Allow registering adapters for JsonElement again (#2789)

This is done for backward compatibility, however registering the adapter still
has no effect because the built-in adapter as higher precedence (like it is
the case for multiple years already).

The Javadoc for the GsonBuilder methods has not been adjusted, to discourage
users from trying to register adapters for JsonElement.

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

@@ -710,7 +710,7 @@ public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
             || typeAdapter instanceof InstanceCreator<?>
             || typeAdapter instanceof TypeAdapter<?>);
 
-    if (isTypeObjectOrJsonElement(type)) {
+    if (hasNonOverridableAdapter(type)) {
       throw new IllegalArgumentException("Cannot override built-in adapter for " + type);
     }
 
@@ -730,9 +730,14 @@ public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
     return this;
   }
 
-  private static boolean isTypeObjectOrJsonElement(Type type) {
-    return type instanceof Class
-        && (type == Object.class || JsonElement.class.isAssignableFrom((Class<?>) type));
+  /** Whether the type has a built-in adapter which cannot be overridden. */
+  private static boolean hasNonOverridableAdapter(Type type) {
+    return type == Object.class;
+    // This should also cover `JsonElement.class.isAssignableFrom(type)`, however for backward
+    // compatibility this is not covered here because really old Gson versions had no built-in
+    // adapter for JsonElement so users registered custom adapters. These adapters don't have any
+    // effect in recent Gson versions. See
+    // https://github.com/google/gson/issues/2787#issuecomment-2581568157
   }
 
   /**
@@ -778,10 +783,6 @@ public GsonBuilder registerTypeHierarchyAdapter(Class<?> baseType, Object typeAd
             || typeAdapter instanceof JsonDeserializer<?>
             || typeAdapter instanceof TypeAdapter<?>);
 
-    if (JsonElement.class.isAssignableFrom(baseType)) {
-      throw new IllegalArgumentException("Cannot override built-in adapter for " + baseType);
-    }
-
     if (typeAdapter instanceof JsonDeserializer || typeAdapter instanceof JsonSerializer) {
       hierarchyFactories.add(TreeTypeAdapter.newTypeHierarchyFactory(baseType, typeAdapter));
     }

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

@@ -28,6 +28,7 @@
 import java.lang.reflect.Type;
 import java.text.DateFormat;
 import java.util.Date;
+import org.junit.Ignore;
 import org.junit.Test;
 
 /**
@@ -251,7 +252,10 @@ public void testSetStrictness() throws IOException {
   public void testRegisterTypeAdapterForObjectAndJsonElements() {
     String errorMessage = "Cannot override built-in adapter for ";
     Type[] types = {
-      Object.class, JsonElement.class, JsonArray.class,
+      Object.class,
+      // TODO: Registering adapter for JsonElement is allowed (for now) for backward compatibility,
+      //   see https://github.com/google/gson/issues/2787
+      // JsonElement.class, JsonArray.class,
     };
     GsonBuilder gsonBuilder = new GsonBuilder();
     for (Type type : types) {
@@ -263,6 +267,22 @@ public void testRegisterTypeAdapterForObjectAndJsonElements() {
     }
   }
 
+  /**
+   * Verifies that (for now) registering adapter for {@link JsonElement} and subclasses is possible,
+   * but has no effect. See {@link #testRegisterTypeAdapterForObjectAndJsonElements()}.
+   */
+  @Test
+  public void testRegisterTypeAdapterForJsonElements() {
+    Gson gson = new GsonBuilder().registerTypeAdapter(JsonArray.class, NULL_TYPE_ADAPTER).create();
+    TypeAdapter<JsonArray> adapter = gson.getAdapter(JsonArray.class);
+    // Does not use registered adapter
+    assertThat(adapter).isNotSameInstanceAs(NULL_TYPE_ADAPTER);
+    assertThat(adapter.toJson(new JsonArray())).isEqualTo("[]");
+  }
+
+  @Ignore(
+      "Registering adapter for JsonElement is allowed (for now) for backward compatibility, see"
+          + " https://github.com/google/gson/issues/2787")
   @Test
   public void testRegisterTypeHierarchyAdapterJsonElements() {
     String errorMessage = "Cannot override built-in adapter for ";
@@ -282,6 +302,20 @@ public void testRegisterTypeHierarchyAdapterJsonElements() {
     gsonBuilder.registerTypeHierarchyAdapter(Object.class, NULL_TYPE_ADAPTER);
   }
 
+  /**
+   * Verifies that (for now) registering hierarchy adapter for {@link JsonElement} and subclasses is
+   * possible, but has no effect. See {@link #testRegisterTypeHierarchyAdapterJsonElements()}.
+   */
+  @Test
+  public void testRegisterTypeHierarchyAdapterJsonElements_Allowed() {
+    Gson gson =
+        new GsonBuilder().registerTypeHierarchyAdapter(JsonArray.class, NULL_TYPE_ADAPTER).create();
+    TypeAdapter<JsonArray> adapter = gson.getAdapter(JsonArray.class);
+    // Does not use registered adapter
+    assertThat(adapter).isNotSameInstanceAs(NULL_TYPE_ADAPTER);
+    assertThat(adapter.toJson(new JsonArray())).isEqualTo("[]");
+  }
+
   @Test
   public void testSetDateFormatWithInvalidPattern() {
     GsonBuilder builder = new GsonBuilder();