new Text();
           
       54.                 @Override
           
       55.                 protected void reduce(StudentCustomKey key, Iterable<Text> values,Context context)
           
       56.                                 throws IOException, InterruptedException {
           
       57.                         String name = "";
           
       58.                         for(Text v : values){
           
       59.                                 if(key.getSource().equals("student")){
           
       60.                                         name = StringUtils.split(v.toString(),',')[1];
           
       61.                                         continue;
           
       62.                                 }
           
       63.                                 newValue.set(name +","+ v.toString());
           
       64.                                 context.write(NullWritable.get(), newValue);
           
       65.                         }
           
       66. 
           
       67.                 }
           
       68. 
           
       69.         }
           
       70.         @Override
           
       71.         public int run(String[] args) throws Exception {
           
       72.                 Job job = Job.getInstance(getConf(),"ReduceJoinJob");
           
       73.                 job.setJarByClass(getClass());
           
       74.                 Configuration conf = job.getConfiguration();
           
       75. 
           
       76.                 MultipleInputs.addInputPath(job, new Path("joinjob/student.txt"), TextInputFormat.class, StudentMapper.class);
           
       77.                 MultipleInputs.addInputPath(job, new Path("joinjob/student_score.txt"), TextInputFormat.class, ScoreMapper.class);
           
       78.                 Path out = new Path("joinjob/output1");
           
       79.                 TextOutputFormat.setOutputPath(job, out);
           
       80.                 FileSystem.get(conf).delete(out, true);
           
       81. 
           
       82.                 job.setOutputFormatClass(TextOutputFormat.class);
           
       83.                 job.setReducerClass(JoinReducer.class);
           
       84. 
           
       85.                 job.setMapOutputKeyClass(StudentCustomKey.class);
           
       86.                 job.setMapOutputValueClass(Text.class);
           
       87.                 job.setOutputKeyClass(NullWritable.class);
           
       88.                 job.setOutputValueClass(Text.class);
           
       89. 
           
       90.                 job.setGroupingComparatorClass(GroupComparator.class);
           
       91.                 return job.waitForCompletion(true)?0:1;
           
       92.         }
           
       93.         public static void main(String [] args){
           
       94.                 int r=0;
           
       95.                 try{
           
       96.                         r= ToolRunner.run(new Configuration(), new ReduceJoinJob() , args);
           
       97.                 }catch(Exception e){
           
       98.                         e.printStackTrace();
           
       99.                 }
           
       100.                 System.exit(r);
            
       101.         }
            
       102. }

       三、打包运行

       
       

   15. 结果如下。
       

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       1. Join,2016001,YY,60
          
       2. Join,2016001,SX,88
          
       3. Join,2016001,YW,91
          
       4. Abigail,2016002,SX,77
          
       5. Abigail,2016002,YW,33
          
       6. Abigail,2016002,YY,56
          
       7. Abby,2016003,YW,69
          
       8. Abby,2016003,SX,84
          
       9. Abby,2016003,YY,34
          
       10. Alexandra,2016004,YW,100
           
       11. Alexandra,2016004,SX,84
           
       12. Alexandra,2016004,YY,89
           
       13. Cathy,2016005,YW,63
           
       14. Cathy,2016005,YY,53
           
       15. Cathy,2016005,SX,43
           
       16. Katherine,2016006,SX,90
           
       17. Katherine,2016006,YW,90
           
       18. Katherine,2016006,YY,90

       相关博文： Hadoop的GroupComparator是如何起做用的（源码分析）

   16. 本文地址：http://blog.itpub.net/30066956/viewspace-2120133/
   
   
6.