Flat file to XML in JAVA

Question

I have a flat file of fixed length fields like this:

ITEM1234LED Light Set
ITEM1235Ratchet Tie 

I'd like to convert it into xml file:

<ITEMS>
  <ITEM>
    <ITEMID>1234</ITEMID>
    <DESCRIPTION>LED Light Set</DESCRIPTION>
  </ITEM>
  <ITEM>
    <ITEMID>1235</ITEMID>
    <DESCRIPTION>Ratchet Tie</DESCRIPTION>
  </ITEM>
</ITEMS>

Which is the best way to achieve this ?

Thank you.

Answer 1

You can use a simple XMLStreamWriter to create the XML document. No need to create a class for the records. Just extract the ID and the description as strings and push these strings to the XML. This works for large files, too. Neither the input file nor the XML document has to be hold entirely in memory.

import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

import javax.xml.stream.FactoryConfigurationError;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamWriter;

public class Items {

  private static final int POS_ID = 4;
  private static final int POS_DESCR = 8;

  public static void main(String[] args) {
    // Files for input and output
    final Path inFile = Paths.get("items.txt");
    final Path outFile = Paths.get("items.xml");

    // Unfortunately, XMLStreamWriter doesn't implement AutoCloseable, 
    // so we cannot use it with try-with-resources.
    XMLStreamWriter xmlWriter = null;
    try(
        // BufferedReader for the input file (assuming UTF-8 for the encoding)
        BufferedReader reader = Files.newBufferedReader(
          inFile, StandardCharsets.UTF_8);

        // BufferedOutputStream, so encoding is handled entirely by 
        // the XMLStreamWriter.
        OutputStream out = new BufferedOutputStream(
          Files.newOutputStream(outFile));
        ) 
    {
      // Use a XMLStreamWriter to create the XML document.
      xmlWriter = XMLOutputFactory.newInstance().createXMLStreamWriter(out);
      xmlWriter.writeStartDocument();
      xmlWriter.writeStartElement("ITEMS");
      String line;
      while((line = reader.readLine()) != null) {
        // Parse the input line with fixed length fields
        final String id = line.substring(POS_ID, POS_DESCR);
        final String descr = line.substring(POS_DESCR);
        xmlWriter.writeStartElement("ITEM");

        xmlWriter.writeStartElement("ITEMID");
        xmlWriter.writeCharacters(id);
        xmlWriter.writeEndElement(); // ITEMID

        xmlWriter.writeStartElement("DESCRIPTION");
        xmlWriter.writeCharacters(descr);
        xmlWriter.writeEndElement(); // DESCRIPTION

        xmlWriter.writeEndElement(); // ITEM
      }
      xmlWriter.writeEndElement(); // ITEMS
      xmlWriter.writeEndDocument();
    } catch (IOException | XMLStreamException | FactoryConfigurationError e) {
      e.printStackTrace();
    } finally {
      // Cleaning up
      if(xmlWriter != null) {
        try {
          xmlWriter.close();
        } catch (XMLStreamException e) {
          e.printStackTrace();
        }
      }
    }
  }
}

Answer 2

1) Create a Java class, that maps to the data in the flat file, for ex:

public class Item {
    private String itemId;
    private String description;

    /**
     * @return the itemId
     */
    public String getItemId() {
        return itemId;
    }

    /**
     * @param itemId the itemId to set
     */
    public void setItemId(String itemId) {
        this.itemId = itemId;
    }

    /**
     * @return the description
     */
    public String getDescription() {
        return description;
    }

    /**
     * @param description the description to set
     */
    public void setDescription(String description) {
        this.description = description;
    }
}

2) Parse the flat file into a list of 'Items' (List of Item objects)

3) Use a good, lightweight framework like 'xStream' and use the appropriate method to serialize Java object to XML file. For example: xStream.toXml(Object obj, Writer out)

PS: This is just a standard way (using well tested frameworks and hence, not re-inventing the wheel), but not the optimal one. Optimally, for performance and less memory footprint, you can parse the flat file and write to a XML file at the same time.

Answer 3

Create java objects to represent your logical data structure.

Parse the flat file and generate the java objects.

Use an XML library (for example JAXB) to serialize that tree of java objects into a file.

Answer 4

You could use any of the following to achieve what you're trying:

JAXB
XSLT 

Or you could use this to read a CSV or a flat file and serialize to an XML (as shown in your question)

Hope this helps!

Answer 5

I think what bchetty mentioned is good, but you do not need ANY XML libraries to output XML.

1. Parse the file into an Collection<> of objects, just like bchetty mentioned. You can use regex, or tools like ANTLR / CookCC / JFlex etc to do the parsing.
2. Then you can just open a PrintWriter for the file.

PrintWriter out = new PrintWriter (file);

out.println ("<ITEMS>");

for (Item item : Items)
{
    out.println ("  <ITEM>");
    out.println ("    <ITEMID>" + item.getItemId() + "</ITEMID>");
    out.println ("    <DESCRIPTION>" + item.getDescription () + "</DESCRIPTION>");
    out.println ("  </ITEM>");
}

out.println ("</ITEMS>");
out.close ();