Make RuntimeTypeAdapterFactory recognize subclasses only conditionally. (#2160)

PR #2139 changed this factory so that if given a certain baseType, it will also
recognize any subtype of that type. That is often the right thing to do, but it
is a change in behaviour, and does in fact break at least one current client of
this code. So instead we introduce a new `recognizeSubclasses()` method that
triggers this behaviour. When the method is not called, we revert to the old
behaviour of only recognizing instances of the exact class `baseType`.

extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java

@@ -30,7 +30,6 @@ import java.io.IOException;
 import java.util.LinkedHashMap;
 import java.util.Map;
 
-
 /**
  * Adapts values whose runtime type may differ from their declaration type. This
  * is necessary when a field's type is not the same type that GSON should create
@@ -138,8 +137,10 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
   private final Map<String, Class<?>> labelToSubtype = new LinkedHashMap<>();
   private final Map<Class<?>, String> subtypeToLabel = new LinkedHashMap<>();
   private final boolean maintainType;
+  private boolean recognizeSubtypes;
 
-  private RuntimeTypeAdapterFactory(Class<?> baseType, String typeFieldName, boolean maintainType) {
+  private RuntimeTypeAdapterFactory(
+      Class<?> baseType, String typeFieldName, boolean maintainType) {
     if (typeFieldName == null || baseType == null) {
       throw new NullPointerException();
     }
@@ -151,7 +152,8 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
   /**
    * Creates a new runtime type adapter using for {@code baseType} using {@code
    * typeFieldName} as the type field name. Type field names are case sensitive.
-   * {@code maintainType} flag decide if the type will be stored in pojo or not.
+   *
+   * @param maintainType true if the type field should be included in deserialized objects
    */
   public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName, boolean maintainType) {
     return new RuntimeTypeAdapterFactory<>(baseType, typeFieldName, maintainType);
@@ -173,6 +175,15 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
     return new RuntimeTypeAdapterFactory<>(baseType, "type", false);
   }
 
+  /**
+   * Ensures that this factory will handle not just the given {@code baseType}, but any subtype
+   * of that type.
+   */
+  public RuntimeTypeAdapterFactory<T> recognizeSubtypes() {
+    this.recognizeSubtypes = true;
+    return this;
+  }
+
   /**
    * Registers {@code type} identified by {@code label}. Labels are case
    * sensitive.
@@ -205,7 +216,13 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
 
   @Override
   public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
-    if (type == null || !baseType.isAssignableFrom(type.getRawType())) {
+    if (type == null) {
+      return null;
+    }
+    Class<?> rawType = type.getRawType();
+    boolean handle =
+        recognizeSubtypes ? baseType.isAssignableFrom(rawType) : baseType.equals(rawType);
+    if (!handle) {
       return null;
     }
 

extras/src/test/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactoryTest.java

@@ -34,8 +34,27 @@ public final class RuntimeTypeAdapterFactoryTest extends TestCase {
 
     CreditCard original = new CreditCard("Jesse", 234);
     assertEquals("{\"type\":\"CreditCard\",\"cvv\":234,\"ownerName\":\"Jesse\"}",
-        //do not give the explicit typeOfSrc, because if this would be in a list
-        //or an attribute, there would also be no hint. See #712
+        gson.toJson(original, BillingInstrument.class));
+    BillingInstrument deserialized = gson.fromJson(
+        "{type:'CreditCard',cvv:234,ownerName:'Jesse'}", BillingInstrument.class);
+    assertEquals("Jesse", deserialized.ownerName);
+    assertTrue(deserialized instanceof CreditCard);
+  }
+
+  public void testRuntimeTypeAdapterRecognizeSubtypes() {
+    // We don't have an explicit factory for CreditCard.class, but we do have one for
+    // BillingInstrument.class that has recognizeSubtypes(). So it should recognize CreditCard, and
+    // when we call gson.toJson(original) below, without an explicit type, it should be invoked.
+    RuntimeTypeAdapterFactory<BillingInstrument> rta = RuntimeTypeAdapterFactory.of(
+        BillingInstrument.class)
+        .recognizeSubtypes()
+        .registerSubtype(CreditCard.class);
+    Gson gson = new GsonBuilder()
+        .registerTypeAdapterFactory(rta)
+        .create();
+
+    CreditCard original = new CreditCard("Jesse", 234);
+    assertEquals("{\"type\":\"CreditCard\",\"cvv\":234,\"ownerName\":\"Jesse\"}",
         gson.toJson(original));
     BillingInstrument deserialized = gson.fromJson(
         "{type:'CreditCard',cvv:234,ownerName:'Jesse'}", BillingInstrument.class);