Add feature for QNAME serialization and deserialization.

This commit fixes FasterXML#4771 by adding serialization and deserialization features to control whether a QName is serialized to a string using the "QName.toString()" method (the only option currently) or if it is serialized to JSON object (a new option to fix FasterXML#4771).

src/main/java/com/fasterxml/jackson/databind/DeserializationFeature.java

@@ -430,6 +430,19 @@ public enum DeserializationFeature implements ConfigFeature
      */
     READ_ENUMS_USING_TO_STRING(false),
 
+    /**
+     * Feature that determines standard deserialization mechanism used for
+     * QName values: if enabled, QNames are assumed to have been serialized using
+     * return value of <code>QName.toString()</code>;
+     * if disabled, it is assumed that the QName was serialized as an object.
+     *<p>
+     * Note: this feature should usually have same value
+     * as {@link SerializationFeature#WRITE_QNAMES_USING_TO_STRING}.
+     *<p>
+     * Feature is disabled by default.
+     */
+    READ_QNAMES_USING_VALUE_OF(true),
+
     /**
      * Feature that allows unknown Enum values to be parsed as {@code null} values.
      * If disabled, unknown Enum values will throw exceptions.

src/main/java/com/fasterxml/jackson/databind/SerializationFeature.java

@@ -292,6 +292,18 @@ public enum SerializationFeature implements ConfigFeature
      */
     WRITE_ENUMS_USING_TO_STRING(false),
 
+    /**
+     * Feature that determines standard serialization mechanism used for
+     * QName values: if enabled, return value of <code>QName.toString()</code>
+     * is used; if disabled, the QName is serialized as an object.
+     *<p>
+     * Note: this feature should usually have same value
+     * as {@link DeserializationFeature#READ_QNAMES_USING_VALUE_OF}.
+     *<p>
+     * Feature is disabled by default.
+     */
+    WRITE_QNAMES_USING_TO_STRING(true),
+
     /**
      * Feature that determines whether Java Enum values are serialized
      * as numbers (true), or textual values (false). If textual values are

src/main/java/com/fasterxml/jackson/databind/ext/CoreXMLDeserializers.java

@@ -7,6 +7,8 @@
 import javax.xml.namespace.QName;
 
 import com.fasterxml.jackson.core.*;
+import com.fasterxml.jackson.core.type.TypeReference;
+
 import com.fasterxml.jackson.databind.*;
 import com.fasterxml.jackson.databind.deser.Deserializers;
 import com.fasterxml.jackson.databind.deser.std.FromStringDeserializer;
@@ -40,7 +42,10 @@ public JsonDeserializer<?> findBeanDeserializer(JavaType type,
     {
         Class<?> raw = type.getRawClass();
         if (raw == QName.class) {
-            return new Std(raw, TYPE_QNAME);
+            if (config == null || config.isEnabled(DeserializationFeature.READ_QNAMES_USING_VALUE_OF)) {
+                return new Std(raw, TYPE_QNAME);
+            }
+            return new QNameObjectDeserializer();
         }
         if (raw == XMLGregorianCalendar.class) {
             return new Std(raw, TYPE_G_CALENDAR);
@@ -149,4 +154,27 @@ protected XMLGregorianCalendar _gregorianFromDate(DeserializationContext ctxt,
             return _dataTypeFactory.newXMLGregorianCalendar(calendar);
         }
     }
+
+    private class QNameObjectDeserializer extends JsonDeserializer<QName> {
+        private static final long serialVersionUID = 1L;
+        public static final TypeReference<Map<String, String>> STRING_MAP_TYPE_REFERENCE = new TypeReference<>() {};
+
+        @Override
+        public QName deserialize(final JsonParser p, final DeserializationContext ctxt)
+            throws IOException
+        {
+            Map<String, String> map;
+            try {
+                map = p.readValueAs(STRING_MAP_TYPE_REFERENCE);
+            } catch (IOException e) {
+                throw new JsonMappingException(p, "Unable to parse the QName as an object.", e);
+            }
+
+            return new QName(
+                map.getOrDefault("namespaceURI", ""),
+                map.getOrDefault("localPart", ""),
+                map.getOrDefault("prefix", "")
+            );
+        }
+    }
 }

src/main/java/com/fasterxml/jackson/databind/ext/CoreXMLSerializers.java

@@ -34,9 +34,15 @@ public JsonSerializer<?> findSerializer(SerializationConfig config,
             JavaType type, BeanDescription beanDesc)
     {
         Class<?> raw = type.getRawClass();
-        if (Duration.class.isAssignableFrom(raw) || QName.class.isAssignableFrom(raw)) {
+        if (Duration.class.isAssignableFrom(raw)){
             return ToStringSerializer.instance;
         }
+        if (QName.class.isAssignableFrom(raw)) {
+            if (config.isEnabled(SerializationFeature.WRITE_QNAMES_USING_TO_STRING)) {
+                return ToStringSerializer.instance;
+            }
+            return null;
+        }
         if (XMLGregorianCalendar.class.isAssignableFrom(raw)) {
             return XMLGregorianCalendarSerializer.instance;
         }

src/test/java/com/fasterxml/jackson/databind/ext/QNameAsObjectReadWrite4771Test.java

@@ -0,0 +1,46 @@
+package com.fasterxml.jackson.databind.ext;
+
+import java.util.stream.Stream;
+import javax.xml.namespace.QName;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.SerializationFeature;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import static com.fasterxml.jackson.databind.testutil.DatabindTestUtil.newJsonMapper;
+
+class QNameAsObjectReadWrite4771Test {
+
+  private final ObjectMapper MAPPER = newJsonMapper();
+
+  @ParameterizedTest
+  @MethodSource("provideAllPerumtationsOfQNameConstructor")
+  void testQNameWithObjectSerialization(QName originalQName) throws JsonProcessingException {
+    String json = MAPPER
+        .disable(SerializationFeature.WRITE_QNAMES_USING_TO_STRING)
+        .writeValueAsString(originalQName);
+
+    QName deserializedQName = MAPPER
+        .disable(DeserializationFeature.READ_QNAMES_USING_VALUE_OF)
+        .readValue(json, QName.class);
+
+    assertEquals(originalQName, deserializedQName);
+  }
+
+  static Stream<Arguments> provideAllPerumtationsOfQNameConstructor() {
+    return Stream.of(
+        Arguments.of(new QName("test-local-part")),
+        Arguments.of(new QName("test-namespace-uri", "test-local-part")),
+        Arguments.of(new QName("test-namespace-uri", "test-local-part", "test-prefix"))
+    );
+  }
+
+}