how to override a service provider in java

问题:

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

回答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.

回答2:

You can just do like this to specify the XMLOutputFactory implementation You want to use:

System.setProperty("javax.xml.stream.XMLOutputFactory", ... full classname You want to use ...);

Source: http://docs.oracle.com/cd/E17802_01/webservices/webservices/docs/1.6/tutorial/doc/SJSXP4.html

Deriving from JAXP, the XMLInputFactory.newInstance() method determines the specific XMLInputFactory implementation class to load by using the following lookup procedure:

1. Use the javax.xml.stream.XMLInputFactory system property.
2. Use the lib/xml.stream.properties file in the JRE directory.
3. Use the Services API, if available, to determine the classname by looking in the META-INF/services/javax.xml.stream.XMLInputFactory files in jars available to the JRE.
4. Use the platform default XMLInputFactory instance.

回答3:

I discovered that if I put the service file under WEB-INF/classes/services/javax.xml.stream.XMLOutputFactory then it will be first in classpath and before jars in WEB-INF/lib. and that's my solution.

回答4:

We had similar issue where parsing would run in local but fail on server. After debugging found server is using reader com.ctc.wstx.evt.WstxEventReader

Whereas on local reader was com.sun.xml.internal.stream.XMLEventReaderImpl

We set following property to resolve it.

System.setProperty("javax.xml.stream.XMLInputFactory", "com.sun.xml.internal.stream.XMLInputFactoryImpl");

回答5:

If your implementation is in a jar then make sure it is before woodstox.jar on the class path, then FactoryFinder will use your implementation.