Using Groovy as a Language for Expressions

The modular architecture of JasperReports provides a way to plug in support for languages other than Java. By default, the library supports Groovy and JavaScript.

Groovy is a full language for the Java 2 Platform. Inside the Groovy language you can use all classes and JARs that are available for Java. The following table compares some typical JasperReports expressions written in Java and Groovy:

Groovy and Java code samples

Expression	Java	Groovy
Field	$F{field_name}	$F{field_name}
Sum of two double fields	new Double($F{f1}.doubleValue() + $F{f2}.doubleValue())	$F{f1} + $F{f2}
Comparison of numbers	new Boolean($F{f}.intValue() == 1)	$F{f} == 1
Comparison of strings	new Boolean($F{f} != null && $F{f}.equals("test"))	$F{f} == "test"

The following is a correct Groovy expression:

new JREmptyDataSource($F{num_of_void_records})

JREmptyDataSource is a class of JasperReports that creates an empty record set (meaning with the all fields set to null). You can see how you can instance this class (a pure Java class) in Groovy without any problem. At the same time, Groovy allows you to use a simple expression like this one:

5+5

The language automatically encapsulates the primitive value 10 (the result of that expression) in a proper object. Actually, you can do more: you can treat this value as an object of type String and create an expression such as:

5 + 5+ ”my value”

Whether or not such an expression resolves to a rational value, it is still a legal expression and the result is an object of type String with the value:

10 my value

Hiding the difference between objects and primitive values, Groovy allows the comparison of different types of objects and primitive values, such as the legal expression:

$F{Name} == “John”

This expression returns true or false, or, again:

$F{Age} > 18	Returns true if the Age object interpreted as a number is greater than 18.
“340” < 100	Always returns false.
“340”.substring(0,2) < 100	Always returns true (since the substring method call produces the string “34”, which is less than 100).

Groovy provides a way to greatly simplify expressions and never complains about null objects that can crash a Java expression throwing a NullPointerException. It really does open the doors of JasperReports to people who don’t know Java.