Howto implement a node index extension to be shared across transform invocations on the same source document (DOMSource)

Hi,

I have a webapplication  where I hold a large XML tree in memory.  On 
this tree
I perform several transforms as response to user requests. 
Each transform declares a <xsl:key> that serves as an index for 
important nodes in the tree.

However, it is expensive to calculate this key and it should be possible 
to reuse it since I know that the source tree has not been modified.


Since modifying the Xalan <xsl:key> implementation is difficult, I tried 
my luck with a
 Java extension approach where I simply store selected nodes in a 
HashMap outside
the transform.

However, when I return a node from the extension then Xalan goes boom! 
if the node
was added in a previous transform.

Does anyone know how to make this work ?


Best regards
Christoffer Dam Bruun

// The Java extension

public class ElementIndex {

   public static Map<String, SingleIndex> indexes = new HashMap<String, 
SingleIndex>();

   public static Node key(String indexId, String key) {
       NodeSet ns = new NodeSet();
       SingleIndex idx = indexes.get(indexId);
       if (null == idx) {
           System.err.println("idx er null");
       }
       List<Node> nl = idx.get(key);
       if (null == nl) {
           return null;
       }
       return nl.get(0);
   }

   public static int add(ExpressionContext ec, String indexId, String 
key, NodeList nodes) {
       SingleIndex idx = indexes.get(indexId);
       if (null == idx) {
           idx = new SingleIndex();
           indexes.put(indexId, idx);
       }
       for (int i = 0; i < nodes.getLength(); i++) {
           Node tmp = nodes.item(i);
           idx.put(key, tmp);
       }
       return 0;
   }

   public static int remove(String indexId) {
       return 0;
   }

   public static int clear(String indexId) {
       return 0;
   }

   public static class SingleIndex {
       String idxName;
       Map<String, ArrayList<Node>> idx = new HashMap<String, 
ArrayList<Node>>();

       public List<Node> get(String key) {
           return idx.get(key);
       }

       public void put(String key, Node n) {
           List<Node> nl = idx.get(key);
           if (null == nl) {
               nl = new ArrayList<Node>();
               idx.put(key, (ArrayList<Node>) nl);
           }
           nl.add(n);
       }

       public void remove(String key, Node n) {
           List<Node> nl = idx.get(key);
           if (null == nl) {
               return;
           }
           nl.remove(n);
       }
   }
}


// Adding to the index:
<xsl:value-of select="idx:add('nidkey', @nid,current())"/>


// Getting nodes from the index
<xsl:variable name="task" select="idx:key('nidkey',@nid)"></xsl:variable>