Converting Wrapped Java Objects

Saxon allows an extension function to return an arbitrary Java object. This will then be wrapped as an XPath item, so that it can be held in a variable and passed subsequently as an argument to another extension function. This second extension function will see the original Java object minus its wrapper, provided it is declared to expect the appropriate Java class.

A wrapped Java object always holds a non-null object value. The Java value null converts to the XPath empty sequence.

A wrapped Java object may be converted to another data type as follows.

• It is converted to a string by calling its toString() method.

• It is atomized by calling its toString() method.

• It is converted to a number by converting it first to a string, and then applying the XPath number() conversion.

• The effective boolean value of a wrapped Java object (like that of a node) is always true.

The type of a wrapped Java object may be declared in a variable declaration or function signature using a type name whose namespace URI is http://saxon.sf.net/java-type, and whose local name is the fully qualified name of the Java class, with any "$" signs replaced by hyphens, and with leading "[" characters replaced by "_-". For example, the sql:connection extension function returns a value of type {http://saxon.sf.net/java-type}java.sql.Connection.

Note that a call on a constructor function (using prefix:new()) always returns a wrapped Java object, regardless of the class. But a call on a static method, instance-level method, or field will return a wrapped Java object only if the result is a class that Saxon does not specifically recognize as one that it can convert to a regular XPath value. Such classes include String, Long, Double, Date, BigInteger, URI, List and so on.

In Saxon 9.4 and earlier releases, a wrapped Java object was considered to be an atomic value (in the XDM type hierarchy, the type representing java.lang.Object was a peer of primitive types such as xs:string and xs:boolean). In Saxon 9.5, this changed so that wrapped objects are a fourth subtype of item(), at the same level in the type hierarchy as nodes, atomic values, and function items. Atomizing a wrapped Java object now produces an instance of xs:string containing the result of the toString() method applied to the underlying object.