Fix #324: support auto-detection, conversion of xsi:type on generat…

…ion (#633)

release-notes/VERSION-2.x

@@ -6,6 +6,9 @@ Project: jackson-dataformat-xml
 
 2.17.0 (not yet released)
 
+#324: Support use of `xsi:type` for polymorphic serialization
+  (`ToXmlGenerator.Feature.AUTO_DETECT_XSI_TYPE`)
+ (requested by @philipzhaoTS)
 #618: `ArrayIndexOutOfBoundsException` thrown for invalid ending XML string
   when using JDK default Stax XML parser
  (reported by Arthur C)

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

@@ -21,6 +21,10 @@
  * Custom specialization of {@link StdTypeResolverBuilder}; needed so that
  * type id property name can be modified as necessary to make it legal
  * XML element or attribute name.
+ *<p>
+ * NOTE: Since 2.17, property name cleansing only applied to default
+ * names (like {@code "@class"} and {@code "@type"}) but not to explicitly
+ * specified ones (where caller presumably knows what to do).
  */
 public class XmlTypeResolverBuilder extends StdTypeResolverBuilder
 {
@@ -32,33 +36,21 @@ 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);
-        }
-        return this;
-    }
-
-    @Override
-    public StdTypeResolverBuilder init(JsonTypeInfo.Value settings, TypeIdResolver idRes) {
-        super.init(settings, idRes);
-        if (_typeProperty != null) {
-            _typeProperty = StaxUtil.sanitizeXmlTypeName(_typeProperty);
-        }
-        return this;
-    }
-
-    @Override
-    public StdTypeResolverBuilder typeProperty(String typeIdPropName)
-    {
-        // ok to have null/empty; will restore to use defaults
-        if (typeIdPropName == null || typeIdPropName.length() == 0) {
-            typeIdPropName = _idType.getDefaultPropertyName();
+    protected String _propName(String propName, JsonTypeInfo.Id idType) {
+        // 30-Jan-2024, tatu: Before 2.17 we used to indiscriminately cleanse
+        //   property name always; with 2.17+ only default ones
+        if (propName == null || propName.isEmpty()) {
+            propName = StaxUtil.sanitizeXmlTypeName(idType.getDefaultPropertyName());
+        } else {
+            // ... alas, there's... a "feature" (read: bug) in `JsonTypeInfo.Value` construction
+            // which will automatically apply default property name if no explicit property
+            // name specific. This means we don't really know if default is being used.
+            // But let's assume that if "propName.equals(defaultPropName)" this is the case.
+            if (propName.equals(idType.getDefaultPropertyName())) {
+                propName = StaxUtil.sanitizeXmlTypeName(propName);
+            }
         }
-        _typeProperty = StaxUtil.sanitizeXmlTypeName(typeIdPropName);
-        return this;
+        return propName;
     }
 
     @Override

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;
@@ -264,6 +277,9 @@ public void initGenerator()  throws IOException
                     _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);
         }
@@ -502,12 +518,22 @@ 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
@@ -523,9 +549,10 @@ public final void writeStringField(String fieldName, String value) throws IOExce
     //    handling...
     //
     //    See [dataformat-xml#4] for more context.
-
+
+    // 30-Jan-2024, tatu: With 2.17 we may want to revisit this.
     /*
-    // @since 2.9
+    @Override
     public WritableTypeId writeTypePrefix(WritableTypeId typeIdDef) throws IOException
     {
         // 03-Aug-2017, tatu: Due to XML oddities, we do need to massage things

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

@@ -0,0 +1,63 @@
+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
+    {
+        // not legal XML but with explicitly specified name is what caller wants
+        // (note: not 100% sure how xsi:type is written as attribute)
+        assertEquals(
+                a2q("<Poly xsi:type='PolyBean'><value>42</value></Poly>"),
+                a2q(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='PolyBean'>"
+                        +"<value>42</value></Poly>"),
+                a2q(XSI_ENABLED_MAPPER.writeValueAsString(new PolyBean())));
+    }
+}