Get DTD from an XML file

How can I get in Java the DTD file name specified in an xml file?

So, if I have:

<!DOCTYPE TEI SYSTEM "dtd-file.dtd"  [
[
<!ENTITY c24r SYSTEM "c2r.jpg" NDATA JPEG>
<!NOTATION JPEG SYSTEM "image/jpeg">
<!ELEMENT figure EMPTY>
<!ATTLIST figure开发者_JAVA百科 entity CDATA #REQUIRED>
]>

I want the string "dtd-file.dtd"

To get the document type's system ID use DocumentType#getSystemId():

Document document = documentBuilder.parse(someXmlInputStream);
String systemId = document.getDoctype().getSystemId();