Converting Method Arguments - General Rules

Having decided which method to call, Saxon has to convert the supplied XPath argument values to the Java objects required by this method.

The conversion is done as follows:

  1. If there is an equivalent XPath type to the declared type of the Java method or constructor, then the supplied value is first converted to this equivalent XPath type using the standard function conversion rules used in all XPath function calls (without 1.0 backwards compatibility mode): for example an untyped attribute node can be supplied to a function that expects a string. The conversion from the equivalent XPath type to the actual Java type is then trivial, for example an xs:string value is converted to the corresponding java.lang.String.
  2. If there is no such equivalent type, the conversion from an XPath value to a Java value follows the rules given below.

The equivalent XPath type is determined as follows:

Where there is no equivalent XPath type, the following rules apply:

If the expected type is Object, the supplied value must either be a singleton, or an empty sequence. If it is an empty sequence, null will be passed. If it is a singleton node, an instance of will be passed. If it is a wrapped Java object, that Java object will be passed. If it is a singleton atomic value, the value will be converted to the nearest equivalent Java object: for example an xs:boolean becomes java.lang.Boolean, an xs:string becomes java.lang.String, and so on. An untyped atomic value is treated as a string. An xs:integer (even if it belongs to a subtype such as xs:short) is converted to a Java BigInteger. The more specialized XML Schema primitive types such as xs:hexBinary and xs:duration are passed in their native Saxon representation (a subclass of net.sf.saxon.value.AtomicValue).

If the expected type is one of the Saxon-specific classes (SequenceIterator, Sequence, Item, AtomicValue, SequenceExtent), then the value is passed unchanged. An error occurs if the supplied value contains more than one item and the expected type does not allow this.

The types ZeroOrMore, OneOrMore, ZeroOrOne, and One can be used to specify the permitted cardinality, and can also be parameterized to define the permitted item type. For example, an argument defined as One<StringValue> is equivalent to the XPath SequenceType string, and allows a single string only. When such declarations are used, the conversion of the supplied argument to the required type follows the standard XPath conversion rules precisely.

If the expected type implements java.util.Collection, Saxon attempts to convert each value in the supplied sequence to the most appropriate Java class, following the same rules as when converting a singleton to java.lang.Object. This process takes no account of parameterized collection types (such as List<String>). If the required collection type accepts an java.util.ArrayList, Saxon will create an ArrayList to hold the values; otherwise it will attempt to instantiate the required type of collection, which will only work if it is a concrete class with a zero-argument public constructor (so it will fail, for example, if the required type is java.util.Set). If an empty sequence is supplied as the argument value, this is converted to an empty Collection.

If the required type is an array, Saxon will attempt to create an array of the required type. This will not always succeed, for example if the array has type X[] where X is an interface rather than a concrete class. If it is an array of items or nodes, the nodes in the supplied sequence will be inserted into the array directly; if it is an array of a type such as integer or double, the sequence will first be atomized.