Adds support for @JsonKey annotation

When serializing the key of a Map, look for a `@JsonKey` annotation.
When present (taking priority over `@JsonValue`), skip the
StdKey:Serializer and attempt to find a serializer for the inner type.

Fixes FasterXML#2871

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

@@ -967,6 +967,23 @@ public PropertyName findNameForSerialization(Annotated a) {
         return null;
     }
 
+    /**
+     * Method for checking whether given method has an annotation
+     * that suggests the return value of annotated method
+     * should be used as "the key" of the object instance; usually
+     * serialized as a primitive value such as String or number.
+     *
+     * @return {@link Boolean#TRUE} if such annotation is found and is not disabled;
+     *   {@link Boolean#FALSE} if disabled annotation (block) is found (to indicate
+     *   accessor is definitely NOT to be used "as value"); or `null` if no
+     *   information found.
+     *
+     * @since TODO
+     */
+    public Boolean hasAsKey(Annotated a) {
+        return null;
+    }
+
     /**
      * Method for checking whether given method has an annotation
      * that suggests that the return value of annotated method

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

@@ -173,6 +173,17 @@ public boolean isNonStaticInnerClass() {
     /**********************************************************
      */
 
+    /**
+     * Method for locating accessor (readable field, or "getter" method)
+     * that has
+     * {@link com.fasterxml.jackson.annotation.JsonKey} annotation,
+     * if any. If multiple ones are found,
+     * an error is reported by throwing {@link IllegalArgumentException}
+     *
+     * @since TODO
+     */
+    public abstract AnnotatedMember findJsonKeyAccessor();
+
     /**
      * Method for locating accessor (readable field, or "getter" method)
      * that has

src/main/java/com/fasterxml/jackson/databind/introspect/BasicBeanDescription.java

@@ -239,6 +239,12 @@ public List<BeanPropertyDefinition> findProperties() {
         return _properties();
     }
 
+    @Override
+    public AnnotatedMember findJsonKeyAccessor() {
+        return (_propCollector == null) ? null
+                : _propCollector.getJsonKeyAccessor();
+    }
+
     @Override
     @Deprecated // since 2.9
     public AnnotatedMethod findJsonValueMethod() {

src/main/java/com/fasterxml/jackson/databind/introspect/JacksonAnnotationIntrospector.java

@@ -1071,6 +1071,15 @@ public PropertyName findNameForSerialization(Annotated a)
         return null;
     }
 
+    @Override
+    public Boolean hasAsKey(Annotated a) {
+        JsonKey ann = _findAnnotation(a, JsonKey.class);
+        if (ann == null) {
+            return null;
+        }
+        return ann.value();
+    }
+
     @Override // since 2.9
     public Boolean hasAsValue(Annotated a) {
         JsonValue ann = _findAnnotation(a, JsonValue.class);

src/main/java/com/fasterxml/jackson/databind/introspect/POJOPropertiesCollector.java

@@ -106,6 +106,11 @@ public class POJOPropertiesCollector
 
     protected LinkedList<AnnotatedMember> _anySetterField;
 
+    /**
+     * Method(s) annotated with 'JsonKey' annotation
+     */
+    protected LinkedList<AnnotatedMember> _jsonKeyAccessors;
+
     /**
      * Method(s) marked with 'JsonValue' annotation
      *<p>
@@ -187,6 +192,23 @@ public Map<Object, AnnotatedMember> getInjectables() {
         return _injectables;
     }
 
+    public AnnotatedMember getJsonKeyAccessor() {
+        if (!_collected) {
+            collectAll();
+        }
+        // If @JsonKey defined, must have a single one
+        if (_jsonKeyAccessors != null) {
+            if (_jsonKeyAccessors.size() > 1) {
+                reportProblem("Multiple 'as-value' properties defined (%s vs %s)",
+                        _jsonKeyAccessors.get(0),
+                        _jsonKeyAccessors.get(1));
+            }
+            // otherwise we won't greatly care
+            return _jsonKeyAccessors.get(0);
+        }
+        return null;
+    }
+
     /**
      * @since 2.9
      */
@@ -384,6 +406,13 @@ protected void _addFields(Map<String, POJOPropertyBuilder> props)
         final boolean transientAsIgnoral = _config.isEnabled(MapperFeature.PROPAGATE_TRANSIENT_MARKER);
 
         for (AnnotatedField f : _classDef.fields()) {
+            // @JsonKey?
+            if (Boolean.TRUE.equals(ai.hasAsKey(f))) {
+                if (_jsonKeyAccessors == null) {
+                    _jsonKeyAccessors = new LinkedList<>();
+                }
+                _jsonKeyAccessors.add(f);
+            }
             // @JsonValue?
             if (Boolean.TRUE.equals(ai.hasAsValue(f))) {
                 if (_jsonValueAccessors == null) {
@@ -596,6 +625,14 @@ protected void _addGetterMethod(Map<String, POJOPropertyBuilder> props,
             _anyGetters.add(m);
             return;
         }
+        // @JsonKey?
+        if (Boolean.TRUE.equals(ai.hasAsKey(m))) {
+            if (_jsonKeyAccessors == null) {
+                _jsonKeyAccessors = new LinkedList<>();
+            }
+            _jsonKeyAccessors.add(m);
+            return;
+        }
         // @JsonValue?
         if (Boolean.TRUE.equals(ai.hasAsValue(m))) {
             if (_jsonValueAccessors == null) {

src/main/java/com/fasterxml/jackson/databind/ser/BasicSerializerFactory.java

@@ -228,18 +228,30 @@ public JsonSerializer<Object> createKeySerializer(SerializerProvider ctxt,
                     ser = StdKeySerializers.getStdKeySerializer(config, keyType.getRawClass(), false);
                     // As per [databind#47], also need to support @JsonValue
                     if (ser == null) {
-                        AnnotatedMember am = beanDesc.findJsonValueAccessor();
-                        if (am != null) {
-                            final Class<?> rawType = am.getRawType();
-                            JsonSerializer<?> delegate = StdKeySerializers.getStdKeySerializer(config,
-                                    rawType, true);
+                        AnnotatedMember keyAm = beanDesc.findJsonKeyAccessor();
+                        if (keyAm != null) {
+                            final Class<?> rawType = keyAm.getRawType();
+                            JsonSerializer<?> delegate = createKeySerializer(ctxt, config.constructType(rawType), null);
                             if (config.canOverrideAccessModifiers()) {
-                                ClassUtil.checkAndFixAccess(am.getMember(),
+                                ClassUtil.checkAndFixAccess(keyAm.getMember(),
                                         config.isEnabled(MapperFeature.OVERRIDE_PUBLIC_ACCESS_MODIFIERS));
                             }
-                            ser = new JsonValueSerializer(am, delegate);
-                        } else {
-                            ser = StdKeySerializers.getFallbackKeySerializer(config, keyType.getRawClass());
+                            ser = new JsonValueSerializer(keyAm, delegate);
+                        }
+                        if (ser == null) {
+                            AnnotatedMember am = beanDesc.findJsonValueAccessor();
+                            if (am != null) {
+                                final Class<?> rawType = am.getRawType();
+                                JsonSerializer<?> delegate = StdKeySerializers.getStdKeySerializer(config,
+                                        rawType, true);
+                                if (config.canOverrideAccessModifiers()) {
+                                    ClassUtil.checkAndFixAccess(am.getMember(),
+                                            config.isEnabled(MapperFeature.OVERRIDE_PUBLIC_ACCESS_MODIFIERS));
+                                }
+                                ser = new JsonValueSerializer(am, delegate);
+                            } else {
+                                ser = StdKeySerializers.getFallbackKeySerializer(config, keyType.getRawClass());
+                            }
                         }
                     }
                 }

src/test/java/com/fasterxml/jackson/databind/jsontype/MapSerializingTest.java

@@ -0,0 +1,79 @@
+package com.fasterxml.jackson.databind.jsontype;
+
+import java.util.Collections;
+import java.util.Map;
+
+import com.fasterxml.jackson.annotation.JsonKey;
+import com.fasterxml.jackson.annotation.JsonValue;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.Assert;
+import org.junit.Ignore;
+import org.junit.Test;
+
+public class MapSerializingTest {
+	class Inner {
+		@JsonKey
+		String key;
+
+		@JsonValue
+		String value;
+
+		Inner(String key, String value) {
+			this.key = key;
+			this.value = value;
+		}
+
+		public String toString() {
+			return "Inner(" + this.key + "," + this.value + ")";
+		}
+
+	}
+
+	class Outer {
+		@JsonKey
+		@JsonValue
+		Inner inner;
+
+		Outer(Inner inner) {
+			this.inner = inner;
+		}
+
+	}
+
+	class NoKeyOuter {
+		@JsonValue
+		Inner inner;
+
+		NoKeyOuter(Inner inner) {
+			this.inner = inner;
+		}
+	}
+
+	@Ignore
+	@Test
+	public void testClassAsKey() throws Exception {
+		ObjectMapper mapper = new ObjectMapper();
+		Outer outer = new Outer(new Inner("innerKey", "innerValue"));
+		Map<Outer, String> map = Collections.singletonMap(outer, "value");
+		String actual = mapper.writeValueAsString(map);
+		Assert.assertEquals("{\"innerKey\":\"value\"}", actual);
+	}
+
+	@Ignore
+	@Test
+	public void testClassAsValue() throws Exception {
+		ObjectMapper mapper = new ObjectMapper();
+		Map<String, Outer> mapA = Collections.singletonMap("key", new Outer(new Inner("innerKey", "innerValue")));
+		String actual = mapper.writeValueAsString(mapA);
+		Assert.assertEquals("{\"key\":\"innerValue\"}", actual);
+	}
+
+	@Ignore
+	@Test
+	public void testNoKeyOuter() throws Exception {
+		ObjectMapper mapper = new ObjectMapper();
+		Map<String, NoKeyOuter> mapA = Collections.singletonMap("key", new NoKeyOuter(new Inner("innerKey", "innerValue")));
+		String actual = mapper.writeValueAsString(mapA);
+		Assert.assertEquals("{\"key\":\"innerValue\"}", actual);
+	}
+}