Your code looks correct. But prefer using the new API:
final SvnOperationFactory svnOperationFactory = new SvnOperationFactory();
try {
final ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
final SvnDiffGenerator diffGenerator = new SvnDiffGenerator();
diffGenerator.setBasePath(new File(""));
final SvnDiff diff = svnOperationFactory.createDiff();
diff.setSources(SvnTarget.fromURL(url1, svnRevision1), SvnTarget.fromURL(url2, svnRevision1));
diff.setDiffGenerator(diffGenerator);
diff.setOutput(byteArrayOutputStream);
diff.run();
} finally {
svnOperationFactory.dispose();
}