How to Retrieve the Line Number of Validation Errors in XML Files Against an XML Schema

Question

How can I get the line number of errors when validating an XML file against a given XML schema?

Answer

Validating XML files against an XML Schema (XSD) is a common task in software development. When validation fails, knowing the exact line number of the error is crucial for debugging. This guide will explain how to capture the line number of validation errors using Java with the help of the JAXB (Java Architecture for XML Binding) framework and the SAX (Simple API for XML) parser.

import javax.xml.XMLConstants;
import javax.xml.transform.Source;
import javax.xml.transform.stream.StreamSource;
import javax.xml.validation.Schema;
import javax.xml.validation.SchemaFactory;
import javax.xml.validation.Validator;
import org.xml.sax.SAXException;
import java.io.File;
import java.io.IOException;

public class XMLValidator {
    public static void main(String[] args) {
        File xmlFile = new File("path/to/xmlfile.xml");
        File xsdFile = new File("path/to/schema.xsd");

        SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
        try {
            Schema schema = schemaFactory.newSchema(xsdFile);
            Validator validator = schema.newValidator();
            validator.setErrorHandler(new ErrorHandler() {
                public void warning(SAXParseException exception) throws SAXException {
                    System.out.println("Warning: " + exception.getMessage() + " at line: " + exception.getLineNumber());
                }

                public void error(SAXParseException exception) throws SAXException {
                    System.out.println("Error: " + exception.getMessage() + " at line: " + exception.getLineNumber());
                }

                public void fatalError(SAXParseException exception) throws SAXException {
                    System.out.println("Fatal Error: " + exception.getMessage() + " at line: " + exception.getLineNumber());
                }
            });
            validator.validate(new StreamSource(xmlFile));
            System.out.println("XML file is valid.");
        } catch (SAXException e) {
            System.out.println("XML file is NOT valid because: " + e.getMessage());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Causes

The XSD file is invalid or not well-formed.
The XML file does not comply with the structure defined in the XSD.
Namespace issues between the XML and XSD.

Solutions

Implement a custom SAX handler to capture validation errors along with their line numbers.
Use `Schema` and `Validator` classes available in the `javax.xml.validation` package.
Ensure that both the XML and XSD files are free from syntax errors before validation.

Common Mistakes

Mistake: Ignoring the XML line number in error messages.

Solution: Always log error line numbers to simplify debugging.

Mistake: Using incorrect XML or XSD paths.

Solution: Double-check the file paths and ensure files exist.

Mistake: Handling exceptions poorly without specific feedback.

Solution: Implement detailed logging in your error handling.

Helpers

XML validation
XML schema validation
line number of XML validation error
SAX parser XML
JAXB XML validation