After setting the property "org.xml.sax.parser" to "org.apache.crimson.parser.Parser" and I instantiated ParserAdapter. I have
type-casted the object to XMLReader and invoked parse(InputSource) method on it.
(Here I created the InputSource for an existing xml file using FileInputStream)

parse method throws the following exception:

java.lang.NullPointerException
        at org.apache.crimson.parser.Parser2.parseInternal(Parser2.java,
Compiled Code)
        at org.apache.crimson.parser.Parser2.parse(Parser2.java, Compiled Code)
        at org.apache.crimson.parser.XMLReaderImpl.parse(XMLReaderImpl.java,
Compiled Code)
        at org.apache.crimson.parser.Parser.parse(Parser.java, Compiled Code)
        at org.xml.sax.helpers.ParserAdapter.parse(ParserAdapter.java, Compiled
Code)
        at LocatorTest.main(LocatorTest.java, Compiled Code)