Is DocumentBuilder.parse() thread safe?

Question

Is the standard Java 1.6 javax.xml.parsers.DocumentBuilder class thread safe? Is it safe to call the parse() method from several threads in parallel?

The JavaDoc doe

Answer 1

Even though DocumentBuilder.parse appears not to mutate the builder it does on the Sun JDK default implementation (based on Apache Xerces). Eccentric design decision. What can you do? I guess use a ThreadLocal:

private static final ThreadLocal<DocumentBuilder> builderLocal =
    new ThreadLocal<DocumentBuilder>() {
        @Override protected DocumentBuilder initialValue() {
            try {
                return
                    DocumentBuilderFactory
                        .newInstance(
                            "xx.MyDocumentBuilderFactory",
                            getClass().getClassLoader()
                        ).newDocumentBuilder();
            } catch (ParserConfigurationException exc) {
                throw new IllegalArgumentException(exc);
            }
        }
    };

(Disclaimer: Not so much as attempted to compile the code.)

Answer 2

You can also check this code to make further optimization https://svn.apache.org/repos/asf/shindig/trunk/java/common/src/main/java/org/apache/shindig/common/xml/XmlUtil.java

Answer 3

There's a reset() method on DocumentBuilder which restores it to the state when it was first created. If you're going the ThreadLocal route, don't forget to call this or you're hosed.