/**
 * Base class for the dynamically generated expression evaluator classes.
 * 
 * @author Lucian Chirita (lucianc@users.sourceforge.net)
 * @version $Id: JREvaluator.java 4595 2011-09-08 15:55:10Z teodord $
 */
public abstract class JREvaluator implements DatasetExpressionEvaluator
{
...
	/**
	 *
	 */
	public Object evaluate(JRExpression expression) throws JRExpressionEvalException
	{
		Object value = null;
		
		if (expression != null)
		{
			try
			{
				value = evaluate(expression.getId());
			}
			catch (NullPointerException e) //NOPMD
			{
			}
			catch (OutOfMemoryError e)
			{
				throw e;
			}
			// we have to catch Throwable because there is no way we could modify the signature
			// of the evaluate method, without breaking backward compatibility of compiled report templates 
			catch (Throwable e) //NOPMD
			{
				throw new JRExpressionEvalException(expression, e);
			}
		}
		
		return value;
	}
...
}

public class R1400DataSource extends ListDataSource<MonthPower> implements JRDataSource {

   /**
   Конструктор
   **/
   public R1400DataSource(...) throws JRException {
     super();
		
      try {		
        // мой код
        // поиск данных в БД, вычисления, агрегирование....
        // достаточно большой код может быть здесь
        ...


      } catch (NullPointerException e) {
 	 throw new JRException(e);
    }

    ... имплементация JRDataSource, немножко методов
}