InputFormat牛逼（6）org.apache.hadoop.mapreduce.lib.db.DBRecordReader

@Public
@Evolving

A RecordReader that reads records from a SQL table. Emits LongWritables containing the record number as key and DBWritables as value.

@InterfaceAudience.Public
@InterfaceStability.Evolving
public class DBRecordReader<T extends DBWritable> extends
    RecordReader<LongWritable, T> {

  private static final Log LOG = LogFactory.getLog(DBRecordReader.class);

  private ResultSet results = null;

  private Class<T> inputClass;

  private Configuration conf;

  private DBInputFormat.DBInputSplit split;

  private long pos = 0;
  
  private LongWritable key = null;
  
  private T value = null;

  private Connection connection;

  protected PreparedStatement statement;

  private DBConfiguration dbConf;

  private String conditions;

  private String [] fieldNames;

  private String tableName;

  /**
   * @param split The InputSplit to read data for
   * @throws SQLException 
   */
  public DBRecordReader(DBInputFormat.DBInputSplit split, 
      Class<T> inputClass, Configuration conf, Connection conn, DBConfiguration dbConfig,
      String cond, String [] fields, String table)
      throws SQLException {
    this.inputClass = inputClass;
    this.split = split;
    this.conf = conf;
    this.connection = conn;
    this.dbConf = dbConfig;
    this.conditions = cond;
    this.fieldNames = fields;
    this.tableName = table;
  }

  protected ResultSet executeQuery(String query) throws SQLException {
    this.statement = connection.prepareStatement(query,
        ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
    return statement.executeQuery();
  }

  /** Returns the query for selecting the records, 
   * subclasses can override this for custom behaviour.*/
  protected String getSelectQuery() {
    StringBuilder query = new StringBuilder();

    // Default codepath for MySQL, HSQLDB, etc. Relies on LIMIT/OFFSET for splits.
    if(dbConf.getInputQuery() == null) {
      query.append("SELECT ");
  
      for (int i = 0; i < fieldNames.length; i++) {
        query.append(fieldNames[i]);
        if (i != fieldNames.length -1) {
          query.append(", ");
        }
      }

      query.append(" FROM ").append(tableName);
      query.append(" AS ").append(tableName); //in hsqldb this is necessary
      if (conditions != null && conditions.length() > 0) {
        query.append(" WHERE (").append(conditions).append(")");
      }

      String orderBy = dbConf.getInputOrderBy();
      if (orderBy != null && orderBy.length() > 0) {
        query.append(" ORDER BY ").append(orderBy);
      }
    } else {
      //PREBUILT QUERY
      query.append(dbConf.getInputQuery());
    }
        
    try {
      query.append(" LIMIT ").append(split.getLength());
      query.append(" OFFSET ").append(split.getStart());
    } catch (IOException ex) {
      // Ignore, will not throw.
    }		

    return query.toString();
  }

  /** {@inheritDoc} */
  public void close() throws IOException {
    try {
      if (null != results) {
        results.close();
      }
      if (null != statement) {
        statement.close();
      }
      if (null != connection) {
        connection.commit();
        connection.close();
      }
    } catch (SQLException e) {
      throw new IOException(e.getMessage());
    }
  }

  public void initialize(InputSplit split, TaskAttemptContext context) 
      throws IOException, InterruptedException {
    //do nothing
  }

  /** {@inheritDoc} */
  public LongWritable getCurrentKey() {
    return key;  
  }

  /** {@inheritDoc} */
  public T getCurrentValue() {
    return value;
  }

  /**
   * @deprecated 
   */
  @Deprecated
  public long getPos() throws IOException {
    return pos;
  }

 
  /** {@inheritDoc} */
  public float getProgress() throws IOException {
    return pos / (float)split.getLength();
  }

  /** {@inheritDoc} */
  public boolean nextKeyValue() throws IOException {
    try {
      if (key == null) {
        key = new LongWritable();
      }
      if (value == null) {
        value = createValue();
      }
      if (null == this.results) {
        // First time into this method, run the query.
        this.results = executeQuery(getSelectQuery());
      }
      if (!results.next())
        return false;

      // Set the key field value as the output key value
      key.set(pos + split.getStart());

      value.readFields(results);

      pos ++;
    } catch (SQLException e) {
      throw new IOException("SQLException in nextKeyValue", e);
    }
    return true;
  }

  //... ...

 }