Approaches
There are a few approaches:
Set the width of textField and set isStretchWithOverflow attribute as true. This solution depends on the font metrics.
Use a Java expression. For example you can use Guava library. In this case you should add import statement to the template.
Example
In this example, only the Title band is used to demonstrate both solutions. An empty datasource is used.
Report Template
Demonstrates both variants of solving the task.
The width of the first textField is enough only for showing 50 symbols. With help isStretchWithOverflow property the text will be split into several rows (the height of textField will increase dynamically).
The second textField's expression (look at textFieldExpression) uses the Guava library.
Joiner.on("\n").join(Splitter.fixedLength(50).split(value))
- The Splitter.fixedLength(int) method allow us to split string by every 50 characters and the Joiner.on(String) method help us to join strings with newline character.
<?xml version="1.0" encoding="UTF-8"?>
<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="break_lines" pageWidth="595" pageHeight="842" whenNoDataType="AllSectionsNoDetail" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20">
<property name="com.jaspersoft.studio.data.defaultdataadapter" value="One Empty Record"/>
<import value="com.google.common.base.*"/>
<parameter name="LONG_TEXT" class="java.lang.String">
<defaultValueExpression><![CDATA["012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789"]]></defaultValueExpression>
</parameter>
<title>
<band height="347" splitType="Stretch">
<textField isStretchWithOverflow="true">
<reportElement x="110" y="10" width="280" height="30"/>
<textFieldExpression><![CDATA[$P{LONG_TEXT}]]></textFieldExpression>
</textField>
<textField isStretchWithOverflow="true">
<reportElement x="20" y="100" width="525" height="30"/>
<textFieldExpression><![CDATA[Joiner.on("\n").join(Splitter.fixedLength(50).split($P{LONG_TEXT}))]]></textFieldExpression>
</textField>
</band>
</title>
</jasperReport>
Output result
The output result in Jaspersoft Studio will be:

A 150-character string is split into 3 rows, each row contains 50 characters.
` tag – Alex K Oct 19 '16 at 14:41