Implement #849: allow adding comments in doc prolog (#851)

Author: cowtowncoder

release-notes/VERSION

@@ -119,6 +119,8 @@ Version: 3.x (for earlier see VERSION-2.x)
  (fix by @cowtowncoder, w/ Claude code)
 #845: Implement `JsonGenerator` methods `writeComment()` and `canWriteComments()`
  (implemented by @cowtowncoder, w/ Claude code)
+#849: Allow writing Comments in Document Prolog (before root element)
+ (implemented by @cowtowncoder)
 
 3.1.2 (11-Apr-2026)
 3.1.1 (27-Mar-2026)

src/main/java/tools/jackson/dataformat/xml/ser/Comment.java

@@ -0,0 +1,28 @@
+package tools.jackson.dataformat.xml.ser;
+
+import javax.xml.stream.XMLStreamException;
+
+import org.codehaus.stax2.XMLStreamWriter2;
+
+import tools.jackson.dataformat.xml.util.ArgUtil;
+
+/**
+ * Value container to represent XML Comment within "prolog"
+ * part of the Document (before XML Root element, after XML
+ * declaration if one written),
+ * to be written using {@link XmlGeneratorInitializer}.
+ *
+ * @since 3.2
+ */
+public record Comment(String content)
+    implements XmlPrologDirective
+{
+    public Comment {
+        content = ArgUtil.nullToEmpty(content);
+    }
+
+    @Override
+    public void write(ToXmlGenerator xmlGen, XMLStreamWriter2 sw) throws XMLStreamException {
+        sw.writeComment(content);
+    }
+}

src/main/java/tools/jackson/dataformat/xml/ser/DTD.java

@@ -4,6 +4,8 @@
 
 import org.codehaus.stax2.XMLStreamWriter2;
 
+import tools.jackson.dataformat.xml.util.ArgUtil;
+
 /**
  * Value container to represent XML Document Type Declaration,
  * to be written using {@link XmlGeneratorInitializer}.
@@ -16,22 +18,10 @@ public record DTD(String rootName,
     implements XmlPrologDirective
 {
     public DTD {
-        rootName = _nonEmptyNonNull("rootName", rootName);
-        systemId = _emptyToNull(systemId);
-        publicId = _emptyToNull(publicId);
-        internalSubset = _emptyToNull(internalSubset);
-    }
-
-    static String _emptyToNull(String str) {
-        return "".equals(str) ? null : str;
-    }
-
-    static String _nonEmptyNonNull(String prop, String str) {
-        if (str == null || str.isEmpty()) {
-            throw new IllegalArgumentException("Illegal argument for '%s': must be non-empty String"
-                    .formatted(prop));
-        }
-        return str;
+        rootName = ArgUtil.nonEmptyNonNull("rootName", rootName);
+        systemId = ArgUtil.emptyToNull(systemId);
+        publicId = ArgUtil.emptyToNull(publicId);
+        internalSubset = ArgUtil.emptyToNull(internalSubset);
     }
 
     @Override

src/main/java/tools/jackson/dataformat/xml/ser/ToXmlGenerator.java

@@ -106,6 +106,14 @@ public class ToXmlGenerator
      */
     protected List<XmlPrologDirective> _prologDirectives;
 
+    /**
+     * Whether linefeed ("pretty-printing") enabled between directives
+     * in Document prolog.
+     *
+     * @since 3.2
+     */
+    protected boolean _lfBetweenPrologDirectives;
+
     /*
     /**********************************************************************
     /* Logical output state
@@ -218,6 +226,22 @@ public ToXmlGenerator(ObjectWriteContext writeCtxt, IOContext ioCtxt,
     /**********************************************************************
      */
 
+    /**
+     * Method called by {@link XmlGeneratorInitializer} to inject
+     * necessary configuration.
+     *
+     * @since 3.2
+     */
+    public void initProlog(boolean lfBetweenPrologDirectives,
+            List<XmlPrologDirective> directives)
+    {
+        if (_initialized) { // sanity check
+            _reportError("Internal error: cannot call `initConfig()` after generator already initialized");
+        }
+        _lfBetweenPrologDirectives = lfBetweenPrologDirectives;
+        _prologDirectives = directives;
+    }
+
     /**
      * Method called by {@link XmlSerializationContext} before writing any output,
      * to optionally output XML declaration and other before-root-element
@@ -230,29 +254,21 @@ public void initGenerator() throws JacksonException
         }
         _initialized = true;
         try {
-            boolean xmlDeclWritten;
-
-            if (XmlWriteFeature.WRITE_XML_1_1.enabledIn(_formatFeatures)
-                    || XmlWriteFeature.WRITE_XML_DECLARATION.enabledIn(_formatFeatures)) {
+            final boolean xml11Decl = XmlWriteFeature.WRITE_XML_1_1.enabledIn(_formatFeatures);
+            if (xml11Decl || XmlWriteFeature.WRITE_XML_DECLARATION.enabledIn(_formatFeatures)) {
 
-                String xmlVersion = XmlWriteFeature.WRITE_XML_1_1.enabledIn(_formatFeatures) ? "1.1" : "1.0";
+                String xmlVersion = xml11Decl ? "1.1" : "1.0";
                 String encoding = "UTF-8";
 
                 if (XmlWriteFeature.WRITE_STANDALONE_YES_TO_XML_DECLARATION.enabledIn(_formatFeatures)) {
                     _xmlWriter.writeStartDocument(xmlVersion, encoding, true);
                 } else {
                     _xmlWriter.writeStartDocument(encoding, xmlVersion);
                 }
-                xmlDeclWritten = true;
-            } else {
-                xmlDeclWritten = false;
-            }
-
-            // as per [dataformat-xml#172], try adding indentation
-            if (xmlDeclWritten && (_xmlPrettyPrinter != null)) {
-                // ... but only if it is likely to succeed:
-                if (!_stax2Emulation) {
-                    _xmlPrettyPrinter.writePrologLinefeed(_xmlWriter);
+                // 20-Apr-2026, tatu: for legacy path, only output prolog lf when pretty-printing
+                //    OR _lfBetweenPrologDirectives passed by initializer
+                if (_lfBetweenPrologDirectives || _xmlPrettyPrinter != null) {
+                    _prologLinefeed();
                 }
             }
             if (XmlWriteFeature.AUTO_DETECT_XSI_TYPE.enabledIn(_formatFeatures)) {
@@ -263,6 +279,10 @@ public void initGenerator() throws JacksonException
             if (_prologDirectives != null) {
                 for (XmlPrologDirective d : _prologDirectives) {
                     d.write(this, _xmlWriter);
+                    // Add linefeed separators b/w directives
+                    if (_lfBetweenPrologDirectives) {
+                        _prologLinefeed();
+                    }
                 }
             }
 
@@ -271,18 +291,16 @@ public void initGenerator() throws JacksonException
         }
     }
 
-    /**
-     * Method called by {@link XmlGeneratorInitializer} to inject
-     * necessary configuration.
-     *
-     * @since 3.2
-     */
-    public void initProlog(List<XmlPrologDirective> directives)
+    // @since 3.2
+    private void _prologLinefeed() throws XMLStreamException
     {
-        if (_initialized) { // sanity check
-            _reportError("Internal error: cannot call `initConfig()` after generator already initialized");
+        if (!_stax2Emulation) {
+            if (_xmlPrettyPrinter != null) {
+                _xmlPrettyPrinter.writePrologLinefeed(_xmlWriter);
+            } else {
+                _xmlWriter.writeSpace("\n");
+            }
         }
-        _prologDirectives = directives;
     }
 
     /*

src/main/java/tools/jackson/dataformat/xml/ser/XmlGeneratorInitializer.java

@@ -16,7 +16,9 @@
  * {@link ObjectWriter#with(GeneratorInitializer)}.
  * It allows output of various document-level things such as
  *<ul>
- * <li>Document Type Declarations (DTD); that is "&lt;!DOCTYPE>" directive 
+ * <li>Document Type Declarations (DTD); that is "&lt;!DOCTYPE>" directive
+ *  </li>
+ * <li>XML Comments (in Document prolog, before the root element)
  *  </li>
  * </ul>
  *<p>
@@ -29,15 +31,44 @@ public class XmlGeneratorInitializer
 {
     protected List<XmlPrologDirective> _directives;
 
+    protected boolean _addLfBetweenPrologDirectives = true;
+
     protected boolean _hasDTD;
 
     @Override
     public void initialize(SerializationConfig config, JsonGenerator g) throws JacksonException {
         if (g instanceof ToXmlGenerator xg) {
-            xg.initProlog(_directives);
+            xg.initProlog(_addLfBetweenPrologDirectives, _directives);
         }
     }
 
+    /**
+     * Method to change whether line-feeds are to be added between Prolog directives
+     * or not: default being they are (enabled).
+     *
+     * @param addLFs Whether line-feeds are to be added or not (default: {@code true})
+     *
+     * @return This initializer for call chaining
+     */
+    public XmlGeneratorInitializer linefeedsBetweenPrologDirectives(boolean addLFs) {
+        _addLfBetweenPrologDirectives = addLFs;
+        return this;
+    }
+
+    /**
+     * Method for adding XML comment; to be written in position added
+     * with respective to other directives
+     * (but always after XML Declaration which must come before any other output;
+     * and before Document Root element)
+     *
+     * @param commentContent (optional) Comment content to include
+     *
+     * @return This initializer for call chaining
+     */
+    public XmlGeneratorInitializer addComment(String commentContent) {
+        return _add(new Comment(commentContent));
+    }
+
     /**
      * Convenience method that constructs {@link DTD} out of arguments
      * and calls {@link #addDTD(DTD)}.
@@ -57,10 +88,10 @@ public XmlGeneratorInitializer addDTD(String rootName,
     }
 
     /**
-     * Method for adding Document Type Declaration (DTD) directive; to write
-     * in order added with respective to other directives (but always after
-     * XML Declaration which most come before any other output; and before
-     * Document Root element)
+     * Method for adding Document Type Declaration (DTD) directive; to
+     * be written in position added with respective to other directives
+     * (but always after XML Declaration which must come before any other output;
+     * and before Document Root element)
      *
      * @param dtd DTD to write
      *

src/main/java/tools/jackson/dataformat/xml/util/ArgUtil.java

@@ -0,0 +1,20 @@
+package tools.jackson.dataformat.xml.util;
+
+public abstract class ArgUtil
+{
+    public static String emptyToNull(String str) {
+        return "".equals(str) ? null : str;
+    }
+
+    public static String nullToEmpty(String str) {
+        return (str == null) ? "" : str;
+    }
+
+    public static String nonEmptyNonNull(String prop, String str) {
+        if (str == null || str.isEmpty()) {
+            throw new IllegalArgumentException("Illegal argument for '%s': must be non-empty String"
+                    .formatted(prop));
+        }
+        return str;
+    }
+}