Just call parse:

xmlReader.parse("foo.xml");

You can also use open streams:

InputStream is = ...;
InputSource s1 = new InputSource(is);
xmlReader.parse(s1);

Reader r = ...;
InputSource s2 = new InputSource(r);
xmlReader.parse(r);

InputSource lets you specify a base URI, encoding, etc.

InputSource s = ...;
s.setSystemId("http://cde.berkeley.edu/bob.xml");
s.setEncoding("UTF-8");