External Styles in JasperReports

Use JasperReport templates. A JasperReports template is one that ends in .jrtx, and may look similar to this (styles.jrtx):

<?xml version="1.0"?>
<!DOCTYPE jasperTemplate
  PUBLIC "-//JasperReports//DTD Template//EN"
  "http://jasperreports.sourceforge.net/dtds/jaspertemplate.dtd">

<jasperTemplate>
    <style name="Report Title" isDefault="false" hAlign="Center" fontSize="24" isBold="true"/>
    <style name="Heading 1" isDefault="false" fontSize="18" isBold="true"/>
    <style name="Heading 2" isDefault="false" fontSize="14" isBold="true"/>
</jasperTemplate>

and then in your .jrxml file, include it as a template:

...
<template><![CDATA["styles.jrtx"]]></template>
...

iReport also understands this, so your styles are imported and shown in iReport correctly (though I did notice sometimes it wouldn't pick them up an a reload or recompile was necessary).

You can also avoid specifying the actual file name in the <template> element by using a parameter passed into your report at runtime

<parameter name="TEMPLATE_FILE" isForPrompting="false" class="java.lang.String"/>

<template><![CDATA[$P{TEMPLATE_FILE}]]></template>

where $P{TEMPLATE_FILE} is the full path to the style resource