Advanced Topics - eXist: A NoSQL Database and Application Development Platform - page 471

We have to return a string that describes which version of eXist this module
became available in. This is just for documentation purposes and is not further
processed.
To implement the actual “Hello World” function, which will be named hello-world ,
we need to extend the abstract class class org.exist.xquery.Function . To assist in
this, eXist provides the abstract subclass org.exist.xquery.BasicFunction , which
makes life much easier by dealing with the necessary mechanics for the XQuery pro‐
filer and extracting argument values that are passed to our function from the XQuery
context . Almost all of the internal module functions already implemented in eXist
extend BasicFunction , and we would recommend that you do the same unless you
need more control over processing (which is unlikely in most use cases). So now that
we have seen the preceding internal module implementation, which tells eXist about
our hello-world function, let's see how we actually implement the function in
Example 16-14 .
Example 16-14. HelloWorld Java XQuery function
public class HelloFunctions extends BasicFunction {
private final static QName qnHelloWorld =
new QName ( "hello-world" , HelloModule . NS , HelloModule . NS_PREFIX );
//signature of our XQuery h:hello-world() function
public final static FunctionSignature FNS_HELLO_WORLD =
new FunctionSignature (
qnHelloWorld ,
"Say \”hello world\”!" ,
null ,
new FunctionReturnSequenceType (
Type . DOCUMENT , Cardinality . ONE , "The hello!"
)
);
//standard constructor, which allows multiple functions to be
//implemented in one class
public HelloFunctions ( final XQueryContext context ,
final FunctionSignature signature ) {
super ( context , signature );
}
//called when the xquery function is executed
@Override
public Sequence eval ( final Sequence [] args ,
final Sequence contextSequence ) throws XPathException {
final Sequence result ;