Saxon-HE Java 扩展 - 如何访问作为参数传递的 xsl 变量的值？

Question

I have created a function using the Saxon documentation which has 3 parameters. The function takes an input string and pads it out to a specific size using an integer and string values.

padStringLeft(inputStr,size,padChar)

If I put this in my XSLT and hard wire the parameters the function works.

<debug1><xsl:value-of select="c4j_XSLT_Ext_padStringLeft:padStringLeft('1',4,'0')" /></debug1>

The output from the above would be '0001'

When I pass the contents of a XSLT variable however and set a debug / break point in my java function I can see that I'm getting param0 as a lazysequence.

<debug2><xsl:value-of select="c4j_XSLT_Ext_padStringLeft:padStringLeft($myvar,4,'0')" /></debug2>

Java function

As my code is attempting to treat it as a string it does not work.

How should I be handling this scenario, how do I access the value or the xsl-variable/param and what if sometimes I want to use a literal string instead of a variable?

public class XSLT_Ext_padStringLeft extends ExtensionFunctionDefinition
{

    @Override
    public SequenceType[] getArgumentTypes()
    {
        return new SequenceType[]{SequenceType.SINGLE_STRING,SequenceType.SINGLE_INTEGER, SequenceType.SINGLE_STRING};
    }

    @Override
    public StructuredQName getFunctionQName()
    {
        return new StructuredQName("c4j_XSLT_Ext_padStringLeft", "http://com.commander4j.Transformation.XSLT_Ext_padStringLeft", "padStringLeft");
    }

    @Override
    public SequenceType getResultType(SequenceType[] arg0)
    {
         return SequenceType.SINGLE_STRING;
    }

    @Override
    public ExtensionFunctionCall makeCallExpression()
    {
           return new ExtensionFunctionCall() {
                @Override
                public Sequence call(XPathContext context, Sequence[] arguments) throws XPathException {
                    
                    String inputStr;
                    try
                    {
                        inputStr = ((StringValue)arguments[0]).getStringValue();
                    } catch (ClassCastException ex)
                    {
                       inputStr = "";   
                    }

                    
                    long size;
                    try
                    {
                        String temp =arguments[1].toString();
                        size = Integer.valueOf(temp);
                    } catch (ClassCastException ex)
                    {
                        size = 1;   
                    }
                    
                    String padStr;
                    try
                    {
                        padStr = ((StringValue)arguments[2]).getStringValue();
                    } catch (ClassCastException ex)
                    {
                        padStr = "";    
                    }
                    
                    String result = inputStr;
                    
                    while (result.length() < size)
                    {
                        result = padStr + result;
                    }

                    return StringValue.makeStringValue(result);
                }
            };
    }

}

Thanks

Dave

Answer 1

In general the parameters are passed as instance of the class net.sf.saxon.om.Sequence , and you should only use the methods on the interface Sequence , rather than examining what particular kind of Sequence it is, because that could change in the future.

If you're expecting a singleton sequence (that is, a single item), call head() to get the first item in the sequence (this will return null if the sequence is empty). You will then have an instance of net.sf.saxon.om.Item . (The Sequence might already be an Item , because an item is a sequence, but you can't rely on that, and calling head() is safer than casting.) If you're expecting a string, you can safely call getStringValue() on this item to get the value as a string.

Also note, Saxon uses lazy evaluation wherever possible, which means that the string might not actually be computed until someone asks for its value. This means that innocent-looking calls like head() and getStringValue() can actually throw exceptions, and you need to be prepared for this.

So in short, you should replace

inputStr = ((StringValue)arguments[0]).getStringValue();

with

inputStr = arguments[0].head().getStringValue();

Answer 2

Also note, Saxon uses lazy evaluation wherever possible, which means that the string might not actually be computed until someone asks for its value. This means that innocent-looking calls like head() and getStringValue() can actually throw exceptions, and you need to be prepared for this.

So if I understand you correctly - when I call Transform to process the XSLT transformation it will call each of my custom java external functions as needed but the reference to

inputStr = arguments[0].head().getStringValue();

could generate an exception?

I would then need to do something within the java function to force it to get the value - or would I let the exception propogate back to the calling Transformation and catch it there?

Dave