Start work on adding "hasDeserializerFor" for Deserializers

Author: cowtowncoder

src/main/java/com/fasterxml/jackson/databind/deser/Deserializers.java

@@ -235,6 +235,9 @@ public JsonDeserializer<?> findMapLikeDeserializer(MapLikeType type,
             TypeDeserializer elementTypeDeserializer, JsonDeserializer<?> elementDeserializer)
         throws JsonMappingException;
 
+    // To be added in 3.0
+//    public boolean hasDeserializerFor(Class<?> valueType);
+
     /*
     /**********************************************************
     /* Helper classes
@@ -248,7 +251,8 @@ public JsonDeserializer<?> findMapLikeDeserializer(MapLikeType type,
      * of methods are not needed (especially enumeration and array deserializers are
      * very rarely overridden).
      */
-    public static class Base implements Deserializers
+    public abstract static class Base
+        implements Deserializers
     {
         @Override
         public JsonDeserializer<?> findEnumDeserializer(Class<?> type,
@@ -266,14 +270,13 @@ public JsonDeserializer<?> findTreeNodeDeserializer(Class<? extends JsonNode> no
             return null;
         }
 
-        @Override // since 2.7
+        @Override
         public JsonDeserializer<?> findReferenceDeserializer(ReferenceType refType,
                 DeserializationConfig config, BeanDescription beanDesc,
                 TypeDeserializer contentTypeDeserializer, JsonDeserializer<?> contentDeserializer)
-            throws JsonMappingException {
-            // 21-Oct-2015, tatu: For backwards compatibility, let's delegate to "bean" variant,
-            //    for 2.7 -- remove work-around from 2.8 or later
-            return findBeanDeserializer(refType, config, beanDesc);
+            throws JsonMappingException
+        {
+            return null;
         }
 
         @Override
@@ -330,5 +333,24 @@ public JsonDeserializer<?> findMapLikeDeserializer(MapLikeType type,
         {
             return null;
         }
+
+        /**
+         * Method that may be called to check whether this deserializer provider would provide
+         * deserializer for values of given type, without attempting to construct (and possibly
+         * fail in some cases) actual deserializer. Mostly needed to support validation
+         * of polymorphic type ids.
+         *<p>
+         * Note: implementations should take care NOT to claim supporting types that they do
+         * not recognize as this could to incorrect assumption of safe support by caller.
+         *<p>
+         * Method added in this implementation since adding new methods for interfaces
+         * before Java 8 is not a good idea: will be added in Jackson 3.0 for `Deserializers`.
+         *
+         * @since 2.11
+         */
+        public boolean hasDeserializerFor(Class<?> valueType) {
+            return false;
+        }
+//      public abstract boolean hasDeserializerFor(Class<?> valueType);
     }
 }

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

@@ -50,6 +50,14 @@ public JsonDeserializer<?> findBeanDeserializer(JavaType type,
         return null;
     }
 
+    @Override // since 2.11
+    public boolean hasDeserializerFor(Class<?> valueType) {
+        return (valueType == QName.class)
+                || (valueType == XMLGregorianCalendar.class)
+                || (valueType == Duration.class)
+                ;
+    }
+
     /*
     /**********************************************************
     /* Concrete deserializers

src/main/java/com/fasterxml/jackson/databind/module/SimpleDeserializers.java

@@ -18,7 +18,8 @@
  * all mappings must be to exact declared deserialization type.
  */
 public class SimpleDeserializers
-   implements Deserializers, java.io.Serializable
+   extends Deserializers.Base
+   implements java.io.Serializable
 {
     private static final long serialVersionUID = 1L;
 
@@ -78,7 +79,7 @@ public void addDeserializers(Map<Class<?>,JsonDeserializer<?>> desers)
     /* Serializers implementation
     /**********************************************************
      */
-    
+
     @Override
     public JsonDeserializer<?> findArrayDeserializer(ArrayType type,
             DeserializationConfig config, BeanDescription beanDesc,
@@ -180,6 +181,12 @@ public JsonDeserializer<?> findMapLikeDeserializer(MapLikeType type,
         return _find(type);
     }
 
+    @Override // since 2.11
+    public boolean hasDeserializerFor(Class<?> valueType) {
+        return (_classMappings != null)
+                && _classMappings.containsKey(new ClassKey(valueType));
+    }
+
     private final JsonDeserializer<?> _find(JavaType type) {
         if (_classMappings == null) {
             return null;

src/test/java/com/fasterxml/jackson/databind/deser/creators/EnumCreatorTest.java

@@ -115,6 +115,8 @@ public JsonDeserializer<?> findEnumDeserializer(final Class<?> type, final Deser
             }
             return null;
         }
+        @Override
+        public boolean hasDeserializerFor(Class<?> valueType) { return false; }
     }
 
     // [databind#745]

src/test/java/com/fasterxml/jackson/databind/deser/creators/TestCreatorNullValue.java

@@ -48,6 +48,8 @@ public JsonDeserializer<?> findBeanDeserializer(JavaType type,
             }
             return new ContainedDeserializer();
         }
+        @Override
+        public boolean hasDeserializerFor(Class<?> valueType) { return false; }
     }
 
     protected static class TestModule extends com.fasterxml.jackson.databind.Module