Not very elegant, but you could do this with the DOM parser and XPath:
public class MergeXmlDemo {
public static void main(String[] args) throws Exception {
// proper error/exception handling omitted for brevity
File file1 = new File("merge1.xml");
File file2 = new File("merge2.xml");
Document doc = merge("/run/host/results", file1, file2);
print(doc);
}
private static Document merge(String expression,
File... files) throws Exception {
XPathFactory xPathFactory = XPathFactory.newInstance();
XPath xpath = xPathFactory.newXPath();
XPathExpression compiledExpression = xpath
.compile(expression);
return merge(compiledExpression, files);
}
private static Document merge(XPathExpression expression,
File... files) throws Exception {
DocumentBuilderFactory docBuilderFactory = DocumentBuilderFactory
.newInstance();
docBuilderFactory
.setIgnoringElementContentWhitespace(true);
DocumentBuilder docBuilder = docBuilderFactory
.newDocumentBuilder();
Document base = docBuilder.parse(files[0]);
Node results = (Node) expression.evaluate(base,
XPathConstants.NODE);
if (results == null) {
throw new IOException(files[0]
+ ": expression does not evaluate to node");
}
for (int i = 1; i < files.length; i++) {
Document merge = docBuilder.parse(files[i]);
Node nextResults = (Node) expression.evaluate(merge,
XPathConstants.NODE);
while (nextResults.hasChildNodes()) {
Node kid = nextResults.getFirstChild();
nextResults.removeChild(kid);
kid = base.importNode(kid, true);
results.appendChild(kid);
}
}
return base;
}
private static void print(Document doc) throws Exception {
TransformerFactory transformerFactory = TransformerFactory
.newInstance();
Transformer transformer = transformerFactory
.newTransformer();
DOMSource source = new DOMSource(doc);
Result result = new StreamResult(System.out);
transformer.transform(source, result);
}
}
This assumes that you can hold at least two of the documents in RAM simultaneously.