Validate instances created by InstanceCreator

gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java

@@ -89,6 +89,19 @@ static String checkInstantiable(Class<?> c) {
     return null;
   }
 
+  private static <T> T useInstanceCreator(InstanceCreator<T> instanceCreator, Type type, Class<?> rawType) {
+    T instance = instanceCreator.createInstance(type);
+    if (instance == null) {
+      throw new RuntimeException("InstanceCreator " + instanceCreator + " returned null for type " + type);
+    }
+
+    if (!rawType.isInstance(instance)) {
+      throw new ClassCastException("InstanceCreator " + instanceCreator + " created instance of wrong type;"
+          + " expected " + rawType.getName() + " but got instance of unrelated type " + instance.getClass().getName());
+    }
+    return instance;
+  }
+
   public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {
     final Type type = typeToken.getType();
     final Class<? super T> rawType = typeToken.getRawType();
@@ -100,7 +113,7 @@ public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {
     if (typeCreator != null) {
       return new ObjectConstructor<T>() {
         @Override public T construct() {
-          return typeCreator.createInstance(type);
+          return useInstanceCreator(typeCreator, type, rawType);
         }
       };
     }
@@ -112,7 +125,7 @@ public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {
     if (rawTypeCreator != null) {
       return new ObjectConstructor<T>() {
         @Override public T construct() {
-          return rawTypeCreator.createInstance(type);
+          return useInstanceCreator(rawTypeCreator, type, rawType);
         }
       };
     }

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

@@ -17,6 +17,7 @@
 package com.google.gson.functional;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
@@ -33,7 +34,7 @@
 import org.junit.Test;
 
 /**
- * Functional Test exercising custom serialization only. When test applies to both
+ * Functional Test exercising custom deserialization only. When test applies to both
  * serialization and deserialization then add it to CustomTypeAdapterTest.
  *
  * @author Inderjeet Singh
@@ -129,4 +130,48 @@ class SubTreeSet<T> extends TreeSet<T> {}
     assertThat(set.first()).isEqualTo("b");
     assertThat(set.getClass()).isEqualTo(SubTreeSet.class);
   }
+
+  private static class CustomClass {}
+
+  @Test
+  public void testInstanceCreatorReturnsNull() {
+    Gson gson = new GsonBuilder()
+        .registerTypeAdapter(CustomClass.class, new InstanceCreator<CustomClass>() {
+          @Override
+          public CustomClass createInstance(Type type) {
+            return null;
+          }
+
+          @Override
+          public String toString() {
+            return "bad-instance-creator";
+          }
+        })
+        .create();
+
+    RuntimeException e = assertThrows(RuntimeException.class, () -> gson.fromJson("{}", CustomClass.class));
+    assertThat(e).hasMessageThat().isEqualTo("InstanceCreator bad-instance-creator returned null for"
+        + " type class " + CustomClass.class.getName());
+  }
+
+  @Test
+  public void testInstanceCreatorReturnsWrongInstance() {
+    Gson gson = new GsonBuilder()
+        .registerTypeAdapter(CustomClass.class, new InstanceCreator<String>() {
+          @Override
+          public String createInstance(Type type) {
+            return "test";
+          }
+
+          @Override
+          public String toString() {
+            return "bad-instance-creator";
+          }
+        })
+        .create();
+
+    ClassCastException e = assertThrows(ClassCastException.class, () -> gson.fromJson("{}", CustomClass.class));
+    assertThat(e).hasMessageThat().isEqualTo("InstanceCreator bad-instance-creator created instance of wrong type;"
+        + " expected " + CustomClass.class.getName() + " but got instance of unrelated type java.lang.String");
+  }
 }