如果我的方法中发生异常，则使用stax解析器编写XML时如何存储或访问以前的数据

Question

In example below when we enter into writeOneMoreElement() method and Exception occurs then how to access previous data that we write on XML. Here we loose every entry if exception occurs on writeOneMoreElement() method.

public class xmlSample {
  public static void main(String[] args) {
    XMLOutputFactory factory = XMLOutputFactory.newInstance();
    try {

        XMLStreamWriter writer1 = factory.createXMLStreamWriter(new FileWriter("E:\\sampleXML.xml"));
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        XMLOutputFactory xmlOutputFactory = new WstxOutputFactory();
        XMLStreamWriter2 xtw = (XMLStreamWriter2) new WstxOutputFactory()
                .createXMLStreamWriter(byteArrayOutputStream, "UTF-8");

        xtw.writeStartDocument("UTF-8", "1.1");
        xtw.setPrefix("itm", "http://adt.cmn.xmlns.commons.platform.actiance.com/core/1.0");

        xtw.writeStartElement("document");
        xtw.writeStartElement("data1");
        xtw.writeCharacters("Sagar");
        xtw.writeEndElement();

        XMLStreamWriter2 writer2 = writeOneMoreElement(xtw);

        writer2.writeStartElement("data2");
        writer2.writeCharacters("Shubham");
        writer2.writeEndElement();

        writeOneMoreElement(writer2);

        writer2.writeEndDocument();
        writer2.close();
        xtw.flush();
        xtw.close();

        System.out.println("XML :" + new String(byteArrayOutputStream.toByteArray()));

    } catch (XMLStreamException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

private static XMLStreamWriter2 writeOneMoreElement(XMLStreamWriter2 writer2)
        throws XMLStreamException, IOException {

    try {
        writer2.writeStartElement("ABC");
        writer2.writeStartElement("data2");
        writer2.writeAttribute("name2", "value2");
        writer2.writeAttribute("otherAttribute", "true");
        writer2.writeEndElement();

        writer2.writeStartElement("data3");
        writer2.writeAttribute("name3", "value3");
        writer2.writeAttribute("otherAttribute", "true");
        writer2.writeEndElement();

        writer2.writeEndElement();

        writer2.flush();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return writer2;
 }
}

Answer 1

One of the rules of Exceptions is that whichever line throws an Exception, all lines after that will not be executed until you reach the catch and finally blocks. So this line System.out.println("XML :" + new String(byteArrayOutputStream.toByteArray())); is never executed, but the values are still available in byteArrayOutputStream before the program ends.

One possible solution is to run the same println() method in the catch block again. Like so:

public static void main(String[] args) {
    XMLOutputFactory factory = XMLOutputFactory.newInstance();
    //Initializing the ByteArrayOutputStream outside the try block, so that it is still available after that scope.
    ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
    try {

        XMLStreamWriter writer1 = factory.createXMLStreamWriter(new FileWriter("E:\\sampleXML.xml"));
        //ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        XMLOutputFactory xmlOutputFactory = new WstxOutputFactory();
        XMLStreamWriter2 xtw = (XMLStreamWriter2) new WstxOutputFactory()
                .createXMLStreamWriter(byteArrayOutputStream, "UTF-8");

        xtw.writeStartDocument("UTF-8", "1.1");
        xtw.setPrefix("itm", "http://adt.cmn.xmlns.commons.platform.actiance.com/core/1.0");

        xtw.writeStartElement("document");
        xtw.writeStartElement("data1");
        xtw.writeCharacters("Sagar");
        xtw.writeEndElement();

        XMLStreamWriter2 writer2 = writeOneMoreElement(xtw);

        writer2.writeStartElement("data2");
        writer2.writeCharacters("Shubham");
        writer2.writeEndElement();

        //Exception thrown here
        writeOneMoreElement(writer2);

        //Unexecuted code from here onwards
        writer2.writeEndDocument();
        writer2.close();
        xtw.flush();
        xtw.close();

        System.out.println("XML :" + new String(byteArrayOutputStream.toByteArray()));

    } catch (XMLStreamException e) {
        e.printStackTrace();
        /*Proof of concept. This line will print out all the values inserted into the ByteArrayOutputStream up to
        the point where the Exception was thrown.*/
        System.out.println("XML :" + new String(byteArrayOutputStream.toByteArray()));
    } catch (IOException e) {
        e.printStackTrace();
    }

}

private static XMLStreamWriter2 writeOneMoreElement(XMLStreamWriter2 writer2)
        throws XMLStreamException/*, IOException*/ {
    //By the way, why does this method throw IOException? It's unnecessary, and you can remove it.
    try {
        writer2.writeStartElement("ABC");
        writer2.writeStartElement("data2");
        writer2.writeAttribute("name2", "value2");
        writer2.writeAttribute("otherAttribute", "true");
        writer2.writeEndElement();

        writer2.writeStartElement("data3");
        writer2.writeAttribute("name3", "value3");
        writer2.writeAttribute("otherAttribute", "true");
        writer2.writeEndElement();

        writer2.writeEndElement();

        writer2.flush();
        //I'm throwing an Exception here on purpose to trigger the catch block.
        throw new XMLStreamException();
    }       
    catch (Exception e) {
        e.printStackTrace();
        // I'm rethrowing the Exception on purpose.
        throw e;
    } 
    //This line won't work for now, but you can change it back.
    //return writer2;
 }

Run the program, and you will get the expected XMLStreamException as well the incomplete data.

XML :<?xml version='1.1' encoding='UTF-8'?><document><data1>Sagar</data1><ABC><data2 name2="value2" otherAttribute="true" /><data3 name3="value3" otherAttribute="true" /></ABC>