From 033d659a118d7206dfa8d9c605bcec601c46f116 Mon Sep 17 00:00:00 2001
From: Martin Grotzke <martin.grotzke@googlemail.com>
Date: Fri, 11 Aug 2017 21:19:18 +0200
Subject: [PATCH] Fix #529: Support serializing the Enum class object

When an `Enum.class` instance is serialized (by `ClassSerializer`), this
causes the creation of the registered default serializer (`EnumSerializer`)
with this call chain:
```
ClassSerializer.write
 -> Kryo.writeClass
  -> DefaultClassResolver.writeClass
   -> Kryo.getDefaultSerializer
    -> ReflectionSerializerFactory.makeSerializer(kryo, EnumSerializer, Enum.class)
```

This is done so that `Kryo.writeClass` can write the class name or the id
of the serializer registration.

Until now, the `EnumSerializer` failed instantiation for a type with `null`
enum constants. This however is the case for `Enum.class` itself.
The creation of `EnumSerializer` for `Enum.class` is now allowed, under the
assumption that this happens in the context of the `Enum.class` serialization,
and that `write`/`read` on the created `EnumSerializer` instance will never
be called. This assumption should be safe, since there should never be an instance
of the abstract `Enum` class.
---
 src/com/esotericsoftware/kryo/Kryo.java                    | 2 +-
 .../kryo/serializers/DefaultSerializers.java               | 7 ++++++-
 test/com/esotericsoftware/kryo/DefaultSerializersTest.java | 2 ++
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/com/esotericsoftware/kryo/Kryo.java b/src/com/esotericsoftware/kryo/Kryo.java
index 6e8993183..8b4507df5 100644
--- a/src/com/esotericsoftware/kryo/Kryo.java
+++ b/src/com/esotericsoftware/kryo/Kryo.java
@@ -490,7 +490,7 @@ public Registration getRegistration (Class type) {
 			if (Proxy.isProxyClass(type)) {
 				// If a Proxy class, treat it like an InvocationHandler because the concrete class for a proxy is generated.
 				registration = getRegistration(InvocationHandler.class);
-			} else if (!type.isEnum() && Enum.class.isAssignableFrom(type)) {
+			} else if (!type.isEnum() && Enum.class.isAssignableFrom(type) && !Enum.class.equals(type)) {
 				// This handles an enum value that is an inner class. Eg: enum A {b{}};
 				registration = getRegistration(type.getEnclosingClass());
 			} else if (EnumSet.class.isAssignableFrom(type)) {
diff --git a/src/com/esotericsoftware/kryo/serializers/DefaultSerializers.java b/src/com/esotericsoftware/kryo/serializers/DefaultSerializers.java
index dd5fc09b2..a01fadf48 100644
--- a/src/com/esotericsoftware/kryo/serializers/DefaultSerializers.java
+++ b/src/com/esotericsoftware/kryo/serializers/DefaultSerializers.java
@@ -394,7 +394,12 @@ static public class EnumSerializer extends Serializer<Enum> {
 
 		public EnumSerializer (Class<? extends Enum> type) {
 			enumConstants = type.getEnumConstants();
-			if (enumConstants == null) throw new IllegalArgumentException("The type must be an enum: " + type);
+			// We allow the serialization of the (abstract!) Enum.class (instead of an actual "user" enum),
+			// which also creates an EnumSerializer instance during Kryo.writeClass with the following trace:
+			// ClassSerializer.write -> Kryo.writeClass -> DefaultClassResolver.writeClass
+			//  -> Kryo.getDefaultSerializer -> ReflectionSerializerFactory.makeSerializer(kryo, EnumSerializer, Enum.class)
+			// This EnumSerializer instance is expected to be never called for write/read.
+			if (enumConstants == null && !Enum.class.equals(type)) throw new IllegalArgumentException("The type must be an enum: " + type);
 		}
 
 		public void write (Kryo kryo, Output output, Enum object) {
diff --git a/test/com/esotericsoftware/kryo/DefaultSerializersTest.java b/test/com/esotericsoftware/kryo/DefaultSerializersTest.java
index 6ec894c67..fc70798b7 100644
--- a/test/com/esotericsoftware/kryo/DefaultSerializersTest.java
+++ b/test/com/esotericsoftware/kryo/DefaultSerializersTest.java
@@ -314,6 +314,7 @@ public void testClassSerializer () {
 		kryo.writeObject(out, void.class);
 		kryo.writeObject(out, ArrayList.class);
 		kryo.writeObject(out, TestEnum.class);
+		kryo.writeObject(out, Enum.class);
 
 		final Input in = new Input(out.getBuffer());
 
@@ -336,6 +337,7 @@ public void testClassSerializer () {
 		assertEquals(void.class, kryo.readObject(in, Class.class));
 		assertEquals(ArrayList.class, kryo.readObject(in, Class.class));
 		assertEquals(TestEnum.class, kryo.readObject(in, Class.class));
+		assertEquals(Enum.class, kryo.readObject(in, Class.class));
 	}
 
 	public void testLocaleSerializer () {