Resolve forwarded type arguments across POJO hierarchy edges

bson/src/main/org/bson/codecs/pojo/PojoBuilderHelper.java

@@ -264,11 +264,23 @@ private static <T> Set<ClassWithParentTypeData<? super T>> getClassHierarchy(fin
         TypeData<?> parentClassTypeData = classTypeData;
         while (currentClass != null && !currentClass.isEnum() && !currentClass.equals(Object.class)) {
             classesToScan.add(new ClassWithParentTypeData<>(currentClass, parentClassTypeData));
-            parentClassTypeData = TypeData.newInstance(currentClass.getGenericSuperclass(), currentClass);
-            for (Class<?> interfaceClass : currentClass.getInterfaces()) {
-                classesToScan.addAll(getClassHierarchy((Class<? super T>) interfaceClass, parentClassTypeData));
+
+            List<TypeVariable<?>> currentTypeParams = asList(currentClass.getTypeParameters());
+            Type[] genericInterfaces = currentClass.getGenericInterfaces();
+            Class<?>[] interfaces = currentClass.getInterfaces();
+            for (int i = 0; i < interfaces.length; i++) {
+                TypeData<?> ifaceResolved = TypeData.newInstance(
+                        genericInterfaces[i], interfaces[i], currentTypeParams, parentClassTypeData);
+                classesToScan.addAll(getClassHierarchy((Class<? super T>) interfaces[i], ifaceResolved));
+            }
+
+            Class<? super T> superClass = currentClass.getSuperclass();
+            if (superClass != null) {
+                parentClassTypeData = TypeData.newInstance(
+                        currentClass.getGenericSuperclass(), superClass,
+                        currentTypeParams, parentClassTypeData);
             }
-            currentClass = currentClass.getSuperclass();
+            currentClass = superClass;
         }
         return classesToScan;
     }

bson/src/main/org/bson/codecs/pojo/TypeData.java

@@ -16,6 +16,7 @@
 
 package org.bson.codecs.pojo;
 
+import javax.annotation.Nullable;
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.lang.reflect.ParameterizedType;
@@ -61,32 +62,70 @@ public static TypeData<?> newInstance(final Field field) {
     }
 
     public static <T> TypeData<T> newInstance(final Type genericType, final Class<T> clazz) {
-        TypeData.Builder<T> builder = TypeData.builder(clazz);
-        if (genericType instanceof ParameterizedType) {
-            ParameterizedType pType = (ParameterizedType) genericType;
+        // No enclosing class context: type variables have nothing to resolve against and erase to Object.
+        return newInstance(genericType, clazz, Collections.<TypeVariable<?>>emptyList(), null);
+    }
+
+    static <T> TypeData<T> newInstance(final Type genericParentType, final Class<T> parentClass,
+                                       final List<TypeVariable<?>> currentClassTypeParameters,
+                                       @Nullable final TypeData<?> currentClassTypeData) {
+        TypeData.Builder<T> builder = TypeData.builder(parentClass);
+        if (genericParentType instanceof ParameterizedType) {
+            ParameterizedType pType = (ParameterizedType) genericParentType;
             for (Type argType : pType.getActualTypeArguments()) {
-                getNestedTypeData(builder, argType);
+                builder.addTypeParameter(resolveTypeArgument(argType, currentClassTypeParameters, currentClassTypeData));
             }
         }
         return builder.build();
     }
 
     @SuppressWarnings({"unchecked", "rawtypes"})
-    private static <T> void getNestedTypeData(final TypeData.Builder<T> builder, final Type type) {
+    private static TypeData<?> resolveTypeArgument(final Type type,
+                                          final List<TypeVariable<?>> currentClassTypeParameters,
+                                          @Nullable final TypeData<?> currentClassTypeData) {
         if (type instanceof ParameterizedType) {
             ParameterizedType pType = (ParameterizedType) type;
             TypeData.Builder paramBuilder = TypeData.builder((Class) pType.getRawType());
             for (Type argType : pType.getActualTypeArguments()) {
-                getNestedTypeData(paramBuilder, argType);
+                paramBuilder.addTypeParameter(resolveTypeArgument(argType, currentClassTypeParameters, currentClassTypeData));
             }
-            builder.addTypeParameter(paramBuilder.build());
-        } else if (type instanceof WildcardType) {
-            builder.addTypeParameter(TypeData.builder((Class) ((WildcardType) type).getUpperBounds()[0]).build());
+            return paramBuilder.build();
         } else if (type instanceof TypeVariable) {
-            builder.addTypeParameter(TypeData.builder(Object.class).build());
+            return resolveTypeVariable((TypeVariable<?>) type, currentClassTypeParameters, currentClassTypeData);
         } else if (type instanceof Class) {
-            builder.addTypeParameter(TypeData.builder((Class) type).build());
+            return TypeData.builder((Class) type).build();
+        } else if (type instanceof WildcardType) {
+            // A wildcard cannot be the top-level type argument of an extends/implements clause (JLS §8.1.4,
+            // §8.1.5), but it can appear nested inside one (e.g. extends Base<List<? extends Number>>) or in a
+            // field/method generic type. Resolve it to its upper bound so any type variable inside the bound is
+            // still substituted against the current context.
+            return resolveTypeArgument(((WildcardType) type).getUpperBounds()[0], currentClassTypeParameters,
+                    currentClassTypeData);
+        } else {
+            // Any other Type (e.g. GenericArrayType) is erased to Object.
+            return TypeData.builder(Object.class).build();
+        }
+    }
+
+    private static TypeData<?> resolveTypeVariable(final TypeVariable<?> type, final List<TypeVariable<?>> currentClassTypeParameters,
+                                           @Nullable final TypeData<?> currentClassTypeData) {
+        if (currentClassTypeData != null) {
+            for (int i = 0; i < currentClassTypeParameters.size(); i++) {
+                //  Given 'class B<T> extends A<T> {}':
+                // - JLS §6.3: the scope of B's type parameter T includes the superclass clause.
+                // - JLS §6.5.5.1: T in "extends A<T>" therefore denotes B's T, not A's.
+                // Both reflection paths represent the same declaration, and the TypeVariable
+                // contract states "all instances representing a type variable must be equal() to
+                // each other".
+                if (currentClassTypeParameters.get(i).equals(type)) {
+                    if (i < currentClassTypeData.getTypeParameters().size()) {
+                        return currentClassTypeData.getTypeParameters().get(i);
+                    }
+                    break;
+                }
+            }
         }
+        return TypeData.builder(Object.class).build();
     }
 
     /**

bson/src/test/unit/org/bson/codecs/pojo/ClassModelTest.java

@@ -20,6 +20,12 @@
 
 import org.bson.codecs.pojo.entities.CollectionNestedPojoModel;
 import org.bson.codecs.pojo.entities.ConcreteAndNestedAbstractInterfaceModel;
+import org.bson.codecs.pojo.entities.ForwardingInterfaceModel;
+import org.bson.codecs.pojo.entities.ForwardingDualInterfaceModel;
+import org.bson.codecs.pojo.entities.ForwardingMixedModel;
+import org.bson.codecs.pojo.entities.ForwardingModel;
+import org.bson.codecs.pojo.entities.ForwardingArrayModel;
+import org.bson.codecs.pojo.entities.ForwardingNestedModel;
 import org.bson.codecs.pojo.entities.GenericHolderModel;
 import org.bson.codecs.pojo.entities.InterfaceBasedModel;
 import org.bson.codecs.pojo.entities.ListGenericExtendedModel;
@@ -266,7 +272,60 @@ public void testSimpleWithStaticModel() {
 
     }
 
-    <T> TypeData.Builder<T> createBuilder(final Class<T> clazz, final Class<?>... types) {
+    @Test
+    public void testForwardingClassChain() {
+        ClassModel<?> classModel = ClassModel.builder(ForwardingModel.class).build();
+
+        assertEquals(1, classModel.getPropertyModels().size());
+        assertEquals(createTypeData(String.class), classModel.getPropertyModel("value").getTypeData());
+    }
+
+    @Test
+    public void testForwardingInterfaceChain() {
+        ClassModel<?> classModel = ClassModel.builder(ForwardingInterfaceModel.class).build();
+
+        assertEquals(1, classModel.getPropertyModels().size());
+        assertEquals(createTypeData(Integer.class), classModel.getPropertyModel("value").getTypeData());
+    }
+
+    @Test
+    public void testForwardingNested() {
+        ClassModel<?> classModel = ClassModel.builder(ForwardingNestedModel.class).build();
+
+        assertEquals(1, classModel.getPropertyModels().size());
+        assertEquals(createTypeData(List.class, String.class), classModel.getPropertyModel("value").getTypeData());
+    }
+
+    @Test
+    public void testForwardingArrayTypeVariableErasedToObject() {
+        // The type argument `T[]` in `extends ForwardingArrayLevel2<T[]>` is a GenericArrayType;
+        // getTypeParameterMap does not handle GenericArrayType, so the `value` property erases to
+        // Object regardless of the concrete binding at the leaf subclass.
+        ClassModel<?> classModel = ClassModel.builder(ForwardingArrayModel.class).build();
+
+        assertEquals(1, classModel.getPropertyModels().size());
+        assertEquals(createTypeData(Object.class), classModel.getPropertyModel("value").getTypeData());
+    }
+
+    @Test
+    public void testForwardingMixedClassAndInterface() {
+        ClassModel<?> classModel = ClassModel.builder(ForwardingMixedModel.class).build();
+
+        assertEquals(2, classModel.getPropertyModels().size());
+        assertEquals(createTypeData(String.class), classModel.getPropertyModel("field1").getTypeData());
+        assertEquals(createTypeData(Integer.class), classModel.getPropertyModel("field2").getTypeData());
+    }
+
+    @Test
+    public void testForwardingDualInterface() {
+        ClassModel<?> classModel = ClassModel.builder(ForwardingDualInterfaceModel.class).build();
+
+        assertEquals(2, classModel.getPropertyModels().size());
+        assertEquals(createTypeData(String.class), classModel.getPropertyModel("field2").getTypeData());
+        assertEquals(createTypeData(Integer.class), classModel.getPropertyModel("field1").getTypeData());
+    }
+
+<T> TypeData.Builder<T> createBuilder(final Class<T> clazz, final Class<?>... types) {
         TypeData.Builder<T> builder = TypeData.builder(clazz);
         List<TypeData<?>> subTypes = new ArrayList<>();
         for (final Class<?> type : types) {

bson/src/test/unit/org/bson/codecs/pojo/PojoRoundTripTest.java

@@ -30,6 +30,10 @@
 import org.bson.codecs.pojo.entities.ConventionModel;
 import org.bson.codecs.pojo.entities.DuplicateAnnotationAllowedModel;
 import org.bson.codecs.pojo.entities.FieldAndPropertyTypeMismatchModel;
+import org.bson.codecs.pojo.entities.ForwardingInterfaceModel;
+import org.bson.codecs.pojo.entities.ForwardingModel;
+import org.bson.codecs.pojo.entities.ForwardingNestedModel;
+import org.bson.codecs.pojo.entities.ForwardingWildcardModel;
 import org.bson.codecs.pojo.entities.GenericHolderModel;
 import org.bson.codecs.pojo.entities.GenericTreeModel;
 import org.bson.codecs.pojo.entities.InterfaceBasedModel;
@@ -526,6 +530,28 @@ private static List<TestData> testCases() {
                 getPojoCodecProviderBuilder(BsonExtraElementsMapModel.class),
                 "{'integerField': 42, 'stringField': 'myString', 'a': 'a', 'b': 'b'}"));
 
+        data.add(new TestData("Forwarding class chain resolves to String",
+                new ForwardingModel("hello"),
+                getPojoCodecProviderBuilder(ForwardingModel.class),
+                "{'value': 'hello'}"));
+
+        data.add(new TestData("Forwarding interface chain resolves to Integer",
+                new ForwardingInterfaceModel(7),
+                getPojoCodecProviderBuilder(ForwardingInterfaceModel.class),
+                "{'value': 7}"));
+
+        data.add(new TestData("Forwarding nested generic chain resolves to List<String>",
+                new ForwardingNestedModel(asList("a", "b", "c")),
+                getPojoCodecProviderBuilder(ForwardingNestedModel.class),
+                "{'value': ['a', 'b', 'c']}"));
+        data.add(new TestData("Forwarding nested wildcard chain resolves to List<? extends ShapeModelAbstract>",
+                new ForwardingWildcardModel(asList(getShapeModelCircle(), getShapeModelRectangle())),
+                getPojoCodecProviderBuilder(ForwardingWildcardModel.class, ShapeModelAbstract.class,
+                        ShapeModelCircle.class, ShapeModelRectangle.class),
+                "{'value': [{'_t': 'org.bson.codecs.pojo.entities.ShapeModelCircle', 'color': 'orange', 'radius': 4.2}, "
+                        + "{'_t': 'org.bson.codecs.pojo.entities.ShapeModelRectangle', 'color': 'green', 'width': 22.1, "
+                        + "'height': 105.0}]}"));
+
         return data;
     }