Question

I m doing a little bit of planning of an application that uses Mondrian OLAP engine with Olap4j and should present/display data to user. I understand all the back-end stuff, but I m not sure how should I display the data in the view layer.

For example olap4j has a formatter that prints the SELECT nicely into the console.

How is the data that I get from olap4j displayed in view layer ? I just went through the olap4j API, and there doesn t seem to be anything for getting the result in a form that can be somehow further processed and displayed. Is this process part of the Pentaho solution ? So that otherwise it is really not easy to present data just from Mondrian OLAP engine and olap4j ?

EDIT: I m used to traditionally get some data from a database into my DTO and display it in view layer. But how do I create DTOs for such a complicated result set ?

Answer 1

You can create your own view layer it s just a little bit tricky.

OlapStatement.executeOlapQuery() returns a CellSet, you will have to work with that. Also read the specifications, it s a good source of information.

Here is an example, that creates List<List<MyCell>> (not the best representation but it s easy to undarstand how it works). This creates a table similar to http://www.olap4j.org/api/index.html?org/olap4j/Position.html (without the "Gender" and "Product" labels).

private final static int COLUMNS = 0; //see Cellset javadoc
private final static int ROWS= 1; //see Cellset javadoc
/**
* Outer list: rows, inner list: elements in a row
*/
private List<List<MyCell>> getListFromCellSet(CellSet cellSet) {
    List<List<MyCell>> toReturn= new ArrayList<List<MyCell>>();
    //Column header
    //See http://www.olap4j.org/api/index.html?org/olap4j/Position.html on how Position works, it helps a lot
    //Every position will be a column in the header
    for (Position pos : cellSet.getAxes().get(COLUMNS).getPositions()) {
        for (int i = 0; i < pos.getMembers().size(); i++) {
            if (toReturn.size() <= i) {
                toReturn.add(i, new ArrayList<MyCell>());
            }
            Member m = pos.getMembers().get(i);
            MyCell myCell = new MyCell(m); //use m.getCaption() for display
            toReturn.get(i).add(myCell );
        }
    }
    //Put empty elements to the beginning of the list, so there will be place for the rows header
    if (cellSet.getAxes().get(ROWS).getPositions().size() > 0) {
        for (int count=0; count < cellSet.getAxes().get(1).getPositions().get(0).getMembers().size(); count++) {
            for (int i = 0; i < toReturn.size(); i++) {
                toReturn.get(i).add(0, new MyCell());
            }
        }
    }
    //Content + row header
    for(int i = 0; i < cellSet.getAxes().get(ROWS).getPositionCount(); i++) {
        List<MyCell> row = new ArrayList<MyCell>();
        //Header
        for (org.olap4j.metadata.Member m : cellSet.getAxes().get(ROWS).getPositions().get(i).getMembers()) {
            row.add(new MyCell(m));
        }
        //Content
        for (int j = 0; j < cellSet.getAxes().get(COLUMNS).getPositionCount(); j++) {
            ArrayList<Integer> list = new ArrayList<Integer>();
            list.add(j); //coordinte
            list.add(i); //coordinte
            row.add(new MyCell(cellSet.getCell(list))); //use cell.getFormattedValue() for display
        }
        toReturn.add(row);
    }
    return toReturn;
}

Create the MyCell class with these constructors:

public class MyCell {   
    ...
    public MyCell(){...}
    public MyCell(Member m){...}
    public MyCell(Cell c){...}  
}

Don t forget to display the filters, use Cellset.getFilterAxis() for that.

You can also check the Rectangular formatter on SourceForge, but it s a bit longer.

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