Added support for JsonAdapter annotation on fields

gson/src/main/java/com/google/gson/annotations/JsonAdapter.java

@@ -72,7 +72,7 @@ import com.google.gson.TypeAdapter;
  */
 // Note that the above example is taken from JsonAdapterANnotationTest.
 @Retention(RetentionPolicy.RUNTIME)
-@Target(ElementType.TYPE)
+@Target({ElementType.TYPE, ElementType.FIELD})
 public @interface JsonAdapter {
 
   Class<? extends TypeAdapter<?>> value();

gson/src/main/java/com/google/gson/internal/bind/JsonAdapterAnnotationTypeAdapterFactory.java

@@ -43,9 +43,15 @@ public final class JsonAdapterAnnotationTypeAdapterFactory implements TypeAdapte
     Class<? super T> clazz = targetType.getRawType();
     JsonAdapter annotation = clazz.getAnnotation(JsonAdapter.class);
     if (annotation == null) return null;
-    Class<? extends TypeAdapter<?>> adapterClass = annotation.value();
+    TypeAdapter adapter = getAnnotationTypeAdapter(constructorConstructor, annotation);
-    ObjectConstructor<? extends TypeAdapter<?>> constructor = constructorConstructor.get(TypeToken.get(adapterClass));
-    TypeAdapter adapter = constructor.construct();
     return adapter;
   }
+
+  static TypeAdapter<?> getAnnotationTypeAdapter(
+      ConstructorConstructor constructorConstructor, JsonAdapter annotation) {
+    Class<? extends TypeAdapter<?>> adapterClass = annotation.value();
+    ObjectConstructor<? extends TypeAdapter<?>> constructor =
+        constructorConstructor.get(TypeToken.get(adapterClass));
+    return constructor.construct();
+  }
 }

gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java

@@ -16,11 +16,18 @@
 
 package com.google.gson.internal.bind;
 
+import java.io.IOException;
+import java.lang.reflect.Field;
+import java.lang.reflect.Type;
+import java.util.LinkedHashMap;
+import java.util.Map;
+
 import com.google.gson.FieldNamingStrategy;
 import com.google.gson.Gson;
 import com.google.gson.JsonSyntaxException;
 import com.google.gson.TypeAdapter;
 import com.google.gson.TypeAdapterFactory;
+import com.google.gson.annotations.JsonAdapter;
 import com.google.gson.annotations.SerializedName;
 import com.google.gson.internal.$Gson$Types;
 import com.google.gson.internal.ConstructorConstructor;
@@ -31,11 +38,6 @@ import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonToken;
 import com.google.gson.stream.JsonWriter;
-import java.io.IOException;
-import java.lang.reflect.Field;
-import java.lang.reflect.Type;
-import java.util.LinkedHashMap;
-import java.util.Map;
 
 /**
  * Type adapter that reflects over the fields and methods of a class.
@@ -76,10 +78,9 @@ public final class ReflectiveTypeAdapterFactory implements TypeAdapterFactory {
       final Gson context, final Field field, final String name,
       final TypeToken<?> fieldType, boolean serialize, boolean deserialize) {
     final boolean isPrimitive = Primitives.isPrimitive(fieldType.getRawType());
-
     // special casing primitives here saves ~5% on Android...
     return new ReflectiveTypeAdapterFactory.BoundField(name, serialize, deserialize) {
-      final TypeAdapter<?> typeAdapter = context.getAdapter(fieldType);
+      final TypeAdapter<?> typeAdapter = getFieldAdapter(context, field, fieldType);
       @SuppressWarnings({"unchecked", "rawtypes"}) // the type adapter and field type always agree
       @Override void write(JsonWriter writer, Object value)
           throws IOException, IllegalAccessException {
@@ -98,6 +99,18 @@ public final class ReflectiveTypeAdapterFactory implements TypeAdapterFactory {
     };
   }
 
+  private TypeAdapter<?> getFieldAdapter(Gson context, Field field, TypeToken<?> fieldType) {
+    TypeAdapter<?> adapter = context.getAdapter(fieldType);
+    // check if the registered adapter is a reflective type adapter. If so, JsonAdapter
+    // annotation should take precedence. Somewhat hackish, but works.
+    if (adapter instanceof Adapter && field.isAnnotationPresent(JsonAdapter.class)) {
+      JsonAdapter annotation = field.getAnnotation(JsonAdapter.class);
+      return JsonAdapterAnnotationTypeAdapterFactory.getAnnotationTypeAdapter(
+          constructorConstructor, annotation);
+    }
+    return adapter;
+  }
+
   private Map<String, BoundField> getBoundFields(Gson context, TypeToken<?> type, Class<?> raw) {
     Map<String, BoundField> result = new LinkedHashMap<String, BoundField>();
     if (raw.isInterface()) {

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

@@ -36,9 +36,9 @@ import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
 
 /**
- * Functional tests for the {@link com.google.gson.annotations.JsonAdapter} annotation.
+ * Functional tests for the {@link com.google.gson.annotations.JsonAdapter} annotation on classes.
  */
-public final class JsonAdapterAnnotationTest extends TestCase {
+public final class JsonAdapterAnnotationOnClassesTest extends TestCase {
 
   public void testJsonAdapterInvoked() {
     Gson gson = new Gson();
@@ -154,8 +154,8 @@ public final class JsonAdapterAnnotationTest extends TestCase {
   // This class is used in JsonAdapter Javadoc as an example
   @JsonAdapter(UserJsonAdapter.class)
   private static class User {
-    public final String firstName, lastName;
+    final String firstName, lastName;
-    private User(String firstName, String lastName) {
+    User(String firstName, String lastName) {
       this.firstName = firstName;
       this.lastName = lastName;
     }