Java: need help with optimizing a part of code

I have a simple code for transforming XML, but it is very time consuming (I have to repeat it many times). Does anyone have a recommendation how to optimize this code? Thanks.

EDIT: This is a new version of the code. I unfortunatelly can't reuse Transformer, since XSLTRuleis in most of the cases different. I'm now reusing TransformerFactory. I'm not reading from files before this so I can't use StreamSource. Largest amount of time is spent on initialization of Transformer.

private static TransformerFactory tFactory = TransformerFactory.newInstance();

public static String transform(String XML, String XSLTRule) throws TransformerException {

    Source xmlInput = new StreamSource(new StringReader(XML));
    Source xslInput = new St开发者_开发知识库reamSource(new StringReader(XSLTRule));

    Transformer transformer = tFactory.newTransformer(xslInput);

    StringWriter resultWriter = new StringWriter();
    Result result = new StreamResult(resultWriter);
    transformer.transform(xmlInput, result);
    return resultWriter.toString();
}

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The first thing you should do is to skip the unnecessary conversion of the XML string to bytes (especially with a hardcoded, potentially incorrect encoding). You can use a StringReader and pass that to the StreamSource constructor. The same for the result: use a StringWriter and avoid the conversion.

Of course, if you call the method after converting your XML from a file (bytes) to a String in the first place (again with a potentially wrong encoding), it would be even better to have the StreamSource read from the file directly.

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It seems like you apply an XSLT to an XML file. To speed things up, you can try compiling the XSLT, like with XSLTC.

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I can only think of a couple of minor things:

• The TransformerFactory could be reused.

• The Transformer could be reused if it is thread confined, and the XSL input is the same each time.

• If you can estimate the output size reasonably accurately, you could create the ByteArrayOutputStream with an initial size hint.

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As stated in Michaels answer, you could potentially speed things up by not loading either the input or output xml entirely into memory yourself and make your api stream based.