issue#2436: Throw exception when registering adapter for Object or JsonElement

UserGuide.md

@@ -405,7 +405,9 @@ gson.registerTypeAdapter(MyType.class, new MyDeserializer());
 gson.registerTypeAdapter(MyType.class, new MyInstanceCreator());
 ```
 
-`registerTypeAdapter` call checks if the type adapter implements more than one of these interfaces and register it for all of them.
+`registerTypeAdapter` call checks 
+1. if the type adapter implements more than one of these interfaces and register it for all of them.
+2. if the type adapter is for Object class or (JsonElements or any of its subclasses), it throws IllegalArgumentException.
 
 #### Writing a Serializer
 

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

@@ -41,6 +41,8 @@
 import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
+
+import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.text.DateFormat;
 import java.util.ArrayDeque;
@@ -337,7 +339,7 @@ public GsonBuilder enableComplexMapKeySerialization() {
    *
    * <p>In general using inner classes with Gson should be avoided; they should be converted to {@code static}
    * nested classes if possible.
-   *
+   *Ø
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
@@ -664,14 +666,16 @@ public GsonBuilder setDateFormat(int dateStyle, int timeStyle) {
    * @param typeAdapter This object must implement at least one of the {@link TypeAdapter},
    * {@link InstanceCreator}, {@link JsonSerializer}, and a {@link JsonDeserializer} interfaces.
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @throws IllegalArgumentException if the Type adapter being registered is for Object class or (JsonElements or any of its subclasses)
    */
   @CanIgnoreReturnValue
   public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
     Objects.requireNonNull(type);
-    $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
+    $Gson$Preconditions.checkArgument((typeAdapter instanceof JsonSerializer<?>
         || typeAdapter instanceof JsonDeserializer<?>
         || typeAdapter instanceof InstanceCreator<?>
-        || typeAdapter instanceof TypeAdapter<?>);
+        || typeAdapter instanceof TypeAdapter<?>)
+        && !isTypeObjectOrJsonElements(type));
     if (typeAdapter instanceof InstanceCreator<?>) {
       instanceCreators.put(type, (InstanceCreator<?>) typeAdapter);
     }
@@ -687,6 +691,12 @@ public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
     return this;
   }
 
+  private boolean isTypeObjectOrJsonElements(Type type) {
+    return (!(type instanceof ParameterizedType) &&
+            (type == Object.class
+            || JsonElement.class.isAssignableFrom((Class<?>) type)));
+  }
+
   /**
    * Register a factory for type adapters. Registering a factory is useful when the type
    * adapter needs to be configured based on the type of the field being processed. Gson
@@ -718,14 +728,16 @@ public GsonBuilder registerTypeAdapterFactory(TypeAdapterFactory factory) {
    * @param typeAdapter This object must implement at least one of {@link TypeAdapter},
    *        {@link JsonSerializer} or {@link JsonDeserializer} interfaces.
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @throws IllegalArgumentException if the Type adapter being registered is for a JsonElements or any of its subclasses
    * @since 1.7
    */
   @CanIgnoreReturnValue
   public GsonBuilder registerTypeHierarchyAdapter(Class<?> baseType, Object typeAdapter) {
     Objects.requireNonNull(baseType);
-    $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
+    $Gson$Preconditions.checkArgument((typeAdapter instanceof JsonSerializer<?>
         || typeAdapter instanceof JsonDeserializer<?>
-        || typeAdapter instanceof TypeAdapter<?>);
+        || typeAdapter instanceof TypeAdapter<?>)
+        && !JsonElement.class.isAssignableFrom(baseType));
     if (typeAdapter instanceof JsonDeserializer || typeAdapter instanceof JsonSerializer) {
       hierarchyFactories.add(TreeTypeAdapter.newTypeHierarchyFactory(baseType, typeAdapter));
     }

gson/src/test/java/com/google/gson/GsonBuilderTest.java

@@ -17,6 +17,7 @@
 package com.google.gson;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.fail;
 
 import com.google.gson.stream.JsonReader;
@@ -254,4 +255,32 @@ public void testSetStrictness() throws IOException {
     assertThat(gson.newJsonReader(new StringReader("{}")).getStrictness()).isEqualTo(STRICTNESS);
     assertThat(gson.newJsonWriter(new StringWriter()).getStrictness()).isEqualTo(STRICTNESS);
   }
+
+  @Test
+  public void testRegisterTypeAdapterForObjectAndJsonElements() {
+    Type[] types = {
+            Object.class,
+            JsonElement.class,
+            JsonArray.class,
+    };
+    GsonBuilder gsonBuilder = new GsonBuilder();
+    for (Type type : types) {
+      assertThrows(IllegalArgumentException.class, () ->  gsonBuilder.registerTypeAdapter(type, NULL_TYPE_ADAPTER));
+    }
+  }
+
+
+  @Test
+  public void testRegisterTypeHierarchyAdapterJsonElements() {
+    Type[] types = {
+            JsonElement.class,
+            JsonArray.class,
+    };
+    GsonBuilder gsonBuilder = new GsonBuilder();
+    for (Type type : types) {
+      assertThrows(IllegalArgumentException.class,
+              () ->  gsonBuilder.registerTypeHierarchyAdapter((Class<?>) type, NULL_TYPE_ADAPTER));
+    }
+    gsonBuilder.registerTypeHierarchyAdapter(Object.class, NULL_TYPE_ADAPTER);
+  }
 }

gson/src/test/java/com/google/gson/GsonTest.java

@@ -93,7 +93,7 @@ public void testClonedTypeAdapterFactoryListsAreIndependent() {
         Collections.<ReflectionAccessFilter>emptyList());
 
     Gson clone = original.newBuilder()
-        .registerTypeAdapter(Object.class, new TestTypeAdapter())
+        .registerTypeAdapter(int.class, new TestTypeAdapter())
         .create();
 
     assertThat(clone.factories).hasSize(original.factories.size() + 1);