how to override a service provider in java

Question

This is more a general question by example: I'm using xstream and woodstox, woodstox comes with a service provider for javax.xml.stream.XMLOutputFactory in woodstox jar registering com.ctc.wstx.stax.WstxOutputFactory. I want to provide my own javax.xml.stream.XMLOutputFactory and still have woodstox jar in the classpath. I know I can provide my own with the system property javax.xml.stream.XMLOutputFactory , but I'm trying to take off the hassle from our dev ops team and do it with a service file in my jar or maybe in my war's META-INF/services folder. looking the code of javax.xml.stream.FactoryFinder how can I make sure that my META-INF/services/javax.xml.stream.XMLOutputFactory file will be the one used by FactoryFinder?

we use xstream with camel and could not find a way to inject the factory to XStreamDataFormat

Answer 1

First: instead of relying on JDK SPI interface, I strongly recommend simplifying your life and NOT using it. It really adds no value over injecting XMLInputFactory and/or XMLOutputFactory yourself. For injection you can use Guice (or Spring); or just pass it manually. Since these factories do not have dependencies of their own, this is easy.

But if choose to (or have to) use XMLInputFactory.newInstance(), you can define a System property for "javax.xml.stream.XMLOutputFactory" and "javax.xml.stream.XMLInputFactory".

So why not use JDK approach? Multiple reasons:

1. It adds overhead: if you are not specifying System property, it will have to scan the whole classpath, and with big app servers this takes 10x-100x as long as most parsing
2. Precedence of implementations is undefined: if you multiple in classpath, which one will you get? Who knows... (and note: it might even change when you add new jars in classpath)
3. You are very likely to get multiple impl via transitive dependencies

Unfortunately, Oracle still seems to insist on adding this known-faulty method for registering service providers. Why? Probably because they do not have a DI lib/framework of their own (Guice is by google, Spring by Springsource), and they tend to be pretty control hungry.