一、提出任务

学生表，包含五个字段（姓名、性别、年龄、手机、专业），有8条记录

使用MR，学生信息按年龄降序排列结果

二、完成任务

（一）准备数据

启动hadoop服务

1、在虚拟机上创建文本文件

创建sortstudent目录，在里面创建student.txt文件

2、上传文件到HDFS指定目录

创建/sortstudent/input目录，执行命令：hdfs dfs -mkdir -p /sortstudent/input
将文本文件student.txt，上传到HDFS的/sortstudent/input目录

（二）实现步骤

1、创建学生实体类

在net.hw.mr包里创建Student类
让学生按照年龄排序，需要让学生实体类实现一个序列化可比较接口 - WritableComparable，这个接口有三个抽象方法要我们去实现。

package net.hw.mr;

import org.apache.hadoop.io.WritableComparable;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

/**
 * 功能：学生实体类
 *      实现序列化可比较接口
 */
public class Student implements WritableComparable<Student> {

    private String name;
    private String gender;
    private int age;
    private String phone;
    private String major;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", gender='" + gender + '\'' +
                ", age=" + age +
                ", phone='" + phone + '\'' +
                ", major='" + major + '\'' +
                '}';
    }

    public int compareTo(Student o) {
        return o.getAge() - this.getAge(); // 降序
    }

    public void write(DataOutput out) throws IOException {
        out.writeUTF(name);
        out.writeUTF(gender);
        out.writeInt(age);
        out.writeUTF(phone);
        out.writeUTF(major);
    }

    public void readFields(DataInput in) throws IOException {
        name = in.readUTF();
        gender = in.readUTF();
        age = in.readInt();
        phone = in.readUTF();
        major = in.readUTF();
    }
}

2、创建学生映射器类

在net.hw.mr里创建StudentMapper类

package net.hw.mr;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

import java.io.IOException;

/**
 * 功能：学生映射器类
 */
public class StudentMapper extends Mapper<LongWritable, Text, Student, NullWritable> {
    @Override
    protected void map(LongWritable key, Text value, Context context) 
            throws IOException, InterruptedException {
        // 获取行内容
        String line = value.toString();
        // 按空格拆分得到字段数组
        String[] fields = line.split(" ");
        // 获取学生信息
        String name = fields[0];
        String gender = fields[1];
        int age = Integer.parseInt(fields[2]);
        String phone = fields[3];
        String major = fields[4];
        // 创建学生对象
        Student student = new Student();
        // 设置学生对象属性
        student.setName(name);
        student.setGender(gender);
        student.setAge(age);
        student.setPhone(phone);
        student.setMajor(major);
        // 写入键值对<student,null>
        context.write(student, NullWritable.get());
    }
}

3、创建学生归并器类

在net.hw.mr包里创建StudentReducer类

package net.hw.mr;

import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;

/**
 * 功能：学生归并器类
 */
public class StudentReducer extends Reducer<Student, NullWritable, Text, NullWritable> {
    @Override
    protected void reduce(Student key, Iterable<NullWritable> values, Context context) 
            throws IOException, InterruptedException {
        // 获取学生对象
        Student student = key;
        // 拼接学生信息
        String studentInfo = student.getName() + "\t" 
                + student.getGender() + "\t"
                + student.getAge() + "\t"
                + student.getPhone() + "\t"
                + student.getMajor();
        // 写入键值对<studentInfo,null>
        context.write(new Text(studentInfo), NullWritable.get());
    }
}

4、创建学生驱动器类

在net.hw.mr包里创建StudentDriver类

package net.hw.mr;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FileStatus;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.net.URI;

/**
 * 功能：学生驱动器类
 */
public class StudentDriver {
    public static void main(String[] args) throws Exception {
        // 创建配置对象
        Configuration conf = new Configuration();
        // 设置数据节点主机名属性
        conf.set("dfs.client.use.datanode.hostname", "true");

        // 获取作业实例
        Job job = Job.getInstance(conf);
        // 设置作业启动类
        job.setJarByClass(StudentDriver.class);

        // 设置Mapper类
        job.setMapperClass(StudentMapper.class);
        // 设置map任务输出键类型
        job.setMapOutputKeyClass(Student.class);
        // 设置map任务输出值类型
        job.setMapOutputValueClass(NullWritable.class);

        // 设置Reducer类
        job.setReducerClass(StudentReducer.class);
        // 设置reduce任务输出键类型
        job.setOutputKeyClass(Student.class);
        // 设置reduce任务输出值类型
        job.setOutputValueClass(NullWritable.class);

        // 定义uri字符串
        String uri = "hdfs://master:9000";
        // 创建输入目录
        Path inputPath = new Path(uri + "/sortstudent/input");
        // 创建输出目录
        Path outputPath = new Path(uri + "/sortstudent/output");

        // 获取文件系统
        FileSystem fs =  FileSystem.get(new URI(uri), conf);
        // 删除输出目录（第二个参数设置是否递归）
        fs.delete(outputPath, true);

        // 给作业添加输入目录（允许多个）
        FileInputFormat.addInputPath(job, inputPath);
        // 给作业设置输出目录（只能一个）
        FileOutputFormat.setOutputPath(job, outputPath);

        // 等待作业完成
        job.waitForCompletion(true);

        // 输出统计结果
        System.out.println("======统计结果======");
        FileStatus[] fileStatuses = fs.listStatus(outputPath);
        for (int i = 1; i < fileStatuses.length; i++) {
            // 输出结果文件路径
            System.out.println(fileStatuses[i].getPath());
            // 获取文件系统数据字节输入流
            FSDataInputStream in = fs.open(fileStatuses[i].getPath());
            // 将结果文件显示在控制台
            IOUtils.copyBytes(in, System.out, 4096, false);
        }
    }
}