Almacenamiento de la salida de datos de Apache Hadoop en la base de datos Mysql

Primero, debe crear una clase que represente los datos que le gustaría almacenar. La clase debe implementar la interfaz DBWritable:

public class DBOutputWritable implements Writable, DBWritable
{
   private String name;
   private int count;

   public DBOutputWritable(String name, int count) {
     this.name = name;
     this.count = count;
   }

   public void readFields(DataInput in) throws IOException {   }

   public void readFields(ResultSet rs) throws SQLException {
     name = rs.getString(1);
     count = rs.getInt(2);
   }

   public void write(DataOutput out) throws IOException {    }

   public void write(PreparedStatement ps) throws SQLException {
     ps.setString(1, name);
     ps.setInt(2, count);
   }
}

Crea objetos de clase previamente definidos en tu Reducer:

public class Reduce extends Reducer<Text, IntWritable, DBOutputWritable, NullWritable> {

   protected void reduce(Text key, Iterable<IntWritable> values, Context ctx) {
     int sum = 0;

     for(IntWritable value : values) {
       sum += value.get();
     }

     try {
       ctx.write(new DBOutputWritable(key.toString(), sum), NullWritable.get());
     } catch(IOException e) {
       e.printStackTrace();
     } catch(InterruptedException e) {
       e.printStackTrace();
     }
   }
}

Finalmente, debe configurar una conexión a su base de datos (no olvide agregar su conector de base de datos en el classpath) y registrar los tipos de datos de entrada/salida de su mapeador y reductor.

public class Main
{
   public static void main(String[] args) throws Exception
   {
     Configuration conf = new Configuration();
     DBConfiguration.configureDB(conf,
     "com.mysql.jdbc.Driver",   // driver class
     "jdbc:mysql://localhost:3306/testDb", // db url
     "user",    // username
     "password"); //password

     Job job = new Job(conf);
     job.setJarByClass(Main.class);
     job.setMapperClass(Map.class); // your mapper - not shown in this example
     job.setReducerClass(Reduce.class);
     job.setMapOutputKeyClass(Text.class); // your mapper - not shown in this example
     job.setMapOutputValueClass(IntWritable.class); // your mapper - not shown in this example
     job.setOutputKeyClass(DBOutputWritable.class); // reducer's KEYOUT
     job.setOutputValueClass(NullWritable.class);   // reducer's VALUEOUT
     job.setInputFormatClass(...);
     job.setOutputFormatClass(DBOutputFormat.class);

     DBInputFormat.setInput(...);

     DBOutputFormat.setOutput(
     job,
     "output",    // output table name
     new String[] { "name", "count" }   //table columns
     );

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