• If a report query is declared, it could be customized to retrieve already aggregated data from the data source (A SQL query could use the GROUP BY clause and statistical functions, for example). The main advantage is that the JR engine needs no more to prepare data before representing it, and the report is running faster in this case. The main disadvantage is that one cannot use the group imbrication, along with group header and footer facilities, so the data presentation can be realized in simplified layouts only.
• If a rich presentation context of data grouping is required, one could use the report <group/> elements. Groups represent a flexible way to organize data on a report. In this case, the engine iterates through records in the data source and retrieves only those according to the grouping criteria.

• Group expression - contains the data grouping criteria. When the value of the group expression changes during the iteration through the data source at report-filling time, a group rupture occurs and the corresponding <groupFooter> and <groupHeader> sections are inserted into the resulting document.
• Group header section - a multiband section containing header additional information.
• Group footer section - a multiband section containing footer additional information.

• The order of groups declared in a report template is important because groups contain each other. One group contains the following group, and so on. When a larger group encounters a rupture, all subsequent groups are reinitialized.
• You can declare as many groups as you want in a report.
• In order to get an accurate data representation, the data in the data source should be already ordered according to the group expressions used in the report. One can either perform data sorting through the report query, or use the <sortField/> element.

• name - Acts as a group identifier. This attribute is mandatory, and should be unique per report.
• isStartNewColumn - Instructs the engine to start a new column when a new group starts. The default value is false.
• isStartNewPage - Instructs the engine to start a new page when a new group starts. The default value is false.
• isResetPageNumber - If true, when a new group starts, the built-in PAGE_NUMBER variable will be reset. The default value is false.
• isReprintHeaderOnEachPage - Instructs the engine to print the group header on each page containing the group data. The default value is false.
• minHeightToStartNewPage - Represents the minimum amount of vertical space at the bottom of the page or column, measured in pixels, that prevents the group from starting a new page of its own.
• footerPosition - Specifies the rendering position of the group footer on the page. Possible values are: Normal, StackAtBottom, ForceAtBottom, CollateAtBottom. The default value is Normal.
• keepTogether - If true, prevents the group from splitting on its first break attempt. The default value is false.

• name - The variable's name used as unique identifier. It is mandatory.
• class - The class name of the variable's values. The default class is java.lang.String
• resetType - Indicates when the variable should be reinitialized during the report-filling process. Possible values are:None, Report, Page, Column, Group. The default value is Report.
• resetGroup - If the resetType is set to Group, indicates the name of the group which triggers the variable reinitializing each time it breaks.
• incrementType - Indicates when the variable should be incremented during the report-filling process. Possible values are:None, Report, Page, Column, Group. The default value is None.
• incrementGroup - If the incrementType is set to Group, indicates the name of the group which triggers the variable increment each time it breaks.
• calculation - Indicates the built-in calculation type to be performed on the variable expression value. Possible values are:Nothing, Count, DistinctCount, Sum, Average, Lowest, Highest, StandardDeviation, Variance, System, First. The default value is Nothing.
• incrementerFactoryClass - The name of the incrementer factory class.

  <queryString><![CDATA[SELECT * FROM Orders WHERE OrderID <= $P{MaxOrderID} ORDER BY ShipCountry]] ></queryString>

  <variable name="FirstLetter" class="java.lang.String" resetType="None">
    <variableExpression><![CDATA[$F{ShipCountry}.substring(0, 1).toUpperCase()]] ></variableExpression>
  </variable>
  <variable name="FreightSumFirstLetterGroup" class="java.lang.Double" resetType="Group" 
    resetGroup="FirstLetterGroup" calculation="Sum">
    <variableExpression><![CDATA[$F{Freight}]] ></variableExpression>
  </variable>
  <variable name="FreightSumCountryGroup" class="java.lang.Double" resetType="Group" 
    resetGroup="CountryGroup" calculation="Sum">
    <variableExpression><![CDATA[$F{Freight}]] ></variableExpression>
  </variable>
  <variable name="FreightSumColumn" class="java.lang.Double" resetType="Column" calculation="Sum">
    <variableExpression><![CDATA[$F{Freight}]] ></variableExpression>
  </variable>
  <variable name="FreightSumPage" class="java.lang.Double" resetType="Page" calculation="Sum">
    <variableExpression><![CDATA[$F{Freight}]] ></variableExpression>
  </variable>
  <variable name="FreightSumReport" class="java.lang.Double" calculation="Sum">
    <variableExpression><![CDATA[$F{Freight}]] ></variableExpression>
  </variable>
  <variable name="DateHighestCountryGroup" class="java.sql.Timestamp" resetType="Group" 
    resetGroup="CountryGroup" calculation="Highest">
    <variableExpression><![CDATA[$F{OrderDate}]] ></variableExpression>
  </variable>
  <variable name="RegionCountCountryGroup" class="java.lang.Integer" resetType="Group" 
    resetGroup="CountryGroup" calculation="Count">
    <variableExpression><![CDATA[$F{ShipRegion}]] ></variableExpression>
  </variable>

<group name="FirstLetterGroup" isStartNewColumn="true" isReprintHeaderOnEachPage="true" 
  minHeightToStartNewPage="200">
  <groupExpression><![CDATA[$V{FirstLetter}]] ></groupExpression>
  <groupHeader>
    <band height="25">
      ...
      <textField>
        <reportElement mode="Opaque" x="130" y="14" width="140" height="11" forecolor="#ff0000" 
          backcolor="#ffdddd" style="Sans_Bold"/>
        <textFieldExpression class="java.lang.String"><![CDATA[$V{FirstLetter}]] ></textFieldExpression>
      </textField>
    </band>
  </groupHeader>
  <groupFooter>
    <band height="15">
      ...
      <textField>
        <reportElement x="45" y="1" width="25" height="11" forecolor="#ff0000" style="Sans_Bold"/>
        <textElement textAlignment="Right"/>
        <textFieldExpression class="java.lang.Integer"><![CDATA[$V{FirstLetterGroup_COUNT}]] ></textFieldExpression>
      </textField>
      ...
      <textField pattern="0.00">
        <reportElement x="170" y="1" width="60" height="11" forecolor="#ff0000" style="Sans_Bold"/>
        <textElement textAlignment="Right"/>
        <textFieldExpression class="java.lang.Double"><![CDATA[$V{FreightSumFirstLetterGroup}]] ></textFieldExpression>
      </textField>
      <textField evaluationTime="Auto">
        <reportElement x="230" y="1" width="40" height="11" forecolor="#ff0000" style="Sans_Bold"/>
        <textElement textAlignment="Right"/>
        <textFieldExpression>
          <![CDATA[
            msg("{0,number,0.00}%", 
            new Double(100d * $V{FreightSumFirstLetterGroup}.doubleValue() / $V{FreightSumReport}.doubleValue()))
          ]] >
        </textFieldExpression>
      </textField>
    </band>
  </groupFooter>
</group>

<group name="CountryGroup" isReprintHeaderOnEachPage="true">
  <groupExpression><![CDATA[$F{ShipCountry}]] ></groupExpression>
  <groupHeader>
  <band height="15">
    ...
    <textField evaluationTime="Group" evaluationGroup="CountryGroup" pattern="EEE, MMM d, yyyy">
      <reportElement x="170" y="2" width="100" height="11" style="Sans_Bold"/>
      <textElement textAlignment="Right"/>
      <textFieldExpression class="java.sql.Timestamp"><![CDATA[$V{DateHighestCountryGroup}]] ></textFieldExpression>
    </textField>
  </band>
  </groupHeader>
  <groupFooter>
  <band height="15">
    ...
    <textField>
      <reportElement mode="Opaque" x="45" y="0" width="25" height="11" backcolor="#c0c0c0" style="Sans_Bold"/>
      <textElement textAlignment="Right"/>
      <textFieldExpression class="java.lang.Integer"><![CDATA[$V{CountryGroup_COUNT}]] ></textFieldExpression>
    </textField>
    ...
    <textField pattern="0.00">
      <reportElement mode="Opaque" x="170" y="0" width="60" height="11" backcolor="#c0c0c0" style="Sans_Bold"/>
      <textElement textAlignment="Right"/>
      <textFieldExpression class="java.lang.Double"><![CDATA[$V{FreightSumCountryGroup}]] ></textFieldExpression>
    </textField>
    <textField evaluationTime="Auto">
      <reportElement mode="Opaque" x="230" y="0" width="40" height="11" backcolor="#c0c0c0" style="Sans_Bold"/>
      <textElement textAlignment="Right"/>
      <textFieldExpression><![CDATA[msg("{0,number,0.00}%", new Double(100d * $V{FreightSumCountryGroup}.doubleValue() / $V{FreightSumFirstLetterGroup}.doubleValue()))]] ></textFieldExpression>
    </textField>
  </band>
  </groupFooter>
</group>

  <group name="BreakGroup">
    <groupExpression><![CDATA[new Boolean($V{BreakGroup_COUNT}.intValue() > 5)]] ></groupExpression>
    <groupHeader>
    <band height="5">
    </band>
    </groupHeader>
    <groupFooter>
    <band height="5">
    </band>
    </groupFooter>
  </group>

• data with correct field type (numeric, currency, date, etc);
• no extra columns/rows added by the grid generation constraints;
• no page margins and other padding settings;
• no page numbers;
• no repetition of page headers;
• data organized in columns/rows in order to allow formula calculations;
• some report sections to be excluded/hidden;
• outline data grouping;
• etc.

JRCsvMetadataExporter exporter = new JRCsvMetadataExporter();
exporter.setParameter(JRCsvMetadataExporterParameter.COLUMN_NAMES, new String[]{"Name,Id"});

• net.sf.jasperreports.export.csv.column.name - this is an important element-level property. If the COLUMN_NAMES parameter (or equivalent property) is not present, this property specifies the name of an exported column and assumes that the current element will be placed in that column. If the COLUMN_NAMES parameter (or equivalent prop) was defined, the name specified by this property must be one of the names in the COLUMN_NAMES parameter. The element will be placed in that column.
• net.sf.jasperreports.export.csv.write.header - a report-level flag property indicating whether the column names should be exported as column headers or not. The equivalent export parameter is WRITE_HEADER. the default value is false.
• net.sf.jasperreports.export.csv.repeat.value - an element-level flag property indicating whether the value associated to that column should be repeated or not when it is missing.
• net.sf.jasperreports.export.csv.data - element-level property containing the exported value for that element. By default is considered the text value of the report element itself.

<detail>
  <band height="13">
  ...
    <textField>
      <reportElement style="OrderIdStyle" x="1" y="0" width="35" height="11">
        <property name="net.sf.jasperreports.export.csv.column.name" value="Order"/>
        ...
      </reportElement>
      ...
    </textField>
    <textField isStretchWithOverflow="true">
      <reportElement positionType="Float" x="40" y="0" width="110" height="11">
        <property name="net.sf.jasperreports.export.csv.column.name" value="ShipCity"/>
        ...
        <propertyExpression name="net.sf.jasperreports.export.csv.data"><![CDATA[$F{ShipCity}]] ></propertyExpression>
        ...
      </reportElement>
    </textField>
    <textField isBlankWhenNull="true">
      <reportElement x="155" y="0" width="25" height="11">
        <property name="net.sf.jasperreports.export.csv.column.name" value="ShipRegion"/>
        ...
      </reportElement>
      ...
    </textField>
    <textField pattern="dd/MM/yyyy">
      <reportElement x="185" y="0" width="50" height="11">
        <property name="net.sf.jasperreports.export.csv.column.name" value="OrderDate"/>
        ...
      </reportElement>
      ...
    </textField>
    <textField pattern="0.00">
      <reportElement x="235" y="0" width="35" height="11">
        <property name="net.sf.jasperreports.export.csv.column.name" value="Freight"/>
        ...
      </reportElement>
      ...
    </textField>
    ...
  </band>
</detail>

• COLUMN_NAMES parameter or the equivalent net.sf.jasperreports.export.xls.column.names.{suffix}
• net.sf.jasperreports.export.xls.column.name
• net.sf.jasperreports.export.xls.write.header
• net.sf.jasperreports.export.xls.repeat.value
• net.sf.jasperreports.export.xls.data

<detail>
  <band height="13">
  ...
    <textField>
      <reportElement style="OrderIdStyle" x="1" y="0" width="35" height="11">
        ...
        <property name="net.sf.jasperreports.export.xls.column.name" value="Order"/>
        ...
      </reportElement>
      ...
    </textField>
    <textField isStretchWithOverflow="true">
      <reportElement positionType="Float" x="40" y="0" width="110" height="11">
        ...
        <property name="net.sf.jasperreports.export.xls.column.name" value="ShipCity"/>
        ...
        <propertyExpression name="net.sf.jasperreports.export.xls.data"><![CDATA[$F{ShipCity}]] ></propertyExpression>
        ...
      </reportElement>
    </textField>
    <textField isBlankWhenNull="true">
      <reportElement x="155" y="0" width="25" height="11">
        ...
        <property name="net.sf.jasperreports.export.xls.column.name" value="ShipRegion"/>
        ...
      </reportElement>
      ...
    </textField>
    <textField pattern="dd/MM/yyyy">
      <reportElement x="185" y="0" width="50" height="11">
        ...
        <property name="net.sf.jasperreports.export.xls.column.name" value="OrderDate"/>
        ...
      </reportElement>
      ...
    </textField>
    <textField pattern="0.00">
      <reportElement x="235" y="0" width="35" height="11">
        ...
        <property name="net.sf.jasperreports.export.xls.column.name" value="Freight"/>
        ...
      </reportElement>
      ...
    </textField>
    ...
  </band>
</detail>

  <style name="Sans_Normal" isDefault="true" fontName="DejaVu Sans" fontSize="8" isBold="false" isItalic="false" isUnderline="false" isStrikeThrough="false"/>

  <style name="OrderIdStyle" style="Sans_Normal">
    <conditionalStyle>
      <conditionExpression><![CDATA[new Boolean($F{OrderID}.intValue() % 10 == 0)]] ></conditionExpression>
      <style forecolor="#ff0000" isBold="true"/>
    </conditionalStyle>
  </style>

<style name="alternateStyle" fontName="Arial" forecolor="red">
  <conditionalStyle>
    <conditionExpression><![CDATA[new Boolean($V{REPORT_COUNT}.intValue() % 2 == 0)]] ></conditionExpression>
    <style forecolor="blue"/>
  </conditionalStyle>
  <conditionalStyle>
    <conditionExpression><![CDATA[new Boolean($F{AMOUNT}.intValue() > 10000)]] ></conditionExpression>
    <style isBold="true"/>
  </conditionalStyle>
  <conditionalStyle>
    <conditionExpression><![CDATA[new Boolean($F{AMOUNT}.intValue() > 20000)]] ></conditionExpression>
    <style isBold="false" isItalic="true"/>
  </conditionalStyle>
</style>

• By default, the style condition expressions are evaluated during the report filling at the moment they are actually used. The conditional expression evaluation is performed with the current values of referenced variables and fields in that moment, regardless of the evaluationTime attribute of the element itself.If the condition expression of the style needs a delayed evaluation, just like the value of the text field or the image element that uses the conditional style, the net.sf.jasperreports.style.evaluation.time.enabled configuration property should be set to true.
• Conditional styles cannot be defined in style templates (see the Templates sample). This is because style templates store styles with properties available for a large group of reports, while conditional styles are styles depending on conditions, variables and parameters specific to each report apart. However, styles defined in style templates can be used as parent styles for conditional styles.
• Other examples of conditional styles can be found in Scriptlet, List and Crosstabs samples.