Split XML into smaller chunks based on the id of the grandchild

I have an xml that should be split into smaller chunks by unique BookId node. Basically I need to filter out each book into separate xml having the same structure of the initial XML.

The purpose of that is - requirement to validate each smaller XML against XSD to determine which Book/PendingBook is not valid.

Note that Books node can contain both Book and PendingBook nodes.

Initial XML:

<Main xmlns="http://some/url/name">
<Books>
<Book>
<IdentifyingInformation>
<ID>
<Year>2021</Year>
<BookId>001</BookId>
<BookDateTime>2021-05-10T12:35:00</BookDateTime>
</ID>
</IdentifyingInformation>
</Book>
<Book>
<IdentifyingInformation>
<ID>
<Year>2020</Year>
<BookId>002</BookId>
<BookDateTime>2021-05-10T12:35:00</BookDateTime>
</ID>
</IdentifyingInformation>
</Book>
<PendingBook>
<IdentifyingInformation>
<ID>
<Year>2020</Year>
<BookId>003</BookId>
<BookDateTime>2021-05-10T12:35:00</BookDateTime>
</ID>
</IdentifyingInformation>
</PendingBook>
<OtherInfo>...</OtherInfo>
</Books>
</Main>

The result should be like next xmls:

Book_001.xml (BookId = 001):

<Main xmlns="http://some/url/name">
<Books>
<Book>
<IdentifyingInformation>
<ID>
<Year>2021</Year>
<BookId>001</BookId>
<BookDateTime>2021-05-10T12:35:00</BookDateTime>
</ID>
</IdentifyingInformation>
</Book>
<OtherInfo>...</OtherInfo>
</Books>
</Main>

Book_002.xml (BookId = 002):

<Main xmlns="http://some/url/name">
<Books>
<Book>
<IdentifyingInformation>
<ID>
<Year>2020</Year>
<BookId>002</BookId>
<BookDateTime>2021-05-10T12:35:00</BookDateTime>
</ID>
</IdentifyingInformation>
</Book>
<OtherInfo>...</OtherInfo>
</Books>
</Main>

PendingBook_003.xml (BookId = 003):

<Main xmlns="http://some/url/name">
<Books>
<PendingBook>
<IdentifyingInformation>
<ID>
<Year>2021</Year>
<BookId>003</BookId>
<BookDateTime>2021-05-10T12:35:00</BookDateTime>
</ID>
</IdentifyingInformation>
</PendingBook>
<OtherInfo>...</OtherInfo>
</Books>
</Main>

So far I fetched only each ID node into smaller xmls. And created root element manually.

Ideally I want to copy all elements from initial xml and put into Books node single Book/PendingBook node.

My java sample:

package com.main;
import java.io.File;
import java.util.ArrayList;
import java.util.List;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.transform.OutputKeys;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
public class ExtractXmls {
/**
* @param args
*/
public static void main(String[] args) throws Exception
{
String inputFile = "C:/pathToXML/Main.xml";
File xmlFile = new File(inputFile);
DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
Document doc = dBuilder.parse(xmlFile);
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true); // never forget this!
XPathFactory xfactory = XPathFactory.newInstance();
XPath xpath = xfactory.newXPath();
XPathExpression allBookIdsExpression = xpath.compile("//Books/*/IdentifyingInformation/ID/BookId/text()");
NodeList bookIdNodes = (NodeList) allBookIdsExpression.evaluate(doc, XPathConstants.NODESET);
//Save all the products
List<String> bookIds = new ArrayList<>();
for (int i = 0; i < bookIdNodes.getLength(); ++i) {
Node bookId = bookIdNodes.item(i);
System.out.println(bookId.getTextContent());
bookIds.add(bookId.getTextContent());
}
//Now we create and save split XMLs
for (String bookId : bookIds)
{
//With such query I can find node based on bookId
String xpathQuery = "//ID[BookId='" + bookId + "']";
xpath = xfactory.newXPath();
XPathExpression query = xpath.compile(xpathQuery);
NodeList bookIdNodesFiltered = (NodeList) query.evaluate(doc, XPathConstants.NODESET);
System.out.println("Found " + bookIdNodesFiltered.getLength() + " bookId(s) for bookId " + bookId);
//We store the new XML file in bookId.xml e.g. 001.xml
Document aamcIdXml = dBuilder.newDocument();
Element root = aamcIdXml.createElement("Main"); //Here I'm recreating root element (don't know if I can avoid it and copy somehow structure of initial xml)
aamcIdXml.appendChild(root);
for (int i = 0; i < bookIdNodesFiltered.getLength(); i++) {
Node node = bookIdNodesFiltered.item(i);
Node copyNode = aamcIdXml.importNode(node, true);
root.appendChild(copyNode);
}
//At the end, we save the file XML on disk
TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
DOMSource source = new DOMSource(aamcIdXml);
StreamResult result =  new StreamResult(new File("C:/pathToXML/" + bookId.trim() + ".xml"));
transformer.transform(source, result);
System.out.println("Done for " + bookId);
}
}
}

You almost got it to work. You could change your XPath in your loop iterating the book IDs to get the Book or PendingBook Element and then use it. Also you need to create Books element in addition to Main and append Book or PendingBook to the newly created Books Element.

The XPath is : //ancestor::*[IdentifyingInformation/ID/BookId=bookId]

It gets the ancestor of the element whose bookId matches to that of the ID in the current iteration i.e. the Book or PendingBook element.

//Now we create and save split XMLs
for (String bookId : bookIds)
{
//With such query I can find node based on bookId
String xpathQuery = "//ancestor::*[IdentifyingInformation/ID/BookId=" + bookId + "]";
xpath = xfactory.newXPath();
XPathExpression query = xpath.compile(xpathQuery);
NodeList bookIdNodesFiltered = (NodeList) query.evaluate(doc, XPathConstants.NODESET);
System.out.println("Found " + bookIdNodesFiltered.getLength() + " bookId(s) for bookId " + bookId);
//We store the new XML file in bookId.xml e.g. 001.xml
Document aamcIdXml = dBuilder.newDocument();
Element root = aamcIdXml.createElement("Main");
Element booksNode = aamcIdXml.createElement("Books");
root.appendChild(booksNode);
//Here I'm recreating root element (don't know if I can avoid it and copy somehow structure of initial xml)
aamcIdXml.appendChild(root);
String bookName = "";
for (int i = 0; i < bookIdNodesFiltered.getLength(); i++) {
Node node = bookIdNodesFiltered.item(i);
Node copyNode = aamcIdXml.importNode(node, true);
bookName = copyNode.getNodeName();
booksNode.appendChild(copyNode);
}
//At the end, we save the file XML on disk
TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
DOMSource source = new DOMSource(aamcIdXml);
StreamResult result =  new StreamResult(new File(bookName + "_" + bookId.trim() + ".xml"));
transformer.transform(source, result);
System.out.println("Done for " + bookId);
}

And also I modified code to name the file as you needed like Book_001.xml.

Consider XSLT, the special purpose language designed to transform XML files including extracting needed nodes. Additionally, you can pass parameters from application layer like Java into XSLT (just like SQL)!

Specifically, iteratively passed in the XPath retrieved BookIds by Java into XSLT named param. By the way, no extensive code re-factoring is needed since you already have the transformer set up to run XSLT!

XSLT (save as .xsl, a special .xml)

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output indent="yes" encoding="UTF-8"/>
<xsl:strip-space elements="*"/>
<!-- INITIALIZE PARAMETER  -->
<xsl:param name="param_bookId"/>
<!-- IDENTITY TRANSFORM -->
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="Books">
<xsl:copy>
<xsl:apply-templates select="Book[descendant::BookId = $param_bookId] |
PendingBook[descendant::BookId = $param_bookId]"/>
<xsl:apply-templates select="OtherInfo"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>

<kbd>Online Demo</kbd>

Java (no rebuild of trees)

// ... same code as reading XML input    ...
// ... same code as creating bookIdNodes ...
String curr_bookId = null;
String outputXML = null;
String xslFile = "C:/Path/To/XSL/Style.xsl";
Source xslt = new StreamSource(new File(xslFile));
// ITERATE THROUGH EACH BOOK ID
for (int i = 0; i < bookIdNodes.getLength(); ++i) {
Node bookId = bookIdNodes.item(i);
System.out.println(bookId.getTextContent());
curr_bookId = bookId.getTextContent();
// CONFIGURE TRANSFORMER
TransformerFactory prettyPrint = TransformerFactory.newInstance();
Transformer transformer = prettyPrint.newTransformer(xslt);
transformer.setParameter("param_bookId", curr_bookId);   // PASS PARAM
transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no");
transformer.setOutputProperty(OutputKeys.METHOD, "xml");
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4");
// TRANSFORM AND OUTPUT FILE TO DISK 
outputXML = "C:/Path/To/XML/BookId_" + curr_bookId + ".xml";
DOMSource source = new DOMSource(doc);
StreamResult result = new StreamResult(new File(outputXML));		
transformer.transform(source, result);
}