5.4.1. Templates API

public interface Templates {
    java.util.Properties getOutputProperties( );
    javax.xml.transform.Transformer newTransformer( )
            throws TransformerConfigurationException;
}

public Templates newTemplates(Source source)
        throws TransformerConfigurationException

As in earlier examples, the Source may obtain the XSLT stylesheet from one of many locations, including a filename, a system identifier, or even a DOM tree. Regardless of the original location, the XSLT processor is supposed to compile the stylesheet into an optimized internal representation.

Whether the stylesheet is actually compiled is up to the implementation, but a safe bet is that performance will continually improve over the next several years as these tools stabilize and vendors have time to apply optimizations.

Figure 5-6. Relationship between Templates and Transformer

5.4.2. A Stylesheet Cache

private static Map cache = new HashMap( );

static class MapEntry {
    long lastModified;  // when the file was modified
    Templates templates;

    MapEntry(long lastModified, Templates templates) {
        this.lastModified = lastModified;
        this.templates = templates;
    }
}

Removing entries from the cache is accomplished by one of two methods:

public static synchronized void flushAll( ) {
    cache.clear( );
}

public static synchronized void flush(String xsltFileName) {
    cache.remove(xsltFileName);
}

public static synchronized Transformer newTransformer(String xsltFileName)
        throws TransformerConfigurationException {

The parameter, an XSLT stylesheet filename, was chosen to facilitate the "last accessed" feature. We use the java.io.File class to determine when the file was last modified, which allows the cache to automatically reload itself as edits are made to the stylesheets. Had we used a system identifier or InputStream instead of a filename, the auto-reload feature could not have been implemented. Next, the File object is created and its lastModified flag is checked:

File xsltFile = new File(xsltFileName);

// determine when the file was last modified on disk
long xslLastModified = xsltFile.lastModified( );

The compiled stylesheet, represented by an instance of MapEntry, is then retrieved from the Map. If the entry is found, its timestamp is compared against the current file's timestamp, thus allowing auto-reload:

MapEntry entry = (MapEntry) cache.get(xsltFileName);

if (entry != null) {
    // if the file has been modified more recently than the
    // cached stylesheet, remove the entry reference
    if (xslLastModified > entry.lastModified) {
        entry = null;
    }
}

Next, we create a new entry in the cache if the entry object reference is still null. This is accomplished by wrapping a StreamSource around the File object, instantiating a TransformerFactory instance, and using that factory to create our Templates object. The Templates object is then stored in the cache so it can be reused by the next client of the cache:

// create a new entry in the cache if necessary
if (entry == null) {
    Source xslSource = new StreamSource(xsltFile);

    TransformerFactory transFact = TransformerFactory.newInstance( );
    Templates templates = transFact.newTemplates(xslSource);

    entry = new MapEntry(xslLastModified, templates);
    cache.put(xsltFileName, entry);
}

return entry.templates.newTransformer( );

Returning a new Transformer is critical because, although the Templates object is thread-safe, the Transformer implementation is not. Each caller gets its own copy of Transformer so multiple clients do not collide with one another.

One potential improvement on this design could be to add a lastAccessed timestamp to each MapEntry object. Another thread could then execute every couple of hours to flush map entries from memory if they have not been accessed for a period of time. In most web applications, this will not be an issue, but if you have a large number of pages and some are seldom accessed, this could be a way to reduce the memory usage of the cache.