用Java解析XML时出现问题

Question

I got some trouble parsing an XML document. For some reason, there are text nodes where I would not expect them to be and therefore my test turns red. The XML file looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<RootNode>
  <PR1>PR1</PR1>
  <ROL>one</ROL>
  <ROL>two</ROL>
  <DG1>DG1</DG1>
  <ROL>three</ROL>
  <ZBK>ZBK</ZBK>
  <ROL>four</ROL>
</RootNode>

Now I have this snippet of code which can reproduce the error:

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document doc = builder.parse(TestHL7Helper.class.getResourceAsStream("TestHL7HelperInput.xml"));
Node root = doc.getFirstChild();
Node pr1 = root.getFirstChild();

Inspecting the root variable yields [RootNode: null] which seems to be right, but then it somehow goes all wrong. The pr1 variable turns out to be a text node [#text:\\n ] - but why does the parser think that the new line and the spaces are a text node? Shouldn't that be ignored? I tried changing the encoding but that did not help either. Any ideas on that?

If I remove all new lines and space and have my XML document in just one line it all works fine...

Answer 1

Actually all text between other nodes forms a text-node itself. So, if you use getFirstChild() you will also retrieve those text-nodes.

In your case all non-text child-nodes have a unique name, so you can get them individually by using getElementsByTagName() :

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document doc = builder.parse(TestHL7Helper.class.getResourceAsStream("TestHL7HelperInput.xml"));
Node root = doc.getFirstChild();
Node pr1 = (root.getElementsByTagName( "PR1" ))[0];

In general I would not rely on the position within the XML-document, but on stuff like tag-names, attributes or ids.

Answer 2

XML supports mixed content meaning elements can have both text and element child nodes. This is to support use cases like the following:

<text>I've bolded the <b>important</b> part.</text>

input.xml

This means that by default a DOM parser will treat the whitespace nodes in the following document as significant (below is a simplified version of your XML document):

<RootNode>
  <PR1>PR1</PR1>
</RootNode>

Demo Code

If you have an XML schema you can set the ignoringElementContentWhitespace property on the DocumentBuilderFactory since then the DOM parser will know if and when the whitespace is significant.

import java.io.File;
import javax.xml.XMLConstants;
import javax.xml.parsers.*;
import javax.xml.validation.*;

import org.w3c.dom.Document;

public class Demo {

    public static void main(String[] args) throws Exception {
        SchemaFactory sf = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
        Schema s = sf.newSchema(new File("src/forum16231687/schema.xsd"));

        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        dbf.setSchema(s);
        dbf.setIgnoringElementContentWhitespace(true);

        DocumentBuilder db = dbf.newDocumentBuilder();
        Document d = db.parse(new File("src/forum16231687/input.xml"));
        System.out.println(d.getDocumentElement().getChildNodes().getLength());
    }

}

schema.xsd

If you create schema.xsd that looks like the following then the demo code will report that the root element has 1 child node.

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema">
    <element name="RootNode">
        <complexType>
            <sequence>
                <element name="PR1" type="string"/>
            </sequence>
        </complexType>
    </element>
</schema>

If you change schema.xsd so that the RootNode has mixed content the demo code will report that the RootNode has 3 child nodes.

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema">
    <element name="RootNode">
        <complexType mixed="true">
            <sequence>
                <element name="PR1" type="string"/>
            </sequence>
        </complexType>
    </element>
</schema>

Answer 3

You can solve this general issue by checking the type of the node:

if (someNode instanceof Element) {
  // ...
}

This can easily form part of a loop, such as:

NodeList childNodes = root.getChildNodes();
for (int i = 0; i < childNodes.getLength(); i++) {
  if (childNodes.item(i).getNodeType() == Node.ELEMENT) {
    Element childElement = (Element) childNodes.item(i);
    // ...
  }
}

Alternatively, use something like XMLBeans to reduce the likelihood of introducing bugs when manually parsing XML. Get a well-tested library to do the work for you!