Merge Two XML Files in Java

Question

I have two XML files of similar structure which I wish to merge into one file. Currently I am using EL4J XML Merge which I came across in this tutorial. However it does not merge as I expect it to for instances the main problem is its not merging the from both files into one element aka one that contains 1, 2, 3 and 4. Instead it just discards either 1 and 2 or 3 and 4 depending on which file is merged first.

So I would be grateful to anyone who has experience with XML Merge if they could tell me what I might be doing wrong or alternatively does anyone know of a good XML API for Java that would be capable of merging the files as I require?

Many Thanks for Your Help in Advance

Edit:

Could really do with some good suggestions on doing this so added a bounty. I've tried jdigital's suggestion but still having issues with XML merge.

Below is a sample of the type of structure of XML files that I am trying to merge.

<run xmloutputversion="1.02">
    <info type="a" />
    <debugging level="0" />
    <host starttime="1237144741" endtime="1237144751">
        <status state="up" reason="somereason"/>
        <something avalue="test" test="alpha" />
        <target>
            <system name="computer" />
        </target>
        <results>
            <result id="1">
                <state value="test" />
                <service value="gamma" />
            </result>
            <result id="2">
                <state value="test4" />
                <service value="gamma4" />
            </result>
        </results>
        <times something="0" />
    </host>
    <runstats>
        <finished time="1237144751" timestr="Sun Mar 15 19:19:11 2009"/>
        <result total="0" />
    </runstats>
</run>

<run xmloutputversion="1.02">
    <info type="b" />
    <debugging level="0" />
    <host starttime="1237144741" endtime="1237144751">
        <status state="down" reason="somereason"/>
        <something avalue="test" test="alpha" />
        <target>
            <system name="computer" />
        </target>
        <results>
            <result id="3">
                <state value="testagain" />
                <service value="gamma2" />
            </result>
            <result id="4">
                <state value="testagain4" />
                <service value="gamma4" />
            </result>
        </results>
        <times something="0" />
    </host>
    <runstats>
        <finished time="1237144751" timestr="Sun Mar 15 19:19:11 2009"/>
        <result total="0" />
    </runstats>
</run>

Expected output

<run xmloutputversion="1.02">
    <info type="a" />
    <debugging level="0" />
    <host starttime="1237144741" endtime="1237144751">
        <status state="down" reason="somereason"/>
        <status state="up" reason="somereason"/>
        <something avalue="test" test="alpha" />
        <target>
            <system name="computer" />
        </target>
        <results>
            <result id="1">
                <state value="test" />
                <service value="gamma" />
            </result>
            <result id="2">
                <state value="test4" />
                <service value="gamma4" />
            </result>
            <result id="3">
                <state value="testagain" />
                <service value="gamma2" />
            </result>
            <result id="4">
                <state value="testagain4" />
                <service value="gamma4" />
            </result>
        </results>
        <times something="0" />
    </host>
    <runstats>
        <finished time="1237144751" timestr="Sun Mar 15 19:19:11 2009"/>
        <result total="0" />
    </runstats>
</run>

Answer 1

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.