InputFormat牛逼（1）org.apache.hadoop.mapreduce.lib.db.DBWritable

@Public
@Stable

Objects that are read from/written to a database should implement DBWritable. DBWritable, is similar to Writable except that the write(PreparedStatement) method takes a PreparedStatement, and readFields(ResultSet) takes a ResultSet.

Implementations are responsible for writing the fields of the object to PreparedStatement, and reading the fields of the object from the ResultSet.

Example:

If we have the following table in the database :
CREATE TABLE MyTable (
   counter        INTEGER NOT NULL,
   timestamp      BIGINT  NOT NULL,
);

then we can read/write the tuples from/to the table with :

public class MyWritable implements Writable, DBWritable {
   // Some data    
   private int counter;
   private long timestamp;
      
   //Writable#write() implementation
   public void write(DataOutput out) throws IOException {
     out.writeInt(counter);
     out.writeLong(timestamp);
   }
      
   //Writable#readFields() implementation
   public void readFields(DataInput in) throws IOException {
     counter = in.readInt();
     timestamp = in.readLong();
   }
      
   public void write(PreparedStatement statement) throws SQLException {
     statement.setInt(1, counter);
     statement.setLong(2, timestamp);
   }
      
   public void readFields(ResultSet resultSet) throws SQLException {
     counter = resultSet.getInt(1);
     timestamp = resultSet.getLong(2);
   }
}

---------------
@InterfaceAudience.Public
@InterfaceStability.Stable
public interface DBWritable {

  /**
   * Sets the fields of the object in the {@link PreparedStatement}.
   * @param statement the statement that the fields are put into.
   * @throws SQLException
   */
public void write(PreparedStatement statement) throws SQLException;

/**
* Reads the fields of the object from the {@link ResultSet}.
* @param resultSet the {@link ResultSet} to get the fields from.
* @throws SQLException
*/
public void readFields(ResultSet resultSet) throws SQLException ;
}