Extend support for deserializing via builder if the builder has type bindings by inferring those bindings from the value type. This behavior is not ideal, but addresses the common case, and is controlled by a MapperFeature.

Author: Ville Koskela

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

@@ -198,6 +198,18 @@ public enum MapperFeature implements ConfigFeature
      */
     USE_STATIC_TYPING(false),
 
+    /**
+     * Feature that enables inferring builder type bindings from the value type
+     * being deserialized. This requires that the generic type declaration on
+     * the value type match that on the builder exactly.
+     *<p>
+     * Feature is disabled by default which means that deserialization does
+     * not support deserializing types via builders with type parameters.
+     *<p>
+     * See: https://github.com/FasterXML/jackson-databind/issues/921
+     */
+    INFER_BUILDER_TYPE_BINDINGS(false),
+
     /*
     /******************************************************
     /* View-related features

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

@@ -138,12 +138,18 @@ public JsonDeserializer<Object> createBeanDeserializer(DeserializationContext ct
     }
 
     @Override
-    public JsonDeserializer<Object> createBuilderBasedDeserializer(DeserializationContext ctxt,
-            JavaType valueType, BeanDescription beanDesc, Class<?> builderClass)
-                    throws JsonMappingException
+    public JsonDeserializer<Object> createBuilderBasedDeserializer(
+            DeserializationContext ctxt, JavaType valueType, BeanDescription beanDesc,
+            Class<?> builderClass)
+        throws JsonMappingException
     {
         // First: need a BeanDescription for builder class
-        JavaType builderType = ctxt.constructType(builderClass);
+        JavaType builderType;
+        if (ctxt.isEnabled(MapperFeature.INFER_BUILDER_TYPE_BINDINGS)) {
+            builderType = ctxt.getTypeFactory().constructParametricType(builderClass, valueType.getBindings());
+        } else {
+            builderType = ctxt.constructType(builderClass);
+        }
         BeanDescription builderDesc = ctxt.getConfig().introspectForBuilder(builderType);
         return buildBuilderBasedDeserializer(ctxt, valueType, builderDesc);
     }

src/main/java/com/fasterxml/jackson/databind/type/TypeFactory.java

@@ -894,7 +894,29 @@ public JavaType constructParametricType(Class<?> parametrized, Class<?>... param
      */
     public JavaType constructParametricType(Class<?> rawType, JavaType... parameterTypes)
     {
-        return _fromClass(null, rawType, TypeBindings.create(rawType, parameterTypes));
+        return constructParametricType(rawType, TypeBindings.create(rawType, parameterTypes));
+    }
+
+    /**
+     * Factory method for constructing {@link JavaType} that
+     * represents a parameterized type. The type's parameters are
+     * specified as an instance of {@link TypeBindings}. This
+     * is useful if you already have the type's parameters such
+     * as those found on {@link JavaType}. For example, you could
+     * call
+     * <pre>
+     *   return TypeFactory.constructParametricType(ArrayList.class, javaType.getBindings());
+     * </pre>
+     * This effectively applies the parameterized types from one
+     * {@link JavaType} to another class.
+     *
+     * @param rawType Actual type-erased type
+     * @param parameterTypes Type bindings for the raw type
+     * @since 3.0
+     */
+    public JavaType constructParametricType(Class<?> rawType, TypeBindings parameterTypes)
+    {
+        return _fromClass(null, rawType, parameterTypes);
     }
 
     /*

src/test/java/com/fasterxml/jackson/failing/BuilderDeserializationTest921.java renamed to src/test/java/com/fasterxml/jackson/databind/deser/builder/BuilderWithTypeParametersTest.java

@@ -1,13 +1,16 @@
-package com.fasterxml.jackson.failing;
+package com.fasterxml.jackson.databind.deser.builder;
 
-import java.util.List;
-
-import com.fasterxml.jackson.annotation.*;
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.core.type.TypeReference;
-import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.BaseMapTest;
+import com.fasterxml.jackson.databind.MapperFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
+import java.util.LinkedHashMap;
+import java.util.List;
 
-public class BuilderDeserializationTest921
+public class BuilderWithTypeParametersTest
     extends BaseMapTest
 {
     public static class MyPOJO {
@@ -77,15 +80,27 @@ public MyGenericPOJOWithCreator<T> build() {
       }
     }
 
-    public void testWithBuilder() throws Exception {
+    public void testWithBuilderInferringBindings() throws Exception {
+        final ObjectMapper mapper = new ObjectMapper();
+        mapper.enable(MapperFeature.INFER_BUILDER_TYPE_BINDINGS);
+        final String json = aposToQuotes("{ 'data': [ { 'x': 'x', 'y': 'y' } ] }");
+        final MyGenericPOJO<MyPOJO> deserialized =
+                mapper.readValue(json, new TypeReference<MyGenericPOJO<MyPOJO>>() {});
+        assertEquals(1, deserialized.data.size());
+        Object ob = deserialized.data.get(0);
+        assertNotNull(ob);
+        assertEquals(MyPOJO.class, ob.getClass());
+    }
+
+    public void testWithBuilderWithoutInferringBindings() throws Exception {
       final ObjectMapper mapper = new ObjectMapper();
       final String json = aposToQuotes("{ 'data': [ { 'x': 'x', 'y': 'y' } ] }");
       final MyGenericPOJO<MyPOJO> deserialized =
           mapper.readValue(json, new TypeReference<MyGenericPOJO<MyPOJO>>() {});
       assertEquals(1, deserialized.data.size());
       Object ob = deserialized.data.get(0);
       assertNotNull(ob);
-      assertEquals(MyPOJO.class, ob.getClass());
+      assertEquals(LinkedHashMap.class, ob.getClass());
     }
 
     public void testWithCreator() throws Exception {

src/test/java/com/fasterxml/jackson/databind/type/TestTypeFactory.java

@@ -175,13 +175,27 @@ public void testParametricTypes()
         assertEquals(t, t2.containedType(1));
         assertNull(t2.containedType(2));
 
-        // and then custom generic type as well
+        // Then using TypeBindings
+        JavaType t3 = tf.constructParametricType(HashSet.class, t.getBindings()); // HashSet<String>
+        assertEquals(CollectionType.class, t3.getClass());
+        assertEquals(1, t3.containedTypeCount());
+        assertEquals(strC, t3.containedType(0));
+        assertNull(t3.containedType(1));
+
+        // Then custom generic type as well
         JavaType custom = tf.constructParametricType(SingleArgGeneric.class, String.class);
         assertEquals(SimpleType.class, custom.getClass());
         assertEquals(1, custom.containedTypeCount());
         assertEquals(strC, custom.containedType(0));
         assertNull(custom.containedType(1));
 
+        // and then custom generic type from TypeBindings
+        JavaType custom2 = tf.constructParametricType(SingleArgGeneric.class, t.getBindings());
+        assertEquals(SimpleType.class, custom2.getClass());
+        assertEquals(1, custom2.containedTypeCount());
+        assertEquals(strC, custom2.containedType(0));
+        assertNull(custom2.containedType(1));
+
         // And finally, ensure that we can't create invalid combinations
         try {
             // Maps must take 2 type parameters, not just one