一、依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>TestHadoop3</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-client</artifactId>
            <version>3.2.1</version>
        </dependency>

        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>1.2.3</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
    </dependencies>

</project>

二、数据库中建表

create table word_count(id int auto_increment primary key,word varchar(8192), count int);

三、定义实体类，实现DBWritable接口和Writable接口

package cn.edu.tju;

import org.apache.hadoop.io.Writable;
import org.apache.hadoop.mapred.lib.db.DBWritable;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class MyDBWritable implements DBWritable, Writable {
    private String word;
    private int count;

    public String getWord() {
        return word;
    }

    public void setWord(String word) {
        this.word = word;
    }

    public int getCount() {
        return count;
    }

    public void setCount(int count) {
        this.count = count;
    }

    @Override
    public void write(PreparedStatement statement) throws SQLException {

        statement.setString(1, word);
        statement.setInt(2, count);
    }

    @Override
    public void readFields(ResultSet resultSet) throws SQLException {
        word = resultSet.getString(1);
        count = resultSet.getInt(2);
    }

    @Override
    public void write(DataOutput out) throws IOException {
        out.writeUTF(word);
        out.writeInt(count);
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        word = in.readUTF();
        count = in.readInt();
    }
}

四、定义mapper

package cn.edu.tju;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import java.io.IOException;

public class WordCountMapper extends Mapper<LongWritable, Text, Text, IntWritable> {
    private IntWritable one = new IntWritable(1);

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String valueString = value.toString();
        String[] values = valueString.split(" ");
        for (String val : values) {
            context.write(new Text(val), one);
        }
    }
}

五、定义reducer

package cn.edu.tju;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.lib.db.DBWritable;
import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;
import java.util.Iterator;

public class WordCountReducer extends Reducer<Text, IntWritable,  DBWritable, NullWritable> {
    @Override
    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
        Iterator<IntWritable> iterator = values.iterator();
        int count = 0;
        while (iterator.hasNext()) {
            count += iterator.next().get();
        }

        MyDBWritable myDBWritable = new MyDBWritable();
        myDBWritable.setCount(count);
        myDBWritable.setWord(key.toString());
        context.write(myDBWritable, NullWritable.get());
    }
}

其中使用了上面定义的MyDBWritable类

六、定义主类，启动hadoop job

package cn.edu.tju;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.db.DBConfiguration;
import org.apache.hadoop.mapreduce.lib.db.DBOutputFormat;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;

public class MyWordCount5 {
    public static void main(String[] args) throws Exception {
        Configuration configuration = new Configuration(true);

        //hdfs路径
        configuration.set("fs.defaultFS", "hdfs://xx.xx.xx.xx:9000/");
        //yarn 运行 还是local 运行
        configuration.set("mapreduce.framework.name", "local");
        //configuration.set("yarn.resourcemanager.hostname", "xx.xx.xx.xx");
        //configuration.set("mapreduce.job.jar","target\\TestHadoop3-1.0-SNAPSHOT.jar");
        //job 创建
        Job job = Job.getInstance(configuration);
        //
        job.setJarByClass(MyWordCount5.class);
        //job name
        job.setJobName("wordcount-" + System.currentTimeMillis());

        //输入数据路径
        FileInputFormat.setInputPaths(job, new Path("/user/root/tju/"));

        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);

        job.setMapperClass(WordCountMapper.class);
        //job.setCombinerClass(MyCombiner.class);
        job.setReducerClass(WordCountReducer.class);

        //job.setOutputFormatClass(MyOutputFormat.class);

        String driverClassName = "com.mysql.jdbc.Driver";
        String url = "jdbc:mysql://xx.xx.xx.xx:3306/test?serverTimezone=UTC&useUnicode=true&characterEncoding=utf-8";
        String username = "root";
        String password = "xxxxxx";

        DBConfiguration.configureDB(job.getConfiguration(), driverClassName, url,
                username, password);


        // 设置输出的表
        DBOutputFormat.setOutput(job, "word_count", "word", "count");

        //等待任务执行完成
        job.waitForCompletion(true);
    }
}

其中DBOutputFormat.setOutput(job, “word_count”, “word”, “count”);这句设置往数据库写数据。任务的输入数据来自hdfs.
七、任务结束后在数据库中查询结果