java multithreading on dom apis

Question

I have a function that does xml parsing. I want to make the function thread safe, but also as optimized (less blocking) as possible.
In short code is something as follows:

public Document doXML(InputStream s)
{
//Some processing.
  DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
  DocumentBuilder parser = factory.newDocumentBuilder();
  Document xmlDoc = parser.parse(is);
  return xmlDoc;

}

But I do not want to create a new DocumentBuilderFactory or DocumentBuilder in each call.
I want to reuse factory and parser, but I am not sure they are thread-safe. So what is the most optimal approach?
1) Cache a DocumentBuilderFactory in a class field and synchronize the factory.newDocumentBuilder(); so that each thread has its own instance of DocumentBuilder
2) Cache a DocumentBuilderFactory and DocumentBuilder and synchronize parser.parse(is); per thread
I think (2) is best, but is it safe to do it? Also can I avoid blocking by synchronized? I would like it to be as fast as possible.

Thanks?

Answer 1

If you are reusing thread (as in a thread pool) you can declare your DocumentBuilderFactory to be thread local. There is the overhead of creating a new set for each thread, but as I said, if you are reuising the subsequent overhead is very low.

final ThreadLocal<DocumentBuilderFactory> documentBuilderFactor = new ThreadLocal<DocumentBuilderFactory>(){
     public DocumentBuilderFactory  initialValue(){
       return  DocumentBuilderFactory.newInstance();
     }
}

public Document doXML(InputStream s)
{
//Some processing.
  DocumentBuilderFactory factory = documentBuilderFactor.get();
  DocumentBuilder parser = factory.newDocumentBuilder();
  Document xmlDoc = parser.parse(is);
  return xmlDoc;

}

Here you will only create one DocumentBuilderFactory for each thread.

I dont know if DocumentBuilder is thread safe when parsing (is it immutable?). But if DocumentBuilder is thread-safe when parsing you can use the same mechanism as I stated.

This resolution would make the overall throughput as fast as possible.

Note: This wasnt tested or compiled just gives an idea of what I am referring to.

Answer 2

2) would be thread safe but your app will only ever be able to parse one doc at a time.

Why not just use the code you have? Does

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder parser = factory.newDocumentBuilder();

have a demonstrably unacceptable overhead?