How to insert a docx into another docx's pre-determined row

My goal is to insert a docx (with keeping the style / formatting) into another docx's specific row. In the second docx there is a word, "placeholder" and first, I have to find this word, and then change it to first docx text, keeping the inserted docx styles and formats.

I have an idea. Maybe I should create a new docx, divide the second docx with the "placeholder", put the first part to the new docx, then put the whole docx, and then put the second part of the second docx. But how can I keep the styles and formats? I don't have images / tablets or anything, just texts and formatting stuff, like lists, tabs, text style, etc.

Currently I am using apache POI and java. (I tried docx4j, but I had less success)
The example code does a simple merging but nothing more. How can I find the "placeholder" word and insert my docx there?

public static void merge(InputStream src1, InputStream src2, OutputStream dest) throws Exception {
    OPCPackage src1Package = OPCPackage.open(src1);
    OPCPackage src2Package = OPCPackage.open(src2);
    XWPFDocument src1Document = new XWPFDocument(src1Package);
    CTBody src1Body = src1Document.getDocument().getBody();
    XWPFDocument src2Document = new XWPFDocument(src2Package);
    CTBody src2Body = src2Document.getDocument().getBody();
    appendBody(src1Body, src2Body);
    src1Document.write(dest);
}

private static void appendBody(CTBody src, CTBody append) throws Exception {
    XmlOptions optionsOuter = new XmlOptions();
    optionsOuter.setSaveOuter();
    String appendString = append.xmlText(optionsOuter);
    String srcString = src.xmlText();
    String prefix = srcString.substring(0, srcString.indexOf(">") + 1);
    String mainPart = srcString.substring(srcString.indexOf(">") + 1, srcString.lastIndexOf("<"));
    String suffix = srcString.substring(srcString.lastIndexOf("<"));
    String addPart = appendString.substring(appendString.indexOf(">") + 1, appendString.lastIndexOf("<"));
    CTBody makeBody = CTBody.Factory.parse(prefix + mainPart + addPart + suffix);
    src.set(makeBody);
}

Re docx4j you can insert a docx at a specific location (eg in a table cell) using MergeDocx in our commercial Docx4j Enterprise.

You can get a trial version from https://www.plutext.com/m/index.php/products

Then see the MergeIntoTableCell sample and documentation.

Other solution is: in my example in mainPart, we can find the text (using indexof / lastindexof / substring are better, than using regex) and add (and replace the text to) the addPart and ready to go.

2 possible problem:

• 1: if we have numbered lists / bulleted lists in addPart, that can be be messy after adding to the other document.
• 2: inserting picture is not possible in this way, it has to be handle in other way.