MapReduce实现TopN效果示例解析

<dependencies>
    <dependency>
        <groupId>org.apache.hadoop</groupId>
        <artifactId>hadoop-client</artifactId>
        <version>3.3.4</version>
    </dependency>
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.22</version>
    </dependency>
</dependencies>
<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <version>3.2.2</version>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <classpathPrefix>lib/</classpathPrefix>
                        <mainClass>com.huan.hadoop.mr.TopNDriver</mainClass>
                    </manifest>
                </archive>
            </configuration>
        </plugin>
    </plugins>
</build>

package com.huan.hadoop.mr;
import lombok.Getter;
import lombok.Setter;
import org.apache.hadoop.io.WritableComparable;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
/**
 * 订单数据
 *
 * @author huan.fu
 * @date 2023/7/13 - 14:20
 */
@Getter
@Setter
public class OrderVo implements WritableComparable<OrderVo> {
    /**
     * 订单编号
     */
    private long orderId;
    /**
     * 订单项
     */
    private String itemId;
    /**
     * 订单项价格
     */
    private long price;
    @Override
    public int compareTo(OrderVo o) {
        // 排序： 根据 订单编号 升序， 如果订单编号相同，则根据 订单项价格 倒序
        int result = Long.compare(this.orderId, o.orderId);
        if (result == 0) {
            // 等于0说明 订单编号 相同，则需要根据 订单项价格 倒序
            result = -Long.compare(this.price, o.price);
        }
        return result;
    }
    @Override
    public void write(DataOutput out) throws IOException {
        // 序列化
        out.writeLong(orderId);
        out.writeUTF(itemId);
        out.writeLong(price);
    }
    @Override
    public void readFields(DataInput in) throws IOException {
        // 反序列化
        this.orderId = in.readLong();
        this.itemId = in.readUTF();
        this.price = in.readLong();
    }
    @Override
    public String toString() {
        return this.getOrderId() + "\t" + this.getItemId() + "\t" + this.getPrice();
    }
}

package com.huan.hadoop.mr;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;
/**
 * 分组： 订单编号相同说明是同一组，否则是不同的组
 *
 * @author huan.fu
 * @date 2023/7/13 - 14:30
 */
public class TopNGroupingComparator extends WritableComparator {
    public TopNGroupingComparator() {
        // 第二个参数为true: 表示可以通过反射创建实例
        super(OrderVo.class, true);
    }
    @Override
    public int compare(WritableComparable a, WritableComparable b) {
        // 订单编号 相同说明是同一个对象，否则是不同的对象
        return ((OrderVo) a).getOrderId() == ((OrderVo) b).getOrderId() ? 0 : 1;
    }
}

package com.huan.hadoop.mr;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import java.io.IOException;
/**
 * map 操作： 输出的key为OrderVo, 输出的value为： price
 *
 * @author huan.fu
 * @date 2023/7/13 - 14:28
 */
public class TopNMapper extends Mapper<LongWritable, Text, OrderVo, LongWritable> {
    private final OrderVo outKey = new OrderVo();
    private final LongWritable outValue = new LongWritable();
    @Override
    protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, OrderVo, LongWritable>.Context context) throws IOException, InterruptedException {
        // 获取一行数据 20230713000010  item-101    10
        String row = value.toString();
        // 根据 \t 进行分割
        String[] cells = row.split("\\s+");
        // 获取订单编号
        long orderId = Long.parseLong(cells[0]);
        // 获取订单项
        String itemId = cells[1];
        // 获取订单项价格
        long price = Long.parseLong(cells[2]);
        // 设置值
        outKey.setOrderId(orderId);
        outKey.setItemId(itemId);
        outKey.setPrice(price);
        outValue.set(price);
        // 写出
        context.write(outKey, outValue);
    }
}

package com.huan.hadoop.mr;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Reducer;
import java.io.IOException;
/**
 * reduce操作： Key（OrderVo）相同的分为一组， 此处 OrderVo 作为key, 分组是根据 TopNGroupingComparator 来实现，
 * 即 订单编号 相同的认为一组
 *
 * @author huan.fu
 * @date 2023/7/13 - 14:29
 */
public class TopNReducer extends Reducer<OrderVo, LongWritable, OrderVo, NullWritable> {
    @Override
    protected void reduce(OrderVo key, Iterable<LongWritable> values, Reducer<OrderVo, LongWritable, OrderVo, NullWritable>.Context context) throws IOException, InterruptedException {
        int topN = 0;
        // 随着每次遍历， key的 orderId 是相同的（因为是根据这个分组的），但是里面的itemId和price是不同的
        for (LongWritable price : values) {
            topN++;
            if (topN > 2) {
                break;
            }
            // 注意： 此处的key每次输出都不一样
            context.write(key, NullWritable.get());
        }
    }
}

package com.huan.hadoop.mr;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
/**
 * @author huan.fu
 * @date 2023/7/13 - 14:29
 */
public class TopNDriver extends Configured implements Tool {
    public static void main(String[] args) throws Exception {
        // 构建配置对象
        Configuration configuration = new Configuration();
        // 使用 ToolRunner 提交程序
        int status = ToolRunner.run(configuration, new TopNDriver(), args);
        // 退出程序
        System.exit(status);
    }
    @Override
    public int run(String[] args) throws Exception {
        // 构建Job对象实例 参数（配置对象，Job对象名称）
        Job job = Job.getInstance(getConf(), "topN");
        // 设置mr程序运行的主类
        job.setJarByClass(TopNDriver.class);
        // 设置mr程序运行的 mapper类型和reduce类型
        job.setMapperClass(TopNMapper.class);
        job.setReducerClass(TopNReducer.class);
        // 指定mapper阶段输出的kv数据类型
        job.setMapOutputKeyClass(OrderVo.class);
        job.setMapOutputValueClass(LongWritable.class);
        // 指定reduce阶段输出的kv数据类型，业务mr程序输出的最终类型
        job.setOutputKeyClass(OrderVo.class);
        job.setOutputValueClass(NullWritable.class);
        // 配置本例子中的输入数据路径和输出数据路径，默认输入输出组件为： TextInputFormat和TextOutputFormat
        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        // 先删除输出目录（方便本地测试）
        FileSystem.get(this.getConf()).delete(new Path(args[1]), true);
        // 设置分组
        job.setGroupingComparatorClass(TopNGroupingComparator.class);
        return job.waitForCompletion(true) ? 0 : 1;
    }
}