Start work on #324

src/main/java/com/fasterxml/jackson/dataformat/xml/XmlTypeResolverBuilder.java

@@ -34,7 +34,6 @@ public XmlTypeResolverBuilder(JsonTypeInfo.Value settings) {
     @Override
     public StdTypeResolverBuilder init(JsonTypeInfo.Id idType, TypeIdResolver idRes)
     {
-
         super.init(idType, idRes);
         if (_typeProperty != null) {
             _typeProperty = StaxUtil.sanitizeXmlTypeName(_typeProperty);
@@ -138,6 +137,8 @@ protected static String decodeXmlClassName(String className)
     protected static class XmlClassNameIdResolver
         extends ClassNameIdResolver
     {
+        private static final long serialVersionUID = 1L;
+
         public XmlClassNameIdResolver(JavaType baseType, TypeFactory typeFactory,
                 PolymorphicTypeValidator ptv)
         {
@@ -159,6 +160,8 @@ public JavaType typeFromId(DatabindContext context, String id) throws IOExceptio
     protected static class XmlMinimalClassNameIdResolver
         extends MinimalClassNameIdResolver
     {
+        private static final long serialVersionUID = 1L;
+
         public XmlMinimalClassNameIdResolver(JavaType baseType, TypeFactory typeFactory,
                 PolymorphicTypeValidator ptv)
         {

src/main/java/com/fasterxml/jackson/dataformat/xml/ser/ToXmlGenerator.java

@@ -93,6 +93,19 @@ public enum Feature implements FormatFeature
          * @since 2.13
          */
         UNWRAP_ROOT_OBJECT_NODE(false),
+
+        /**
+         * Feature that enables automatic conversion of logical property
+         * name {@code "xsi:type"} into matching XML name where "type"
+         * is the local name and "xsi" prefix is bound to URI
+         * {@link XMLConstants#W3C_XML_SCHEMA_INSTANCE_NS_URI},
+         * and output is indicated to be done as XML Attribute.
+         * This is mostly desirable for Polymorphic handling where it is difficult
+         * to specify XML Namespace for type identifier
+         *
+         * @since 2.17
+         */
+        AUTO_DETECT_XSI_TYPE(false),
         ;
 
         final boolean _defaultState;
@@ -247,20 +260,26 @@ public void initGenerator()  throws IOException
         }
         _initialized = true;
         try {
+            boolean xmlDeclWritten;
             if (Feature.WRITE_XML_1_1.enabledIn(_formatFeatures)) {
                 _xmlWriter.writeStartDocument("UTF-8", "1.1");
+                xmlDeclWritten = true;
             } else if (Feature.WRITE_XML_DECLARATION.enabledIn(_formatFeatures)) {
                 _xmlWriter.writeStartDocument("UTF-8", "1.0");
+                xmlDeclWritten = true;
             } else {
-                return;
+                xmlDeclWritten = false;
             }
             // as per [dataformat-xml#172], try adding indentation
-            if (_xmlPrettyPrinter != null) {
+            if (xmlDeclWritten && (_xmlPrettyPrinter != null)) {
                 // ... but only if it is likely to succeed:
                 if (!_stax2Emulation) {
                     _xmlPrettyPrinter.writePrologLinefeed(_xmlWriter);
                 }
             }
+            if (Feature.AUTO_DETECT_XSI_TYPE.enabledIn(_formatFeatures)) {
+                _xmlWriter.setPrefix("xsi", XMLConstants.W3C_XML_SCHEMA_INSTANCE_NS_URI);
+            }
         } catch (XMLStreamException e) {
             StaxUtil.throwAsGenerationException(e, this);
         }
@@ -487,23 +506,35 @@ public void writeRepeatedFieldName() throws IOException
     /* JsonGenerator method overrides
     /**********************************************************
      */
-
-    /* Most overrides in this section are just to make methods final,
-     * to allow better inlining...
-     */
+
+    @Override
+    public void writeFieldName(SerializableString name) throws IOException
+    {
+        writeFieldName(name.getValue());
+    }
 
     @Override
     public final void writeFieldName(String name) throws IOException
     {
         if (_writeContext.writeFieldName(name) == JsonWriteContext.STATUS_EXPECT_VALUE) {
             _reportError("Can not write a field name, expecting a value");
         }
-        // Should this ever get called?
-        String ns = (_nextName == null) ? "" : _nextName.getNamespaceURI();
-        _nameToEncode.namespace = ns;
-        _nameToEncode.localPart = name;
-        _nameProcessor.encodeName(_nameToEncode);
-        setNextName(new QName(_nameToEncode.namespace, _nameToEncode.localPart));
+
+        String ns;
+        // 30-Jan-2024, tatu: Surprise!
+        if (Feature.AUTO_DETECT_XSI_TYPE.enabledIn(_formatFeatures)
+                && "xsi:type".equals(name)) {
+            setNextName(new QName(XMLConstants.W3C_XML_SCHEMA_INSTANCE_NS_URI,
+                    "type", "xsi"));
+            setNextIsAttribute(true);
+        } else {
+            // Should this ever get called?
+            ns = (_nextName == null) ? "" : _nextName.getNamespaceURI();
+            _nameToEncode.namespace = ns;
+            _nameToEncode.localPart = name;
+            _nameProcessor.encodeName(_nameToEncode);
+            setNextName(new QName(_nameToEncode.namespace, _nameToEncode.localPart));
+        }
     }
 
     @Override
@@ -519,7 +550,7 @@ public final void writeStringField(String fieldName, String value) throws IOExce
     //    handling...
     //
     //    See [dataformat-xml#4] for more context.
-    
+
     /*
     // @since 2.9
     public WritableTypeId writeTypePrefix(WritableTypeId typeIdDef) throws IOException
@@ -640,12 +671,6 @@ public final void _handleEndObject() throws IOException
     /**********************************************************
      */
 
-    @Override
-    public void writeFieldName(SerializableString name) throws IOException
-    {
-        writeFieldName(name.getValue());
-    }
-
     @Override
     public void writeString(String text) throws IOException
     {

src/test/java/com/fasterxml/jackson/dataformat/xml/ser/XsiTypeWriteTest.java

@@ -0,0 +1,60 @@
+package com.fasterxml.jackson.dataformat.xml.ser;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.annotation.JsonRootName;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import com.fasterxml.jackson.annotation.JsonTypeInfo.As;
+import com.fasterxml.jackson.annotation.JsonTypeInfo.Id;
+import com.fasterxml.jackson.dataformat.xml.XmlMapper;
+import com.fasterxml.jackson.dataformat.xml.XmlTestBase;
+
+// [dataformat-xml#324]
+public class XsiTypeWriteTest extends XmlTestBase
+{
+    @JsonRootName("Typed")
+    static class TypeBean {
+        @JsonProperty("xsi:type")
+        public String typeId = "abc";
+    }
+
+    @JsonRootName("Poly")
+    @JsonTypeInfo(use = Id.SIMPLE_NAME, include = As.PROPERTY, property="xsi:type")
+    static class PolyBean {
+        public int value = 42;
+    }
+
+    private final XmlMapper NO_XSI_MAPPER = XmlMapper.builder()
+            .configure(ToXmlGenerator.Feature.AUTO_DETECT_XSI_TYPE, false)
+            .build();
+
+    private final XmlMapper XSI_ENABLED_MAPPER = XmlMapper.builder()
+            .configure(ToXmlGenerator.Feature.AUTO_DETECT_XSI_TYPE, true)
+            .build();
+
+    public void testExplicitXsiTypeWriteDisabled() throws Exception
+    {
+        assertEquals("<Typed><xsi:type>abc</xsi:type></Typed>",
+                NO_XSI_MAPPER.writeValueAsString(new TypeBean()));
+    }
+
+    public void testExplicitXsiTypeWriteEnabled() throws Exception
+    {
+        assertEquals(
+                a2q("<Typed xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xsi:type='abc'/>"),
+                a2q(XSI_ENABLED_MAPPER.writeValueAsString(new TypeBean())));
+    }
+
+    public void testXsiTypeAsTypeIdWriteDisabled() throws Exception
+    {
+        assertEquals("<Poly><xsi_type>abc</xsi_type><value>42</value></Poly>",
+                NO_XSI_MAPPER.writeValueAsString(new PolyBean()));
+    }
+
+    public void testXsiTypeAsTypeIdWriteEnabled() throws Exception
+    {
+        assertEquals(
+                a2q("<Poly xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xsi:type='abc'>"
+                        +"<value>42</value></Poly>"),
+                a2q(XSI_ENABLED_MAPPER.writeValueAsString(new PolyBean())));
+    }
+}