Super class generics failing to find return type (#313)

* #311

AbstractJacksonFieldSnipplet.java
- Updated to deal with superclass generic return type

JacksonResponseFieldSnippetTest.java
- Updated test objects to add super classes and interfaces
- Added genericSuperMethod() test

* #311 More Generics fixes

AbstractJacksonFieldSnippet.java
- Handle the cases there MethodParameter is a TypeVariable

JacksonResponseFieldSnippet.java
- Handle the cases where the return type is a TypeVariable
-- public E getItem()

JacksonResponseFieldSnippetTest.java
- Tests cases for TypeVariable return type

* Formatting feedback

Author: cmercer

spring-auto-restdocs-core/src/main/java/capital/scalable/restdocs/payload/AbstractJacksonFieldSnippet.java

@@ -7,9 +7,9 @@
  * 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.
@@ -28,6 +28,7 @@
 
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
 import java.util.Collection;
 import java.util.Map;
 import java.util.stream.Stream;
@@ -40,6 +41,8 @@
 import capital.scalable.restdocs.snippet.StandardTableSnippet;
 import com.fasterxml.jackson.databind.JsonMappingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import org.apache.commons.lang3.StringUtils;
+import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.MethodParameter;
 import org.springframework.restdocs.operation.Operation;
 import org.springframework.web.method.HandlerMethod;
@@ -87,13 +90,35 @@ protected FieldDescriptors createFieldDescriptors(Operation operation,
 
     protected Type firstGenericType(MethodParameter param) {
         Type type = param.getGenericParameterType();
-        if (type instanceof ParameterizedType) {
+        if (type instanceof TypeVariable) {
+            TypeVariable tv = (TypeVariable) type;
+            return findTypeFromTypeVariable(tv, param.getContainingClass());
+        } else if (type instanceof ParameterizedType) {
+            ParameterizedType parameterizedType = (ParameterizedType) type;
+            Type actualArgument = parameterizedType.getActualTypeArguments()[0];
+            if (actualArgument instanceof Class) {
+                return actualArgument;
+            } else if (actualArgument instanceof TypeVariable) {
+                TypeVariable typeVariable = (TypeVariable)actualArgument;
+                return findTypeFromTypeVariable(typeVariable, param.getContainingClass());
+            }             
             return ((ParameterizedType) type).getActualTypeArguments()[0];
         } else {
             return Object.class;
         }
     }
 
+    protected Type findTypeFromTypeVariable(TypeVariable typeVariable, Class<?> clazz) {
+        String variableName = typeVariable.getName();
+        Map<TypeVariable, Type> typeMap = GenericTypeResolver.getTypeVariableMap(clazz);
+        for (TypeVariable tv : typeMap.keySet()) {
+            if (StringUtils.equals(tv.getName(), variableName)) {
+                return typeMap.get(tv);
+            }
+        }
+        return Object.class;
+    }
+
     protected abstract Type getType(HandlerMethod method);
 
     protected abstract boolean shouldFailOnUndocumentedFields();

spring-auto-restdocs-core/src/main/java/capital/scalable/restdocs/payload/JacksonResponseFieldSnippet.java

@@ -7,9 +7,9 @@
  * 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.
@@ -25,6 +25,7 @@
 import java.lang.reflect.GenericArrayType;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
 import java.util.Map;
 
 import capital.scalable.restdocs.jackson.FieldDescriptors;
@@ -83,6 +84,8 @@ protected Type getType(final HandlerMethod method) {
             }
         } else if (REACTOR_FLUX_CLASS.equals(returnType.getCanonicalName())) {
             return (GenericArrayType) () -> firstGenericType(method.getReturnType());
+        } else if (method.getReturnType().getGenericParameterType() instanceof TypeVariable) {
+            return firstGenericType(method.getReturnType());
         } else {
             return returnType;
         }

spring-auto-restdocs-core/src/test/java/capital/scalable/restdocs/payload/JacksonResponseFieldSnippetTest.java

@@ -432,6 +432,46 @@ public void comment() throws Exception {
                         .row("field4", "String", "true", "Method 4."));
     }
 
+    @Test
+    public void genericSuperMethodCollection() throws Exception{
+        HandlerMethod handlerMethod = createHandlerMethod("getItemsGeneric");
+        mockFieldComment(Item.class, "field1", "A string");
+        mockFieldComment(Item.class, "field2", "A decimal");
+
+        new JacksonResponseFieldSnippet().document(operationBuilder
+                .attribute(HandlerMethod.class.getName(), handlerMethod)
+                .attribute(ObjectMapper.class.getName(), mapper)
+                .attribute(JavadocReader.class.getName(), javadocReader)
+                .attribute(ConstraintReader.class.getName(), constraintReader)
+                .build());
+
+        assertThat(this.generatedSnippets.snippet(AUTO_RESPONSE_FIELDS)).is(
+                tableWithHeader("Path", "Type", "Optional", "Description")
+                        .row("[].field1", "String", "true", "A string.")
+                        .row("[].field2", "Decimal", "true", "A decimal."));
+
+    }
+
+    @Test
+    public void genericSuperMethodSingleItem() throws Exception {
+        HandlerMethod handlerMethod = createHandlerMethod("getItemGeneric");
+        mockFieldComment(Item.class, "field1", "A string");
+        mockFieldComment(Item.class, "field2", "A decimal");
+
+        new JacksonResponseFieldSnippet().document(operationBuilder
+                .attribute(HandlerMethod.class.getName(), handlerMethod)
+                .attribute(ObjectMapper.class.getName(), mapper)
+                .attribute(JavadocReader.class.getName(), javadocReader)
+                .attribute(ConstraintReader.class.getName(), constraintReader)
+                .build());
+
+        assertThat(this.generatedSnippets.snippet(AUTO_RESPONSE_FIELDS)).is(
+                tableWithHeader("Path", "Type", "Optional", "Description")
+                        .row("field1", "String", "true", "A string.")
+                        .row("field2", "Decimal", "true", "A decimal."));
+    }
+
+
     private void mockConstraintMessage(Class<?> type, String fieldName, String comment) {
         when(constraintReader.getConstraintMessages(type, fieldName))
                 .thenReturn(singletonList(comment));
@@ -477,8 +517,32 @@ private HandlerMethod createHandlerMethod(String responseEntityItem)
         return new HandlerMethod(new TestResource(), responseEntityItem);
     }
 
+    public interface IGenericTestResource<T> {
+
+        List<T> getItemsGeneric();
+    }
+
+    public static abstract class GenericTestResource<E> implements IGenericTestResource<E>{
+
+        abstract E createGeneric();
+
+        @Override
+        public List<E> getItemsGeneric() {
+            return Collections.singletonList(createGeneric());
+        }
+
+        public E getItemGeneric() {
+            return createGeneric();
+        }
+    }
+
     // actual method responses do not matter, they are here just for the illustration
-    private static class TestResource {
+    private static class TestResource extends GenericTestResource<Item>{
+
+        @Override
+        Item createGeneric() {
+            return new Item("test");
+        }
 
         public Item getItem() {
             return new Item("test");