Understanding Fields

A print is commonly created starting from a data source that provides a set of records composed of a series of fields. This behavior is exactly like obtaining the results of an SQL query.

Jaspersoft Studio displays available fields as children of the Fields node in the document outline view. To create a field, right-click the Fields node and select Create Field. The new field is included as an undefined entry on the Properties tab. You can configure the field properties by selecting it.

Press the Object button to name your field, enter a description, and choose a class.

Field Properties Tab - Object

Press the Advanced button to enter advanced properties for the field.

Field Properties Tab - Advanced

A field is identified by a unique name, a type, and an optional description. You can also define a set of name/value pair properties for each field. These custom properties are not generally used by JasperReports, but they can be used by external applications or by some custom modules of JasperReports (such as a special query executor).

Jaspersoft Studio determines the value for a field based on your data source. For example, when using a SQL query to fill a report, Jaspersoft Studio assumes that the name of the field is the same as the name of a field in the query result set. You must ensure that the field name and type match the field name and type in the data source. You can systematically declare and configure large numbers of fields using the tools provided by Jaspersoft Studio. Since the number of fields in a report can be quite large (possibly reaching the hundreds), Jaspersoft Studio provides different tools for handling declaration fields retrieved from particular types of data sources.

Inside each report expression (like the one used to set the content of a textfield) Jaspersoft Studio specifies a field object, using the following syntax:

$F{<field name>}

where <field name> must be replaced with the name of the field. When using a field expression (for example, calling a method on it), keep in mind that it can have a value of null, so you should check for that condition. An example of a Java expression that checks for a null value is:

($F{myField} != null) ? $F{myFiled}.doSomething() : null

This method is generally valid for all the objects, not just fields. Using Groovy or JavaScript this is rarely a problem, since those languages handle a null value exception in a more transparent way, usually returning an empty string.

In some cases a field can be a complex object, like a JavaBean, not just a simple value like a String or an Integer. A trick to convert a generic object to a String is to concatenate it to a empty string this way:

$F{myfield}+ “”

All Java objects can be converted to a string; the result of this expression depends on the individual object implementation (specifically, by the implementation of the toString() method). If the object is null, the result returns the literal text string “null” as a value.

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