IDEA手写GroupingComparator分组案例实操

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需求：在每个订单中，找出最贵的商品。
需求分析：
可以将Map阶段读取到的所有订单数据按照id升序排序，如果id相同再按照金额降序排序，发送到Reduce。
在Reduce端利用groupingComparator将订单id相同的kv聚合成组，然后取第一个即是该订单中最贵商品。

OrderBean类：

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package com.atguigu.groupingcomparator;

import org.apache.hadoop.io.WritableComparable;

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

public class OrderBean implements WritableComparable<OrderBean> {
    /**
     * 订单Bean
     */
    private String orderId;//订单ID
    private String productId;//商品ID
    private double price;//成交金额

    @Override
    public String toString() {
        return orderId + '\t' + productId + '\t' + price;
    }

    public String getOrderId() {
        return orderId;
    }

    public void setOrderId(String orderId) {
        this.orderId = orderId;
    }

    public String getProductId() {
        return productId;
    }

    public void setProductId(String productId) {
        this.productId = productId;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public int compareTo(OrderBean o) {
        int compare = this.orderId.compareTo(o.orderId);//先按照订单ID分区排序
        if(compare == 0){
            return Double.compare(o.price,this.price);//id相同再按照金额降序排序
        }else{
            return compare;
        }
    }

    @Override
    public void write(DataOutput dataOutput) throws IOException {
        dataOutput.writeUTF(orderId);
        dataOutput.writeUTF(productId);
        dataOutput.writeDouble(price);
    }

    @Override
    public void readFields(DataInput dataInput) throws IOException {
        this.orderId = dataInput.readUTF();
        this.productId = dataInput.readUTF();
        this.price = dataInput.readDouble();
    }
}

OrderComparator类：

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package com.atguigu.groupingcomparator;

import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;

public class OrderComparator extends WritableComparator {

    /**
     * 需要有两个空对象，不然反序列化时没有对象接收数据，会报空指针异常
     * 所以在构造函数阶段就设置createInstanes为true,生成空对象，后面才可以进行对象属性的比较
     */
    protected OrderComparator() {
        super(OrderBean.class,true);
    }

    @Override
    public int compare(WritableComparable a, WritableComparable b) {
        OrderBean oa = (OrderBean) a;
        OrderBean ob = (OrderBean) b;

        return oa.getOrderId().compareTo(ob.getOrderId());
    }
}

OrderMapper类：

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package com.atguigu.groupingcomparator;

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;

public class OrderMapper extends Mapper<LongWritable, Text,OrderBean, NullWritable> {
    private OrderBean orderbean = new OrderBean();

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String[] fields = value.toString().split("\t");
        orderbean.setOrderId(fields[0]);
        orderbean.setProductId(fields[1]);
        orderbean.setPrice(Double.parseDouble(fields[2]));
        context.write(orderbean,NullWritable.get());
    }
}

OrderReducer类：

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package com.atguigu.groupingcomparator;

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

import java.io.IOException;

public class OrderReducer extends Reducer<OrderBean, NullWritable,OrderBean,NullWritable> {
    @Override
    protected void reduce(OrderBean key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {
        /**
         * 只有一个key,默认就是排序后的每组的第一个OrderBean
         * 本质上，只有一个键对象key和一个值对象value
         * 所有数据都是通过KV值反序列化不断更新复用对象,达到遍历所有对象的需要
         */
        context.write(key,NullWritable.get());
        /**
         * 如果想要取得每个组的前两名，可写为以下代码
         */
//        Iterator<NullWritable> iterator = values.iterator();
//        for (int i = 0;i<2;++i) {
//            if(iterator.hasNext()) {
//                context.write(key,iterator.next());
//            }
//        }
    }
}

OrderDriver类：

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package com.atguigu.groupingcomparator;

import org.apache.hadoop.fs.Path;
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 java.io.IOException;

public class OrderDriver {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        Job job = Job.getInstance();

        job.setJarByClass(OrderDriver.class);
        job.setMapperClass(OrderMapper.class);
        job.setReducerClass(OrderReducer.class);

        job.setMapOutputKeyClass(OrderBean.class);
        job.setMapOutputValueClass(NullWritable.class);

        job.setGroupingComparatorClass(OrderComparator.class);

        job.setOutputKeyClass(OrderBean.class);
        job.setOutputValueClass(NullWritable.class);

        FileInputFormat.setInputPaths(job, new Path("f:\\input"));
        FileOutputFormat.setOutputPath(job, new Path("f:\\output"));

        boolean b = job.waitForCompletion(true);
        System.exit(b ? 0 : 1);
    }
}

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