How do I hide a table column based on its content? - jasper-reports

Question

I have a report which contains a table. I would like to hide the columns of that table based on the result set of the query that backs the table. As an example, here is the XML for one of the columns:
<jr:column width="80">
<printWhenExpression>$F{Total1_header} != null</printWhenExpression>
<jr:columnHeader height="30" rowSpan="1">
<textField isStretchWithOverflow="true" isBlankWhenNull="true">
<reportElement x="0" y="0" width="80" height="30" style="table_CH"/>
<textElement textAlignment="Center" verticalAlignment="Middle"/>
<textFieldExpression>$F{Total1_header}</textFieldExpression>
</textField>
</jr:columnHeader>
<jr:detailCell style="table_TD" height="20" rowSpan="1">
<textField isStretchWithOverflow="true" isBlankWhenNull="true">
<reportElement x="0" y="0" width="80" height="20"/>
<textElement textAlignment="Center" verticalAlignment="Middle"/>
<textFieldExpression>$F{Total1}</textFieldExpression>
</textField>
</jr:detailCell>
</jr:column>
For some reason, my report is complaining about the printWhenExpression. It claims that $F{Total1_header} does not exist. It does not complain however about the instance of $F{Total1_header} in the textFieldExpression.
I can't figure out why the field is avaiable for the textFieldExpression, but not the printWhenExpression.

Answer 1

Column has 'Column print when' property to hide/show the column by an expression.
Another way to delete columns is to modify the jasper design at runtime, then report compilation and then procees as usually. This method makes it possible to distribute unused width to remaining columns.
Roughly goes like this:
JasperDesign design = ...
JRDesignComponentElement tableElement = (JRDesignComponentElement) design.getTitle().getElementByKey(tableKey);
StandardTable tableComponent = (StandardTable) tableElement.getComponent();
col = tableComponent.getColumns().get(0);
tableComponent.removeColumn(col);
// Then compile the JasperDesign
JasperReport result=JasperCompileManager.compileReport(design);

Answer 2

For future readers
You should use this
<printWhenExpression><![CDATA[$F{Total1_header} != null]]></printWhenExpression>
Instead of
<printWhenExpression>$F{Total1_header} != null</printWhenExpression>