implemented support for deserialization to deserialize to more specific type if an instance creator returns a sub-class.

gson/src/main/java/com/google/gson/JsonDeserializationContextDefault.java

@@ -79,7 +79,8 @@ final class JsonDeserializationContextDefault implements JsonDeserializationCont
       JsonDeserializationContext context) throws JsonParseException {
     JsonObjectDeserializationVisitor<T> visitor = new JsonObjectDeserializationVisitor<T>(
         json, typeOfT, navigatorFactory, objectConstructor, deserializers, context);
-    ObjectNavigator on = navigatorFactory.create(new ObjectTypePair(json.getAsObject(), typeOfT, true));
+    ObjectNavigator on = 
+      navigatorFactory.create(new ObjectTypePair(json.getAsObject(), typeOfT, true));
     on.accept(visitor);
     Object target = visitor.getTarget();
     return (T) target;

gson/src/main/java/com/google/gson/JsonDeserializationVisitor.java

@@ -66,16 +66,17 @@ abstract class JsonDeserializationVisitor<T> implements ObjectNavigator.Visitor
 
   @SuppressWarnings("unchecked")
   public final boolean visitUsingCustomHandler(ObjectTypePair objTypePair) {
-    Type objType = objTypePair.getType();
-    JsonDeserializer deserializer = deserializers.getHandlerFor(objType);
-    if (deserializer != null) {
+    Pair<JsonDeserializer<?>, ObjectTypePair> pair = objTypePair.getMatchingHandler(deserializers);
+    if (pair == null) {
+      return false;
+    }    
     if (!json.isJsonNull()) {
+      JsonDeserializer deserializer = pair.getFirst();
+      Type objType = pair.getSecond().getType();
       target = (T) deserializer.deserialize(json, objType, context);
     }
     return true;
   }
-    return false;
-  }
 
   final Object visitChildAsObject(Type childType, JsonElement jsonChild) {
     JsonDeserializationVisitor<?> childVisitor =
@@ -92,7 +93,7 @@ abstract class JsonDeserializationVisitor<T> implements ObjectNavigator.Visitor
   }
 
   private Object visitChild(Type type, JsonDeserializationVisitor<?> childVisitor) {
-    ObjectNavigator on = factory.create(new ObjectTypePair(null, type, true));
+    ObjectNavigator on = factory.create(new ObjectTypePair(null, type, false));
     on.accept(childVisitor);
     // the underlying object may have changed during the construction phase
     // This happens primarily because of custom deserializers

gson/src/main/java/com/google/gson/JsonSerializationVisitor.java

@@ -149,7 +149,6 @@ final class JsonSerializationVisitor implements ObjectNavigator.Visitor {
     return childVisitor.getJsonElement();
   }
 
-  @SuppressWarnings("unchecked")
   public boolean visitUsingCustomHandler(ObjectTypePair objTypePair) {
     try {
       Object obj = objTypePair.getObject();
@@ -176,7 +175,7 @@ final class JsonSerializationVisitor implements ObjectNavigator.Visitor {
    */
   @SuppressWarnings("unchecked")
   private JsonElement findAndInvokeCustomSerializer(ObjectTypePair objTypePair) {
-    Pair<JsonSerializer, ObjectTypePair> pair = objTypePair.getMatchingSerializer(serializers);
+    Pair<JsonSerializer<?>,ObjectTypePair> pair = objTypePair.getMatchingHandler(serializers);
     if (pair == null) {
       return null;
     }
@@ -192,7 +191,6 @@ final class JsonSerializationVisitor implements ObjectNavigator.Visitor {
     }
   }
 
-  @SuppressWarnings("unchecked")
   public boolean visitFieldUsingCustomHandler(Field f, Type declaredTypeOfField, Object parent) {
     try {
       Preconditions.checkState(root.isJsonObject());

gson/src/main/java/com/google/gson/ObjectNavigator.java

@@ -102,14 +102,14 @@ final class ObjectNavigator {
       if (objectToVisit == null) {
         return;
       }
-
+      objTypePair.setObject(objectToVisit);
       visitor.start(objTypePair);
       try {
         if (objTypeInfo.isArray()) {
           visitor.visitArray(objectToVisit, objTypePair.getType());
         } else if (objTypeInfo.getActualType() == Object.class
             && isPrimitiveOrString(objectToVisit)) {
-          // TODO(Joel): this is only used for deserialization of "primitves"
+          // TODO(Joel): this is only used for deserialization of "primitives"
           //             we should rethink this!!!
           visitor.visitPrimitive(objectToVisit);
           objectToVisit = visitor.getTarget();

gson/src/main/java/com/google/gson/ObjectTypePair.java

@@ -23,7 +23,7 @@ import java.lang.reflect.Type;
  * @author Inderjeet Singh
  */
 final class ObjectTypePair {
-  private final Object obj;
+  private Object obj;
   private final Type type;
   private final boolean preserveType;
 
@@ -37,26 +37,28 @@ final class ObjectTypePair {
     return obj;
   }
 
+  void setObject(Object obj) {
+    this.obj = obj;
+  }
+
   Type getType() {
     return type;
   }
   
-  @SuppressWarnings("unchecked")
-  Pair<JsonSerializer, ObjectTypePair> getMatchingSerializer(
-      ParameterizedTypeHandlerMap<JsonSerializer<?>> serializers) {
-    Preconditions.checkNotNull(obj);
-    JsonSerializer serializer = null;
-    if (!preserveType) {
-      // First try looking up the serializer for the actual type
+  <HANDLER> Pair<HANDLER, ObjectTypePair> getMatchingHandler(
+      ParameterizedTypeHandlerMap<HANDLER> handlers) {
+    HANDLER handler = null;
+    if (!preserveType && obj != null) {
+      // First try looking up the handler for the actual type
       ObjectTypePair moreSpecificType = toMoreSpecificType();    
-      serializer = serializers.getHandlerFor(moreSpecificType.type);
-      if (serializer != null) {
-        return new Pair<JsonSerializer, ObjectTypePair>(serializer, moreSpecificType);
+      handler = handlers.getHandlerFor(moreSpecificType.type);
+      if (handler != null) {
+        return new Pair<HANDLER, ObjectTypePair>(handler, moreSpecificType);
       }
     }
     // Try the specified type
-    serializer = serializers.getHandlerFor(type);
-    return serializer == null ? null : new Pair<JsonSerializer, ObjectTypePair>(serializer, this);
+    handler = handlers.getHandlerFor(type);
+    return handler == null ? null : new Pair<HANDLER, ObjectTypePair>(handler, this);
   }
 
   ObjectTypePair toMoreSpecificType() {    
@@ -122,4 +124,8 @@ final class ObjectTypePair {
     }
     return preserveType == other.preserveType;
   }
+
+  public boolean isPreserveType() {
+    return preserveType;
+  }
 }

gson/src/test/java/com/google/gson/functional/InstanceCreatorTest.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright (C) 2009 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.gson.functional;
+
+import java.lang.reflect.Type;
+
+import junit.framework.TestCase;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.InstanceCreator;
+import com.google.gson.common.TestTypes.Base;
+import com.google.gson.common.TestTypes.ClassWithBaseField;
+import com.google.gson.common.TestTypes.Sub;
+
+/**
+ * Functional Test exercising custom serialization only.  When test applies to both 
+ * serialization and deserialization then add it to CustomTypeAdapterTest.
+ *
+ * @author Inderjeet Singh
+ */
+public class InstanceCreatorTest extends TestCase {
+  
+  public void testInstanceCreatorReturnsBaseType() {
+    Gson gson = new GsonBuilder()
+      .registerTypeAdapter(Base.class, new InstanceCreator<Base>() {
+       public Base createInstance(Type type) {
+         return new Base();
+       }
+      })
+      .create();
+    String json = "{baseName:'BaseRevised',subName:'Sub'}";
+    Base base = gson.fromJson(json, Base.class);
+    assertEquals("BaseRevised", base.baseName);
+  }
+  
+  public void testInstanceCreatorReturnsSubTypeForTopLevelObject() {
+    Gson gson = new GsonBuilder()
+    .registerTypeAdapter(Base.class, new InstanceCreator<Base>() {
+      public Base createInstance(Type type) {
+        return new Sub();
+      }
+    })
+    .create();
+    String json = "{baseName:'Base',subName:'SubRevised'}";
+    Base base = gson.fromJson(json, Base.class);
+    assertFalse("SubRevised".equals(((Sub)base).subName));
+  }
+  
+  public void testInstanceCreatorReturnsSubTypeForField() {
+    Gson gson = new GsonBuilder()
+    .registerTypeAdapter(Base.class, new InstanceCreator<Base>() {
+      public Base createInstance(Type type) {
+        return new Sub();
+      }
+    })
+    .create();
+    String json = "{base:{baseName:'Base',subName:'SubRevised'}}";
+    ClassWithBaseField target = gson.fromJson(json, ClassWithBaseField.class);
+    assertTrue(target.base instanceof Sub);
+    assertEquals("SubRevised", ((Sub)target.base).subName);
+  }
+}