Java Remove empty XML tags

Question

I'm looking for a simple Java snippet to remove empty tags from a (any) XML structure

<xml>
    <field1>bla</field1>
    <field2></field2>
    <field3/>
    <structure1>
       <field4>bla</field4>
       <field5></field5>
    <structure1>
</xml>

should turn into;

<xml>
    <field1>bla</field1>
    <structure1>
       <field4>bla</field4>
    </structure1>
</xml>

Answer 1

With XSLT you could transform your XML to ignore the empty tags and re-write the document.

Answer 2

This XSLT stylesheet should do what you're looking for:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"&gt;
  <xsl:template match="@*|node()">
    <xsl:if test=". != '' or ./@* != ''">
      <xsl:copy>
        <xsl:apply-templates select="@*|node()"/>
      </xsl:copy>
    </xsl:if>
  </xsl:template>
</xsl:stylesheet>

It should also preserve elements which are empty but have attributes which aren't. If you don't want this behaviour then change:

<xsl:if test=". != '' or ./@* != ''">

To: <xsl:if test=". != ''">

If you want to know how to apply XSLT in Java, there should be plenty of tutorials out there on the Interwebs. Good luck!

Answer 3

I was wondering whether it would be easy to do this with the XOM library and gave it a try.

It turned out to be quite easy:

import nu.xom.*;

import java.io.File;
import java.io.IOException;

public class RemoveEmptyTags {

    public static void main(String[] args) throws IOException, ParsingException {
        Document document = new Builder().build(new File("original.xml"));
        handleNode(document.getRootElement());
        System.out.println(document.toXML()); // empty elements now removed
    }

    private static void handleNode(Node node) {
        if (node.getChildCount() == 0 && "".equals(node.getValue())) {
            node.getParent().removeChild(node);
            return;
        }
        // recurse the children
        for (int i = 0; i < node.getChildCount(); i++) { 
            handleNode(node.getChild(i));
        }
    }
}

This probably won't handle all corner cases properly, like a completely empty document. And what to do about elements that are otherwise empty but have attributes?

(This answer is part of my evaluation of XOM as a potential replacement to dom4j.)

Answer 4

As a side note: The different states of a tag actually have meaning:

• Open-Closed Tag: The element exists and its value is an empty string
• Single-Tag: The element exists, but the value is null or nil
• Missing Tag: The element does not exist

So, by removing empty Open-Closed tags and Single-Tags, you're merging them with the group of missing tags and thus lose information.

Answer 5

If the xml is feed as a String; regex could be used to filter out empty elements:

<(\\w+)></\\1>|<\\w+/>

This will find empty elements.

data.replaceAll(re, "")

data in this case a variable holding your xml string.
Not saying this would be the best of solutions, but it is possible...