Klasses in HotSpot have a pointer to the Java mirror which the compiler considers to be non-null. This means that you should never be able to see the Klass before the _java_mirror has been set. When building ArrayKlasses, cross links are made between the element Class and the array Class. This creates an access to the newly created ArrayKlass before its _java_mirror has been set. If another thread see this value you can get a crash.