How a system knows what class it should use to create instance of TransformerFactory?

[Evgeniy Strokin \(rT\)]

Hi,
I have a simple question:
When we write:
TransformerFactory tFactory =
TransformerFactory.newInstance();
How the system know which class it should use?
I tried print out System property
System.getProperty("javax.xml.transform.TransformerFactory")
but it returns null, but everything works perfectly,
it uses
org.apache.xalan.processor.TransformerFactoryImpl.
And how I can change it to another implementation?

Thanks, 
Jenya

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