Thread: Merge 20+ xml files with 7000+nodes into one using java

im trying to merge more than 20 xml files into one, any of them contains more than 7000 nodes code that im using for that is like this

private void createFile(String month, String year) throws Exception {
 
    Properties prop = new Properties();
    prop.load(new FileInputStream("Config.properties"));
    String edb = prop.getProperty("EDB");
    String bbbCode = prop.getProperty("BBB");
    String InFolder = prop.getProperty("IN");
    String OutFolder = prop.getProperty("OUT");
    String Particija = prop.getProperty("DISK");
    DateFormat dateFormat = new SimpleDateFormat("ddMMyyyy");
    Date date = new Date();
    String ImeFile = "O3" + dateFormat.format(date) + "0000001.UJP." + bbbCode + ".xml";
    String MesecGodina = month + "/" + year;
    DateFormat dateTimeFormat = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");
    Date dateTime = new Date();
    String DatPratka = dateTimeFormat.format(dateTime);
    try {
 
        DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder docBuilder = docFactory.newDocumentBuilder();
        // root elements
        Document doc = docBuilder.newDocument();
        Element rootElement = doc.createElement("UplatiIsplatiMesecIzvestaj");
        doc.appendChild(rootElement);
        // PrireduvacVodecki elements
        Element PrireduvacVodecki = doc.createElement("PrireduvacVodecki");
        rootElement.appendChild(PrireduvacVodecki);
        // edbElement elements
        Element edbElement = doc.createElement("O3_Edb");
        edbElement.appendChild(doc.createTextNode(edb));
        PrireduvacVodecki.appendChild(edbElement);
        // imeFileElement elements
        Element imeFileElement = doc.createElement("O3_ImeFile");
        imeFileElement.appendChild(doc.createTextNode(ImeFile));
        PrireduvacVodecki.appendChild(imeFileElement);
        // mesecGodinaElement elements
        Element mesecGodinaElement = doc.createElement("O3_MesecGodina");
        mesecGodinaElement.appendChild(doc.createTextNode(MesecGodina));
        PrireduvacVodecki.appendChild(mesecGodinaElement);
        // datPratkaElement elements
        Element datPratkaElement = doc.createElement("O3_DatPratka");
        datPratkaElement.appendChild(doc.createTextNode(DatPratka));
        PrireduvacVodecki.appendChild(datPratkaElement);
        // rbrPratkaElement elements
        Element rbrPratkaElement = doc.createElement("O3_RbrPratka");
        rbrPratkaElement.appendChild(doc.createTextNode("0000001"));
        PrireduvacVodecki.appendChild(rbrPratkaElement);
 
        File dir = new File(Particija + ":\\" + InFolder + "\\");
        File[] rootFiles = dir.listFiles();
        int stavki = 0;
        for (File rootFile : rootFiles) {
            display.setText(display.getText() + rootFile + "\n");
            InputStream in = new FileInputStream(rootFile);
            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
            DocumentBuilder db = dbf.newDocumentBuilder();
            Document docc = db.parse(new InputSource(in));
            docc.getDocumentElement().normalize();
            int y = docc.getElementsByTagName("UplataIsplata").getLength();
            display.setText(display.getText() + Integer.toString(y) + "\n");
            stavki = stavki + y;
            for (int i = 0; i < y; i++) {
                // UplataIsplata elements
                Element UplataIsplata = doc.createElement("UplataIsplata");
                rootElement.appendChild(UplataIsplata);
 
                Element idElement = doc.createElement("O3_Id");
                idElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Id").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(idElement);
 
                Element sifraUplMestoElement = doc.createElement("O3_Sifra_uplmesto");
                sifraUplMestoElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Sifra_uplmesto").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(sifraUplMestoElement);
 
                Element sifraKompElement = doc.createElement("O3_Sifra_komp");
                sifraKompElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Sifra_komp").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(sifraKompElement);
 
                Element serNumTicketElement = doc.createElement("O3_Serial_num_ticket");
                serNumTicketElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Serial_num_ticket").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(serNumTicketElement);
 
                Element codeTicketElement = doc.createElement("O3_Code_ticket");
                codeTicketElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Code_ticket").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(codeTicketElement);
 
                Element vremeUplataElement = doc.createElement("O3_Vreme_uplata");
                vremeUplataElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Vreme_uplata").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(vremeUplataElement);
 
                Element iznosUplataElement = doc.createElement("O3_Iznos_uplata");
                iznosUplataElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Iznos_uplata").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(iznosUplataElement);
 
                Element iznosMoznaDobivkaElement = doc.createElement("O3_Iznos_mozna_dobivka");
                iznosMoznaDobivkaElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Iznos_mozna_dobivka").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(iznosMoznaDobivkaElement);
 
                Element iznosIsplatenElement = doc.createElement("O3_Iznos_isplaten");
                iznosIsplatenElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Iznos_isplaten").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(iznosIsplatenElement);
 
                Element brojKombinaciiElement = doc.createElement("O3_Broj_kombinacii");
                brojKombinaciiElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Broj_kombinacii").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(brojKombinaciiElement);
 
                Element vkupenKoefElement = doc.createElement("O3_Vkupen_koef");
                vkupenKoefElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Vkupen_koef").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(vkupenKoefElement);
 
                Element vidTransakcijaElement = doc.createElement("O3_Vid_transakcija");
                vidTransakcijaElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Vid_transakcija").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(vidTransakcijaElement);
 
                Element statusTicketElement = doc.createElement("O3_Status_ticket");
                statusTicketElement.appendChild(doc.createTextNode(docc.getElementsByTagName("O2_Status_ticket").item(i).getChildNodes().item(0).getNodeValue()));
                UplataIsplata.appendChild(statusTicketElement);
 
            }
        }
 
        // vkBrStavkiElement elements
        Element vkBrStavkiElement = doc.createElement("O3_VkBrStavki");
        vkBrStavkiElement.appendChild(doc.createTextNode(Integer.toString(stavki)));
        PrireduvacVodecki.appendChild(vkBrStavkiElement);
        // write the content into xml file
        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        Transformer transformer = transformerFactory.newTransformer();
        DOMSource source = new DOMSource(doc);
        StreamResult result = new StreamResult(new File(Particija + ":\\" + OutFolder + "\\" + ImeFile));
        transformer.setOutputProperty(OutputKeys.ENCODING, "WINDOWS-1251");
        transformer.transform(source, result);
 
        JOptionPane.showMessageDialog(mainPanel, "File Saved!", "XML Merge Information", JOptionPane.INFORMATION_MESSAGE);
        jButton1.setEnabled(true);
        mainPanel.setCursor(java.awt.Cursor.getPredefinedCursor(java.awt.Cursor.DEFAULT_CURSOR));
 
    } catch (ParserConfigurationException pce) {
        pce.printStackTrace();
    } catch (TransformerException tfe) {
        tfe.printStackTrace();
    }
}

but after some time of creating file program stops working.
i got this exception Exception in thread "AWT-EventQueue-0" java.lang.OutOfMemoryError: Java heap space –
how to change this code to write to file part by part not from memory.

I would actually create a for loop, First detect how many files there are... Next you want to read the file one by one, Store each line using a scanner or some other file reader method to store line by line into an array, You could also try using a vector so you wouldnt have to specify the size. Add the lines to the array, using a while next line != null loop. Wen the file is finished reading close that stream, then the for loop starts again for the next file opening a new stream.

Once you have stored all the lines into the array, vector, arraylist etc.. you can then just create a new xml file and overright another xml file if you are wanting to overright the xml file... or create a new file and write the lines in the array using another for loop to detect the array size.. currentLine < array.length etc.

And write line by line to the new file.