8313874: JNI NewWeakGlobalRef throws exception for null arg

Reviewed-by: dholmes, kbarrett, shade

src/hotspot/share/prims/jni.cpp

@@ -2875,7 +2875,7 @@ JNI_ENTRY(jweak, jni_NewWeakGlobalRef(JNIEnv *env, jobject ref))
   HOTSPOT_JNI_NEWWEAKGLOBALREF_ENTRY(env, ref);
   Handle ref_handle(thread, JNIHandles::resolve(ref));
   jweak ret = JNIHandles::make_weak_global(ref_handle, AllocFailStrategy::RETURN_NULL);
-  if (ret == nullptr) {
+  if (ret == nullptr && ref_handle.not_null()) {
     THROW_OOP_(Universe::out_of_memory_error_c_heap(), nullptr);
   }
   HOTSPOT_JNI_NEWWEAKGLOBALREF_RETURN(ret);

test/hotspot/jtreg/runtime/jni/ReturnJNIWeak/ReturnJNIWeak.java

@@ -123,9 +123,19 @@ private static void testClear() throws Exception {
         }
     }
 
+    // Verify passing a null value returns null and doesn't throw.
+    private static void testNullValue() {
+        System.out.println("running testNullValue");
+        registerObject(null);
+        if (getObject() != null) {
+            throw new RuntimeException("expected null");
+        }
+    }
+
     public static void main(String[] args) throws Exception {
         testSanity();
         testSurvival();
         testClear();
+        testNullValue();
     }
 }