Xalan: Transformation – Performance Enhancement

The major problem I face in my programming is the time taken to transform a source xml to result (html, pdf etc.) using stylesheet. Sometimes we use transformer provided without looking into details of transformer’s features and properties. Practically, we used to to find a short cut method to get transform done, unless and otherwise there was a specific need for performance in our project plan. We generally try to write a stylesheet. Use sun provided architecture classes for transformation then convert our XML document into html documents. This is not only me, I can say almost all programmers follow. We never think about features and properties provided by a particular API.

Here I am covering some performance enhancement tips to be followed while doing transformation.

XALAN – JAVA DTM

Introduction:

The Document Table Model (DTM) is an interface to a Document Model designed specifically for the needs of our XPath and XSLT implementations. The motivation behind this model is to optimize performance and minimize storage. In Document object model, it stores everything in memory as a String. Also it instantiates all dependent objects for a DOM object. String is an immutable object so there is a performance concern when the xml document is large in size. So to avoid this type of tree structure in memory, DTM avoids the overhead of instantiating the objects the standard DOM requires to represent a tree of nodes. DTM uses unique integer "handles" to identify nodes, integer ID values to represent URLs, local names, and expanded names, and integer index and length references to a string buffer to represent the text value of each node.

In general, the "read" APIs to DTM resemble those of the W3C Document Object Model (DOM) interface. However, in place of the DOM object tree of nodes, DTM uses integer arrays and string pools to represent the structure and content of the XML document to be transformed. DTM also structures the document's contents slightly differently, to better match the XPath data model; some details and constraints present in a standard DOM are suppressed, and a few XPath-specific features are added.

DTM is a read-only model, and so it does not have capability to create/modify nodes as the DOM's write or create-node operations.

The details of constructing a DTM vary depending on which implementation of this API you are using. Two reference implementations are currently available:

SAX2DTM (built via a SAX stream)

DOM2DTM (which provides DTM access to an existing DOM)

Both DTMs can be built incrementally (see incremental transforms). When operating incrementally, the DTM allows the Xalan-Java processor to begin reading the DTM and performing the transformation while the DTM is still being assembled (for example, while the parser is still parsing the XML source), and attempts to do only as much work as is needed to support the read requests actually made by the XPath or XSLT processor.

For the convenience of user-written extensions, a proxy mechanism presents the contents of the DTM as a read-only subset of the DOM.

DTM Performance Setting

Xalan-Java implements two DTM performance features that you can control with the TransformerFactory setAttribute(String name, Object value) method.

Att

Both of these DTM settings are described below:

·        http://xml.apache.org/xalan/features/incremental:

Set this feature to true to enable incremental transformations. If set to false (the default), the transform and the parse are performed on the same thread.

Note: When this feature is set to true: If the parser is Xerces, we perform an incremental transform on a single thread using the Xerces "parse on demand" feature. If the parser is not Xerces, we run the transform in one thread and the parse in another. Exception: if the parser is not Xerces and the XML source is a DOMSource, setting this feature to true has no effect.

 Note: The incremental feature is not currently supported by the XSLT Compiling processor, XSLTC. I will be updating this section when this feature is supported by the XSLTC.

Example of setting incremental transforms to true (for the XSLT Interpretive processor):

javax.xml.transform.TransformerFactory tFactory =

            javax.xml.transform.TransformerFactory.newInstance();

            // setAttribute() takes a String and an Object.           

            tFactory.setAttribute

            ("http://xml.apache.org/xalan/features/incremental",

             java.lang.Boolean.TRUE);

·        http://xml.apache.org/xalan/features/optimize:

Set this feature to true(the default), this feature enables optimizations that may involve structural rewrites of the stylesheet. Any tool that requires direct access to the stylesheet structure should set this feature to false.