Tuesday, September 15, 2009

Cache Jersey JAXB Marshaller and Unmarshaller Instances

If you create new JAXB Marshaller and Unmarshaller instances often then performance suffers. To avoid unnecessary creations you should cache and reuse your Marshaller and Unmarshaller instances.

The reason performance is affected is that creation of each new Marshaller or Unmarshaller instance results (indirectly) in the creation of a new SAXParserFactory deep in the bowels of JAXB. This creation of a new SAXParserFactory for each Marshaller or Unmarshaller can be very expensive. SAXParserFactory creation involves a service lookup that scans the classpath for “javax.xml.parsers.SAXParserFactory”. If your classpath includes directories or includes jars that are indexed then a lot of file system activity can occur and affect your performance.

Ideally it would be nice if JAXB did not create so many SAXParserFactory instances. Factory creation can be expensive and it is not a good strategy to perform factory lookups so often. Why does JAXB create so many SAXParserFactory instances? Is it a bug in JAXB? Typically when such code is found in applications it is considered bad coding practice.

After thinking about it some I came to the conclusion that it probably isn’t a bug in JAXB. These are smart guys and I couldn’t imagine them making this kind of choice on accident. Though I don’t know the exact reasoning I assume it relates to the fact that JAXB is a low-level library that can be used from many different contexts and it is difficult for JAXB to make a choice that meets all use cases.

Unfortunately this behavior is deep in the bowels of JAXB in protected or private methods and does not appear easy to change externally. In the case of Unmarshaller, for example, the factory creation is found in the protected method AbstractUnmarshallerImpl.getXMLReader(). Your best bet to avoid the expense is to simply not create Marshaller or Unmarshaller instances more often than necessary.

The fix this problem in my Jersey-based REST service I created my own custom Jersey ContextResolver for Marshaller and Unmarshaller. The custom resolver maintains a thread-based cache of Marshaller and Unmarshaller instances and thereby avoids unnecessary creation of fresh instances and the corresponding expense of the excessive SAXParserFactory creations.

1 comment:

  1. Hello David,

    Found your article informative and I guess I can benefit from it too. Well, also wanted to get your opinion on an Jersey based rest app that I am designing..
    I am using JAXB generated classes as transfer objects and supporting both XML and JSON. I also have simple response and error transfer objevt types and I wanted to handle all content conversions at one place, so was thinking of having a filter (which would filter out unspported requests and also convert all objects to appropriate String (xml or json) using jaxb marshaller/unmarshaller and jackson..
    Do you think having a filter is a good idea or do you think some other approach would be better