private final static DocumentBuilder BUILDER;
static {
final DocumentBuilderFactory factory =
DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true);
factory.setIgnoringComments(true);
DocumentBuilder builder = null;
try {
builder = factory.newDocumentBuilder();
} catch (ParserConfigurationException e) {
e.printStackTrace();
}
BUILDER = builder;
}
When I used this construct by multiple threads, I stumbled across the following exception:
org.xml.sax.SAXException: FWK005 parse may not be called while parsing
A quick search with google came up with results like this.Bad news: Parsing XML in Java is not thread safe. An immediate solution is synchronization. However, synchronization adds a lot of overhead and is actually only needed, if calls are actually coming from different threads. A better solution for me was to use ThreadLocal. My current soultion in JVoiceXML is the following:
private static final ThreadLocal
= new ThreadLocal
@Override
protected DocumentBuilder initialValue() {
final DocumentBuilderFactory factory =
DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true);
factory.setIgnoringComments(true);
DocumentBuilder builder = null;
try {
builder = factory.newDocumentBuilder();
} catch (ParserConfigurationException e) {
e.printStackTrace();
}
return builder;
};
};
The implementation of ThreadLocal usage is very fast. However, creation of new objects is also very fast in current JVMs. I still have to check if caching really makes sense in this scenario, espacially, in the light of possible memory leaks as indicated by Nancy Deschenes.
