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Use external functions with XQuery

When using an XQuery expression that uses external functions, declare the function signatures either in the XQuery prolog as external functions or using an XStaticContext instance. Supply (or bind) a Java implementation for each function using an XDynamicContext instance.

1. When preparing an XQuery expression that uses external functions, declare the function signatures in the XQuery prolog or using an XStaticContext instance.

   Functions declared in the XStaticContext are only visible to the main module. For an external function to be visible to a library module, it must be declared in the prolog of that library module.

   The XStaticContext interface has two declareFunction methods that each have three parameters—one for the name, one for the return type of the function, and an array for the types of the arguments. The name is always provided as a QName object, but the types can be QNames or XSequenceTypes. The function name, return type, and argument types must uniquely identify the function.

   XStaticContext declareFunction methods.

   This table explains when to use each form of the declareFunction method.

   Method Signature	Purpose
   declareFunction(QName name, QName type, QName[] argTypes)	Use when the return value and arguments of the function are all single atomic values The type QNames must refer to built-in types or global types declared in a schema that has been registered on the XFactory instance used to create the XStaticContext instance. If a QName refers to a non-atomic type, the processor will treat it as the type element(*, ns:type), where ns:type is the given QName. The XTypeConstants interface has convenient constants available that provide a QName object for each built-in type.
   declareFunction(QName name, XSequenceType type, XSequenceType[] argTypes)	Use when any of the arguments or the return value of the function is a node or a sequence of atomic values or nodes

   The following XQuery expression uses three functions, two of which are declared in the prolog.

   declare namespace xs = "http://www.w3.org/2001/XMLSchema";
   declare namespace trig = "http://www.example.org/trigonometry";
   declare function trig:arctan($ratio as xs:double) as xs:double external;
   declare function trig:sin($angle as xs:double) as xs:double external;
   
   <ramps> {
       for $ramp in ramps/ramp
       let $angleRadians := trig:arctan($ramp/height div $ramp/base)
       let $angle := trig:toDegrees($angleRadians)
       let $length := $ramp/height div trig:sin($angleRadians)
       return         element ramp
           {
               $ramp/height,
               $ramp/base,
               element angle { $angle },
               element length { $length }
           }}
   </ramps>

   Assuming the query above is available using the xquerySource Source object, the code below prepares the query. The function that was not declared in the query itself, trig:toDegrees, is declared on the XStaticContext instance.

   // Create the factory
   XFactory factory = XFactory.newInstance();
   
   // Create a new static context XStaticContext staticContext = factory.newStaticContext();
   
   // Declare a namespace for the functions
   staticContext.declareNamespace("trig", "http://www.example.org/trigonometry");
   
   // Create a QName for the trig:toDegrees function QName toDegreesQName = new QName("http://www.example.org/trigonometry", "toDegrees");
   
   // Declare the function on the static context staticContext.declareFunction(toDegreesQName, XTypeConstants.DOUBLE_QNAME, new QName[]{XTypeConstants.DOUBLE_QNAME});
   
   // Create an XQuery executable for the query XQueryExecutable executable = factory.prepareXQuery(xquerySource, staticContext);

2. To execute an XQuery expression that uses external functions, supply (or bind) the Java methods that implement the functions using an XDynamicContext instance.

   The bindings for external functions will be available to the main module and to any library modules that have an external function declaration in their prolog for that function.

   Use Java reflection to obtain a java.lang.reflect.Method object for the function. If the method is an instance method, an instance object is required when binding this function.

   An error is raised if we do not supply a Java method for a function used when executing the XQuery expression.

   The XDynamicContext has two bindFunction methods. Each requires a QName object corresponding to the name of the function and a Method object identifying the Java method that will provide the implementation for the function.

   XDynamicContext bindFunction methods.

   This table explains when to use each form of the XDynamicContext bindFunction methods.

   Method Name	Purpose
   bindFunction(QName qname, Method method)	Use when binding a static method
   bindFunction(QName qname, Method method, Object instanceObject)	Use when binding an instance method

   The following example executes the XQuery expression prepared in the first example, first binding methods for the functions it uses. In this example, the static atan, sin, and toDegrees methods of the java.lang.Math class are used to provide the implementations for the external functions.

   // Create a new dynamic context XDynamicContext dynamicContext = factory.newDynamicContext();
   
   // Retrieve the java.lang.reflect.Method object for the trig:toDegrees function Method toDegreesMethod = Math.class.getMethod("toDegrees", Double.TYPE);
   
   // Bind the function to the dynamic context dynamicContext.bindFunction(toDegreesQName, toDegreesMethod);
   
   // Create QNames for the trig:arctan and trig:sin functions
   QName arctanQName = new QName("http://www.example.org/trigonometry", "arctan");
   QName sinQName = new QName("http://www.example.org/trigonometry", "sin");
   
   // Retrieve the java.lang.reflect.Method objects for the trig:arctan and trig:sin functions
   // then bind them to the dynamic context Method arctanMethod = Math.class.getMethod("atan", Double.TYPE);
   Method sinMethod = Math.class.getMethod("sin", Double.TYPE);
   dynamicContext.bindFunction(arctanQName, arctanMethod);
   dynamicContext.bindFunction(sinQName, sinMethod);
   
   // Create an XML input document
   String xml = "<ramps>" +
   "<ramp><base>4</base><height>4</height></ramp>" +
   "<ramp><base>4</base><height>3</height></ramp>" +
   "<ramp><base>10</base><height>2</height></ramp>" +
   "</ramps>";
   StreamSource source = new StreamSource(new StringReader(xml));
   
   // Execute the query XSequenceCursor result = executable.execute(source, dynamicContext);
    
   // Serialize the result to System.out result.exportItem(new StreamResult(System.out));

Subtopics

• Use query-declared external functions with XQuery
  As an alternative to binding Java methods to functions in a query using the API, Java external functions can be declared directly within a query. The only additional configuration required is for bound Java classes to exist on the classpath during query execution.

Related concepts:

The XFactory class

Related

Use static and dynamic contexts
Use sequence types
Map XML types to Java types
Use query-declared external functions with XQuery

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