对Reduce阶段的数据根据某一个或几个字段进行分组。
分组排序步骤：
（1）自定义类继承WritableComparator
（2）重写compare()方法
@Override
public int compare(WritableComparable a, WritableComparable b) {
// 比较的业务逻辑
return result;
}
（3）创建一个构造将比较对象的类传给父类
protected OrderGroupingComparator() {
super(OrderBean.class, true);
}
3.3.10 GroupingComparator分组案例实操
1．需求
有如下订单数据
表4-2 订单数据
现在需要求出每一个订单中最贵的商品。
（1）输入数据
（2）期望输出数据
1 222.8
2 722.4
3 232.8
2．需求分析
（1）利用“订单id和成交金额”作为key，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。
（2）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，如图4-18所示。
图4-18 过程分析
3．代码实现
（1）定义订单信息OrderBean类
|
package com.atguigu.mapreduce.order; import java.io.DataInput; import java.io.DataOutput; import java.io.IOException; import org.apache.hadoop.io.WritableComparable;
public class OrderBean implements WritableComparable<OrderBean> {
private int order_id; // 订单id号 private double price; // 价格
public OrderBean() { super(); }
public OrderBean(int order_id, double price) { super(); this.order_id = order_id; this.price = price; }
@Override public void write(DataOutput out) throws IOException { out.writeInt(order_id); out.writeDouble(price); }
@Override public void readFields(DataInput in) throws IOException { order_id = in.readInt(); price = in.readDouble(); }
@Override public String toString() { return order_id + “\t” + price; }
public int getOrder_id() { return order_id; }
public void setOrder_id(int order_id) { this.order_id = order_id; }
public double getPrice() { return price; }
public void setPrice(double price) { this.price = price; }
// 二次排序 @Override public int compareTo(OrderBean o) {
int result;
if (order_id > o.getOrder_id()) { result = 1; } else if (order_id < o.getOrder_id()) { result = -1; } else { // 价格倒序排序 result = price > o.getPrice() ? -1 : 1; }
return result; } } |
（2）编写OrderSortMapper类
|
package com.atguigu.mapreduce.order; import java.io.IOException; import org.apache.hadoop.io.LongWritable; import org.apache.hadoop.io.NullWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.Mapper;
public class OrderMapper extends Mapper<LongWritable, Text, OrderBean, NullWritable> {
OrderBean k = new OrderBean();
@Override protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
// 1 获取一行 String line = value.toString();
// 2 截取 String[] fields = line.split(“\t”);
// 3 封装对象 k.setOrder_id(Integer.parseInt(fields[0])); k.setPrice(Double.parseDouble(fields[2]));
// 4 写出 context.write(k, NullWritable.get()); } } |
（3）编写OrderSortGroupingComparator类
|
package com.atguigu.mapreduce.order; import org.apache.hadoop.io.WritableComparable; import org.apache.hadoop.io.WritableComparator;
public class OrderGroupingComparator extends WritableComparator {
protected OrderGroupingComparator() { super(OrderBean.class, true); }
@Override public int compare(WritableComparable a, WritableComparable b) {
OrderBean aBean = (OrderBean) a; OrderBean bBean = (OrderBean) b;
int result; if (aBean.getOrder_id() > bBean.getOrder_id()) { result = 1; } else if (aBean.getOrder_id() < bBean.getOrder_id()) { result = -1; } else { result = 0; }
return result; } } |
（4）编写OrderSortReducer类
|
package com.atguigu.mapreduce.order; import java.io.IOException; import org.apache.hadoop.io.NullWritable; import org.apache.hadoop.mapreduce.Reducer;
public class OrderReducer extends Reducer<OrderBean, NullWritable, OrderBean, NullWritable> {
@Override protected void reduce(OrderBean key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {
context.write(key, NullWritable.get()); } } |
（5）编写OrderSortDriver类
|
package com.atguigu.mapreduce.order; import java.io.IOException; import org.apache.hadoop.conf.Configuration; 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;
public class OrderDriver {
public static void main(String[] args) throws Exception, IOException {
// 输入输出路径需要根据自己电脑上实际的输入输出路径设置 args = new String[]{“e:/input/inputorder” , “e:/output1”};
// 1 获取配置信息 Configuration conf = new Configuration(); Job job = Job.getInstance(conf);
// 2 设置jar包加载路径 job.setJarByClass(OrderDriver.class);
// 3 加载map/reduce类 job.setMapperClass(OrderMapper.class); job.setReducerClass(OrderReducer.class);
// 4 设置map输出数据key和value类型 job.setMapOutputKeyClass(OrderBean.class); job.setMapOutputValueClass(NullWritable.class);
// 5 设置最终输出数据的key和value类型 job.setOutputKeyClass(OrderBean.class); job.setOutputValueClass(NullWritable.class);
// 6 设置输入数据和输出数据路径 FileInputFormat.setInputPaths(job, new Path(args[0])); FileOutputFormat.setOutputPath(job, new Path(args[1]));
// 8 设置reduce端的分组 job.setGroupingComparatorClass(OrderGroupingComparator.class);
// 7 提交 boolean result = job.waitForCompletion(true); System.exit(result ? 0 : 1); } } |
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