Fix #529: Support serializing the Enum class object

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)) {

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) {

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 () {