Jackson is "the Java JSON library" or "the best JSON parser for Java". Or simply as "JSON for Java".

Jackson also allows you to parse or generate XML messages with the Jackson Dataformat XML Extension.

Jackson Dataformat XML Source Code files are provided in the source packge (jackson-dataformat-xml-2.14.0-sources.jar). You can download it at Jackson Maven Website.

You can also browse Jackson Dataformat XML Source Code below:

⏎ com/fasterxml/jackson/dataformat/xml/util/DefaultXmlPrettyPrinter.java

package com.fasterxml.jackson.dataformat.xml.util;

import java.io.IOException;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.Arrays;

import javax.xml.XMLConstants;
import javax.xml.stream.XMLStreamException;

import org.codehaus.stax2.XMLStreamWriter2;

import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.core.util.Instantiatable;

import com.fasterxml.jackson.dataformat.xml.XmlPrettyPrinter;
import com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator;

/**
 * Indentation to use with XML is different from JSON, because JSON
 * requires use of separator characters and XML just basic whitespace.
 *<p>
 * Note that only a subset of methods of {@link PrettyPrinter} actually
 * get called by {@link ToXmlGenerator}; because of this, implementation
 * is bit briefer (and uglier...).
 */
public class DefaultXmlPrettyPrinter
    implements XmlPrettyPrinter, Instantiatable<DefaultXmlPrettyPrinter>,
        java.io.Serializable
{
    private static final long serialVersionUID = 1L; // since 2.6

    /**
     * Interface that defines objects that can produce indentation used
     * to separate object entries and array values. Indentation in this
     * context just means insertion of white space, independent of whether
     * linefeeds are output.
     */
    public interface Indenter
    {
        public void writeIndentation(JsonGenerator g, int level) throws IOException;

        public void writeIndentation(XMLStreamWriter2 sw, int level) throws XMLStreamException;

        /**
         * @return True if indenter is considered inline (does not add linefeeds),
         *   false otherwise
         */
        public boolean isInline();
    }

    /*
    /**********************************************************
    /* Configuration
    /**********************************************************
     */

    /**
     * By default, let's use only spaces to separate array values.
     */
    protected Indenter _arrayIndenter = new FixedSpaceIndenter();

    /**
     * By default, let's use linefeed-adding indenter for separate
     * object entries. We'll further configure indenter to use
     * system-specific linefeeds, and 2 spaces per level (as opposed to,
     * say, single tabs)
     */
    protected Indenter _objectIndenter = new Lf2SpacesIndenter();

    // // // Config, other white space configuration

    /**
     * By default we will add spaces around colons used to
     * separate object fields and values.
     * If disabled, will not use spaces around colon.
     */
    protected boolean _spacesInObjectEntries = true;

    /*
    /**********************************************************
    /* State
    /**********************************************************
    */
    
    /**
     * Number of open levels of nesting. Used to determine amount of
     * indentation to use.
     */
    protected transient int _nesting = 0;

    /**
     * Marker flag set on start element, and cleared if an end element
     * is encountered. Used for suppressing indentation to allow empty
     * elements.
     * 
     * @since 2.3
     */
    protected transient boolean _justHadStartElement;
    
    /*
    /**********************************************************
    /* Life-cycle (construct, configure)
    /**********************************************************
    */

    public DefaultXmlPrettyPrinter() { }

    protected DefaultXmlPrettyPrinter(DefaultXmlPrettyPrinter base)
    {
        _arrayIndenter = base._arrayIndenter;
        _objectIndenter = base._objectIndenter;
        _spacesInObjectEntries = base._spacesInObjectEntries;
        _nesting = base._nesting;
    }

    public void indentArraysWith(Indenter i)
    {
        _arrayIndenter = (i == null) ? new NopIndenter() : i;
    }

    public void indentObjectsWith(Indenter i)
    {
        _objectIndenter = (i == null) ? new NopIndenter() : i;
    }

    public void spacesInObjectEntries(boolean b) { _spacesInObjectEntries = b; }

    /*
    /**********************************************************
    /* Instantiatable impl
    /**********************************************************
     */
    
    @Override
    public DefaultXmlPrettyPrinter createInstance() {
        return new DefaultXmlPrettyPrinter(this);
    }

    /*
    /**********************************************************
    /* PrettyPrinter impl
    /**********************************************************
     */

    @Override
    public void writeRootValueSeparator(JsonGenerator gen) throws IOException {
        // Not sure if this should ever be applicable; but if multiple roots were allowed, we'd use linefeed
        gen.writeRaw('\n');
    }
    
    /*
    /**********************************************************
    /* Array values
    /**********************************************************
     */
    
    @Override
    public void beforeArrayValues(JsonGenerator gen) throws IOException {
        // never called for ToXmlGenerator
    }

    @Override
    public void writeStartArray(JsonGenerator gen) throws IOException {
        // anything to do here?
    }

    @Override
    public void writeArrayValueSeparator(JsonGenerator gen)  throws IOException {
        // never called for ToXmlGenerator
    }

    @Override
    public void writeEndArray(JsonGenerator gen, int nrOfValues) throws IOException {
        // anything to do here?
    }
    
    /*
    /**********************************************************
    /* Object values
    /**********************************************************
     */

    @Override
    public void beforeObjectEntries(JsonGenerator gen)
        throws IOException, JsonGenerationException
    {
        // never called for ToXmlGenerator
    }

    @Override
    public void writeStartObject(JsonGenerator gen) throws IOException
    {
        if (!_objectIndenter.isInline()) {
            if (_nesting > 0) {
                _objectIndenter.writeIndentation(gen, _nesting);
            }
            ++_nesting;
        }
        _justHadStartElement = true;
        ((ToXmlGenerator) gen)._handleStartObject();
    }

    @Override
    public void writeObjectEntrySeparator(JsonGenerator gen) throws IOException {
        // never called for ToXmlGenerator
    }

    @Override
    public void writeObjectFieldValueSeparator(JsonGenerator gen) throws IOException {
        // never called for ToXmlGenerator
    }
    
    @Override
    public void writeEndObject(JsonGenerator gen, int nrOfEntries) throws IOException
    {
        if (!_objectIndenter.isInline()) {
            --_nesting;
        }
        // for empty elements, no need for linefeeds etc:
        if (_justHadStartElement) {
            _justHadStartElement = false;
        } else {
            _objectIndenter.writeIndentation(gen, _nesting);
        }
        ((ToXmlGenerator) gen)._handleEndObject();
    }
    
    /*
    /**********************************************************
    /* XML-specific additions
    /**********************************************************
     */

    @Override
    public void writeStartElement(XMLStreamWriter2 sw,
            String nsURI, String localName) throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            if (_justHadStartElement) {
                _justHadStartElement = false;
            }
            _objectIndenter.writeIndentation(sw, _nesting);
            ++_nesting;
        }
        sw.writeStartElement(nsURI, localName);
        _justHadStartElement = true;        
    }

    @Override
    public void writeEndElement(XMLStreamWriter2 sw, int nrOfEntries) throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            --_nesting;
        }
        // for empty elements, no need for linefeeds etc:
        if (_justHadStartElement) {
            _justHadStartElement = false;
        } else {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeEndElement();
    }
    
    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
    		String nsURI, String localName, String text, boolean isCData)
  		throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        if(isCData) {
            sw.writeCData(text);
        } else {
            sw.writeCharacters(text);
        }
        sw.writeEndElement();
        _justHadStartElement = false;
    }

    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
    		String nsURI, String localName,
    		char[] buffer, int offset, int len, boolean isCData)
        throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        if(isCData) {
            sw.writeCData(buffer, offset, len);
        } else {
            sw.writeCharacters(buffer, offset, len);
        }
        sw.writeEndElement();
        _justHadStartElement = false;
    }
	
    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
    		String nsURI, String localName, boolean value)
  		throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeBoolean(value);
        sw.writeEndElement();
        _justHadStartElement = false;
    }
    
    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
            String nsURI, String localName, int value)
        throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeInt(value);
        sw.writeEndElement();
        _justHadStartElement = false;
    }

    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
            String nsURI, String localName, long value)
        throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeLong(value);
        sw.writeEndElement();
        _justHadStartElement = false;
    }

    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
            String nsURI, String localName, double value)
  		throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeDouble(value);
        sw.writeEndElement();
        _justHadStartElement = false;
    }

    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
    		String nsURI, String localName, float value)
  		throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeFloat(value);
        sw.writeEndElement();
        _justHadStartElement = false;
    }
	
    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
            String nsURI, String localName, BigInteger value)
        throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeInteger(value);
        sw.writeEndElement();
        _justHadStartElement = false;
    }

    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
    		String nsURI, String localName, BigDecimal value)
  		throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeDecimal(value);
        sw.writeEndElement();
        _justHadStartElement = false;
    }

    // method definition changed in 2.12
    @Override
    public void writeLeafElement(XMLStreamWriter2 sw,
            String nsURI, String localName,
            org.codehaus.stax2.typed.Base64Variant base64variant,
            byte[] data, int offset, int len)
        throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeStartElement(nsURI, localName);
        sw.writeBinary(base64variant, data, offset, len);
        sw.writeEndElement();
        _justHadStartElement = false;
    }

    @Override
    public void writeLeafNullElement(XMLStreamWriter2 sw,
            String nsURI, String localName)
        throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeEmptyElement(nsURI, localName);
        _justHadStartElement = false;
    }

    // @since 2.12
    public void writeLeafXsiNilElement(XMLStreamWriter2 sw,
            String nsURI, String localName)
        throws XMLStreamException
    {
        if (!_objectIndenter.isInline()) {
            _objectIndenter.writeIndentation(sw, _nesting);
        }
        sw.writeEmptyElement(nsURI, localName);
        sw.writeAttribute("xsi", XMLConstants.W3C_XML_SCHEMA_INSTANCE_NS_URI, "nil", "true");
        _justHadStartElement = false;    
    }

    @Override // since 2.7
    public void writePrologLinefeed(XMLStreamWriter2 sw) throws XMLStreamException
    {
        // 06-Dec-2015, tatu: Alternatively could try calling `writeSpace()`...
        sw.writeRaw(Lf2SpacesIndenter.SYSTEM_LINE_SEPARATOR);
    }

    /*
    /**********************************************************
    /* Helper classes
    /* (note: copied from jackson-core to avoid dependency;
    /* allow local changes)
    /**********************************************************
     */

    /**
     * Dummy implementation that adds no indentation whatsoever
     */
    protected static class NopIndenter
        implements Indenter, java.io.Serializable
    {
        private static final long serialVersionUID = 1L;

        public NopIndenter() { }
        @Override public void writeIndentation(JsonGenerator jg, int level) { }
        @Override public boolean isInline() { return true; }
        @Override public void writeIndentation(XMLStreamWriter2 sw, int level) { }
    }

    /**
     * This is a very simple indenter that only every adds a
     * single space for indentation. It is used as the default
     * indenter for array values.
     */
    protected static class FixedSpaceIndenter
        implements Indenter, java.io.Serializable
    {
        private static final long serialVersionUID = 1L;

        public FixedSpaceIndenter() { }

        @Override
        public void writeIndentation(XMLStreamWriter2 sw, int level)
            throws XMLStreamException
        {
            sw.writeRaw(" ");
        }
        
        @Override
        public void writeIndentation(JsonGenerator g, int level) throws IOException
        {
            g.writeRaw(' ');
        }

        @Override
        public boolean isInline() { return true; }
    }

    /**
     * Default linefeed-based indenter uses system-specific linefeeds and
     * 2 spaces for indentation per level.
     */
    protected static class Lf2SpacesIndenter
        implements Indenter, java.io.Serializable
    {
        private static final long serialVersionUID = 1L;

        final static String SYSTEM_LINE_SEPARATOR;
        static {
            String lf = null;
            try {
                lf = System.getProperty("line.separator");
            } catch (Throwable t) { } // access exception?
            SYSTEM_LINE_SEPARATOR = (lf == null) ? "\n" : lf;
        }

        final static int SPACE_COUNT = 64;
        final static char[] SPACES = new char[SPACE_COUNT];
        static {
            Arrays.fill(SPACES, ' ');
        }

        public Lf2SpacesIndenter() { }

        @Override
        public boolean isInline() { return false; }

        @Override
        public void writeIndentation(XMLStreamWriter2 sw, int level) throws XMLStreamException
        {
            sw.writeRaw(SYSTEM_LINE_SEPARATOR);
            level += level; // 2 spaces per level
            while (level > SPACE_COUNT) { // should never happen but...
            	sw.writeRaw(SPACES, 0, SPACE_COUNT); 
                level -= SPACES.length;
            }
            sw.writeRaw(SPACES, 0, level);
        }
        
        @Override
        public void writeIndentation(JsonGenerator jg, int level) throws IOException
        {
            jg.writeRaw(SYSTEM_LINE_SEPARATOR);
            level += level; // 2 spaces per level
            while (level > SPACE_COUNT) { // should never happen but...
                jg.writeRaw(SPACES, 0, SPACE_COUNT); 
                level -= SPACES.length;
            }
            jg.writeRaw(SPACES, 0, level);
        }
    }
}

⏎ com/fasterxml/jackson/dataformat/xml/util/DefaultXmlPrettyPrinter.java

Or download all of them as a single archive file:

File name: jackson-dataformat-xml-2.14.0-sources.jar
File size: 98015 bytes
Release date: 2022-11-05
Download 

⇒ Download Jackson Dataformat Binary Packages

⇐ Jackson Dataformat Extensions

⇑ Downloading and Reviewing jackson-*.jar

⇑⇑ Jackson - Java JSON library