Friday, June 17, 2016

Compile xsl files and store in cache to improve XSLT performance

Common code found online to do XSLT transformation. (removed non essential pieces for brevity)

TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer(new StreamSource(new File(xsltPath)));
transformer.transform(new StreamSource(new File(sourceFilePath)), new StreamResult(new File(resultPath)));

The code works. But if a xslt file is relatively big and  
needs to be used over and over again to transform 
a lot of files, for example, in the batch mode, 
it may not perform well. 

The following shows a way to cache the compiled version of an xsl file, which is a 'Templates' object. This object is thread safe.

Code snippet to cache the 'Templates' object.

static final Map<String, Templates> cacheTemplates = new ConcurrentHashMap<String, Templates>();

       static TransformerFactory transformFactory = null;

       static {

     private static void init() {
         try {
             transformFactory =TransformerFactory.newInstance();
        catch(Exception e) {
            throw new RuntimeException(e);

     public static void cacheCompiled( String xsl) {
File file = null;
  StreamSource source= null;
                Templates  templates = null;
try {
file = new File( xsl);
source = new StreamSource(file);
                         templates = transformFactory.newTemplates(source); //create this once for a file, save in a cache.
cacheTemplates .put(xsl, templates );
} catch (Exception e) {
throw new RuntimeException(e);
} finally {

The above 'templates' object is basically a coompiled version of the original xsl file.  If the original file is relatively big, for example, 20KB, it takes more than 2 seconds on my local machine to transform a small file.  Without caching the templates, it takes more than 2 seconds every time.  With caching,  it takes about 0.1 seconds  for every transformation after the first time.

The basic code is like this:

//get the Templates object from cache based on the xsl file name, then get a Transformer object

Transformer transformer = templates.newTransformer();

transformer.transform(new StreamSource(new File(sourceFilePath)),
new StreamResult(new File(resultPath)));

The 'transformer' object mentioned above is not thread safe.

The SAXON parser seems becoming more popular, and the Xalan parser seems fading away.

The home edition of the SAXON parser, which is free, may be good enough for a lot of applications.

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