Improve TypeToken creation validation

gson/src/main/java/com/google/gson/internal/$Gson$Types.java

@@ -154,7 +154,10 @@ public static Class<?> getRawType(Type type) {
       return Object.class;
 
     } else if (type instanceof WildcardType) {
-      return getRawType(((WildcardType) type).getUpperBounds()[0]);
+      Type[] bounds = ((WildcardType) type).getUpperBounds();
+      // Currently the JLS only permits one bound for wildcards so using first bound is safe
+      assert bounds.length == 1;
+      return getRawType(bounds[0]);
 
     } else {
       String className = type == null ? "null" : type.getClass().getName();
@@ -163,7 +166,7 @@ public static Class<?> getRawType(Type type) {
     }
   }
 
-  static boolean equal(Object a, Object b) {
+  private static boolean equal(Object a, Object b) {
     return a == b || (a != null && a.equals(b));
   }
 
@@ -225,10 +228,6 @@ public static boolean equals(Type a, Type b) {
     }
   }
 
-  static int hashCodeOrZero(Object o) {
-    return o != null ? o.hashCode() : 0;
-  }
-
   public static String typeToString(Type type) {
     return type instanceof Class ? ((Class<?>) type).getName() : type.toString();
   }
@@ -238,19 +237,19 @@ public static String typeToString(Type type) {
    * IntegerSet}, the result for when supertype is {@code Set.class} is {@code Set<Integer>} and the
    * result when the supertype is {@code Collection.class} is {@code Collection<Integer>}.
    */
-  static Type getGenericSupertype(Type context, Class<?> rawType, Class<?> toResolve) {
-    if (toResolve == rawType) {
+  private static Type getGenericSupertype(Type context, Class<?> rawType, Class<?> supertype) {
+    if (supertype == rawType) {
       return context;
     }
 
     // we skip searching through interfaces if unknown is an interface
-    if (toResolve.isInterface()) {
+    if (supertype.isInterface()) {
       Class<?>[] interfaces = rawType.getInterfaces();
       for (int i = 0, length = interfaces.length; i < length; i++) {
-        if (interfaces[i] == toResolve) {
+        if (interfaces[i] == supertype) {
           return rawType.getGenericInterfaces()[i];
-        } else if (toResolve.isAssignableFrom(interfaces[i])) {
-          return getGenericSupertype(rawType.getGenericInterfaces()[i], interfaces[i], toResolve);
+        } else if (supertype.isAssignableFrom(interfaces[i])) {
+          return getGenericSupertype(rawType.getGenericInterfaces()[i], interfaces[i], supertype);
         }
       }
     }
@@ -259,17 +258,17 @@ static Type getGenericSupertype(Type context, Class<?> rawType, Class<?> toResol
     if (!rawType.isInterface()) {
       while (rawType != Object.class) {
         Class<?> rawSupertype = rawType.getSuperclass();
-        if (rawSupertype == toResolve) {
+        if (rawSupertype == supertype) {
           return rawType.getGenericSuperclass();
-        } else if (toResolve.isAssignableFrom(rawSupertype)) {
-          return getGenericSupertype(rawType.getGenericSuperclass(), rawSupertype, toResolve);
+        } else if (supertype.isAssignableFrom(rawSupertype)) {
+          return getGenericSupertype(rawType.getGenericSuperclass(), rawSupertype, supertype);
         }
         rawType = rawSupertype;
       }
     }
 
     // we can't resolve this further
-    return toResolve;
+    return supertype;
   }
 
   /**
@@ -279,10 +278,13 @@ static Type getGenericSupertype(Type context, Class<?> rawType, Class<?> toResol
    *
    * @param supertype a superclass of, or interface implemented by, this.
    */
-  static Type getSupertype(Type context, Class<?> contextRawType, Class<?> supertype) {
+  private static Type getSupertype(Type context, Class<?> contextRawType, Class<?> supertype) {
     if (context instanceof WildcardType) {
       // wildcards are useless for resolving supertypes. As the upper bound has the same raw type, use it instead
-      context = ((WildcardType)context).getUpperBounds()[0];
+      Type[] bounds = ((WildcardType)context).getUpperBounds();
+      // Currently the JLS only permits one bound for wildcards so using first bound is safe
+      assert bounds.length == 1;
+      context = bounds[0];
     }
     checkArgument(supertype.isAssignableFrom(contextRawType));
     return resolve(context, contextRawType,
@@ -306,9 +308,6 @@ public static Type getArrayComponentType(Type array) {
   public static Type getCollectionElementType(Type context, Class<?> contextRawType) {
     Type collectionType = getSupertype(context, contextRawType, Collection.class);
 
-    if (collectionType instanceof WildcardType) {
-      collectionType = ((WildcardType)collectionType).getUpperBounds()[0];
-    }
     if (collectionType instanceof ParameterizedType) {
       return ((ParameterizedType) collectionType).getActualTypeArguments()[0];
     }
@@ -440,7 +439,7 @@ private static Type resolve(Type context, Class<?> contextRawType, Type toResolv
     return toResolve;
   }
 
-  static Type resolveTypeVariable(Type context, Class<?> contextRawType, TypeVariable<?> unknown) {
+  private static Type resolveTypeVariable(Type context, Class<?> contextRawType, TypeVariable<?> unknown) {
     Class<?> declaredByRaw = declaringClassOf(unknown);
 
     // we can't reduce this further
@@ -522,6 +521,10 @@ public Type getOwnerType() {
           && $Gson$Types.equals(this, (ParameterizedType) other);
     }
 
+    private static int hashCodeOrZero(Object o) {
+      return o != null ? o.hashCode() : 0;
+    }
+
     @Override public int hashCode() {
       return Arrays.hashCode(typeArguments)
           ^ rawType.hashCode()

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

@@ -120,8 +120,7 @@ public MapTypeAdapterFactory(ConstructorConstructor constructorConstructor,
       return null;
     }
 
-    Class<?> rawTypeOfSrc = $Gson$Types.getRawType(type);
-    Type[] keyAndValueTypes = $Gson$Types.getMapKeyAndValueTypes(type, rawTypeOfSrc);
+    Type[] keyAndValueTypes = $Gson$Types.getMapKeyAndValueTypes(type, rawType);
     TypeAdapter<?> keyAdapter = getKeyAdapter(gson, keyAndValueTypes[0]);
     TypeAdapter<?> valueAdapter = gson.getAdapter(TypeToken.get(keyAndValueTypes[1]));
     ObjectConstructor<T> constructor = constructorConstructor.get(typeToken);

gson/src/main/java/com/google/gson/reflect/TypeToken.java

@@ -37,17 +37,20 @@
  * <p>
  * {@code TypeToken<List<String>> list = new TypeToken<List<String>>() {};}
  *
- * <p>This syntax cannot be used to create type literals that have wildcard
- * parameters, such as {@code Class<?>} or {@code List<? extends CharSequence>}.
+ * <p>Capturing a type variable as type argument of a {@code TypeToken} should
+ * be avoided. Due to type erasure the runtime type of a type variable is not
+ * available to Gson and therefore it cannot provide the functionality one
+ * might expect, which gives a false sense of type-safety at compilation time
+ * and can lead to an unexpected {@code ClassCastException} at runtime.
  *
  * @author Bob Lee
  * @author Sven Mawson
  * @author Jesse Wilson
  */
 public class TypeToken<T> {
-  final Class<? super T> rawType;
-  final Type type;
-  final int hashCode;
+  private final Class<? super T> rawType;
+  private final Type type;
+  private final int hashCode;
 
   /**
    * Constructs a new type literal. Derives represented class from type
@@ -59,7 +62,7 @@ public class TypeToken<T> {
    */
   @SuppressWarnings("unchecked")
   protected TypeToken() {
-    this.type = getSuperclassTypeParameter(getClass());
+    this.type = getTypeTokenTypeArgument();
     this.rawType = (Class<? super T>) $Gson$Types.getRawType(type);
     this.hashCode = type.hashCode();
   }
@@ -68,23 +71,33 @@ protected TypeToken() {
    * Unsafe. Constructs a type literal manually.
    */
   @SuppressWarnings("unchecked")
-  TypeToken(Type type) {
+  private TypeToken(Type type) {
     this.type = $Gson$Types.canonicalize($Gson$Preconditions.checkNotNull(type));
     this.rawType = (Class<? super T>) $Gson$Types.getRawType(this.type);
     this.hashCode = this.type.hashCode();
   }
 
   /**
-   * Returns the type from super class's type parameter in {@link $Gson$Types#canonicalize
+   * Verifies that {@code this} is an instance of a direct subclass of TypeToken and
+   * returns the type argument for {@code T} in {@link $Gson$Types#canonicalize
    * canonical form}.
    */
-  static Type getSuperclassTypeParameter(Class<?> subclass) {
-    Type superclass = subclass.getGenericSuperclass();
-    if (superclass instanceof Class) {
-      throw new RuntimeException("Missing type parameter.");
+  private Type getTypeTokenTypeArgument() {
+    Type superclass = getClass().getGenericSuperclass();
+    if (superclass instanceof ParameterizedType) {
+      ParameterizedType parameterized = (ParameterizedType) superclass;
+      if (parameterized.getRawType() == TypeToken.class) {
+        return $Gson$Types.canonicalize(parameterized.getActualTypeArguments()[0]);
+      }
+    }
+    // Check for raw TypeToken as superclass
+    else if (superclass == TypeToken.class) {
+      throw new IllegalStateException("TypeToken must be created with a type argument: new TypeToken<...>() {}; "
+          + "When using code shrinkers (ProGuard, R8, ...) make sure that generic signatures are preserved.");
     }
-    ParameterizedType parameterized = (ParameterizedType) superclass;
-    return $Gson$Types.canonicalize(parameterized.getActualTypeArguments()[0]);
+
+    // User created subclass of subclass of TypeToken
+    throw new IllegalStateException("Must only create direct subclasses of TypeToken");
   }
 
   /**

gson/src/test/java/com/google/gson/reflect/TypeTokenTest.java

@@ -27,23 +27,24 @@
 /**
  * @author Jesse Wilson
  */
-@SuppressWarnings({"deprecation"})
 public final class TypeTokenTest extends TestCase {
-
+  // These fields are accessed using reflection by the tests below
   List<Integer> listOfInteger = null;
   List<Number> listOfNumber = null;
   List<String> listOfString = null;
   List<?> listOfUnknown = null;
   List<Set<String>> listOfSetOfString = null;
   List<Set<?>> listOfSetOfUnknown = null;
 
+  @SuppressWarnings({"deprecation"})
   public void testIsAssignableFromRawTypes() {
     assertTrue(TypeToken.get(Object.class).isAssignableFrom(String.class));
     assertFalse(TypeToken.get(String.class).isAssignableFrom(Object.class));
     assertTrue(TypeToken.get(RandomAccess.class).isAssignableFrom(ArrayList.class));
     assertFalse(TypeToken.get(ArrayList.class).isAssignableFrom(RandomAccess.class));
   }
 
+  @SuppressWarnings({"deprecation"})
   public void testIsAssignableFromWithTypeParameters() throws Exception {
     Type a = getClass().getDeclaredField("listOfInteger").getGenericType();
     Type b = getClass().getDeclaredField("listOfNumber").getGenericType();
@@ -56,6 +57,7 @@ public void testIsAssignableFromWithTypeParameters() throws Exception {
     assertFalse(TypeToken.get(b).isAssignableFrom(a));
   }
 
+  @SuppressWarnings({"deprecation"})
   public void testIsAssignableFromWithBasicWildcards() throws Exception {
     Type a = getClass().getDeclaredField("listOfString").getGenericType();
     Type b = getClass().getDeclaredField("listOfUnknown").getGenericType();
@@ -69,6 +71,7 @@ public void testIsAssignableFromWithBasicWildcards() throws Exception {
     // assertTrue(TypeToken.get(b).isAssignableFrom(a));
   }
 
+  @SuppressWarnings({"deprecation"})
   public void testIsAssignableFromWithNestedWildcards() throws Exception {
     Type a = getClass().getDeclaredField("listOfSetOfString").getGenericType();
     Type b = getClass().getDeclaredField("listOfSetOfUnknown").getGenericType();
@@ -102,4 +105,56 @@ public void testParameterizedFactory() {
     Type listOfListOfString = TypeToken.getParameterized(List.class, listOfString).getType();
     assertEquals(expectedListOfListOfListOfString, TypeToken.getParameterized(List.class, listOfListOfString));
   }
+
+  private static class CustomTypeToken extends TypeToken<String> {
+  }
+
+  public void testTypeTokenNonAnonymousSubclass() {
+    TypeToken<?> typeToken = new CustomTypeToken();
+    assertEquals(String.class, typeToken.getRawType());
+    assertEquals(String.class, typeToken.getType());
+  }
+
+  /**
+   * User must only create direct subclasses of TypeToken, but not subclasses
+   * of subclasses (...) of TypeToken.
+   */
+  public void testTypeTokenSubSubClass() {
+    class SubTypeToken<T> extends TypeToken<String> {}
+    class SubSubTypeToken1<T> extends SubTypeToken<T> {}
+    class SubSubTypeToken2 extends SubTypeToken<Integer> {}
+
+    try {
+      new SubTypeToken<Integer>() {};
+      fail();
+    } catch (IllegalStateException expected) {
+      assertEquals("Must only create direct subclasses of TypeToken", expected.getMessage());
+    }
+
+    try {
+      new SubSubTypeToken1<Integer>();
+      fail();
+    } catch (IllegalStateException expected) {
+      assertEquals("Must only create direct subclasses of TypeToken", expected.getMessage());
+    }
+
+    try {
+      new SubSubTypeToken2();
+      fail();
+    } catch (IllegalStateException expected) {
+      assertEquals("Must only create direct subclasses of TypeToken", expected.getMessage());
+    }
+  }
+
+  @SuppressWarnings("rawtypes")
+  public void testTypeTokenRaw() {
+    try {
+      new TypeToken() {};
+      fail();
+    } catch (IllegalStateException expected) {
+      assertEquals("TypeToken must be created with a type argument: new TypeToken<...>() {}; "
+          + "When using code shrinkers (ProGuard, R8, ...) make sure that generic signatures are preserved.",
+          expected.getMessage());
+    }
+  }
 }