Why Unsafe.allocateInstance(Class.class) failed?

问题:

I'm now using unsafe. When I run the following code:

unsafe.allocateInstance(Class.class)

There happen's

Exception in thread "main" java.lang.IllegalAccessException: java.lang.Class

Since Class is a non-abstract class, why it so special? And is there any way to construct an 'empty' Class like allocateInstance?

回答1:

Because there is an explicit check inside HotSpot JVM to ensure that java.lang.Class cannot be instantiated through JNI, Unsafe etc. See instanceKlass.cpp:

void InstanceKlass::check_valid_for_instantiation(bool throwError, TRAPS) {
  if (is_interface() || is_abstract()) {
    ResourceMark rm(THREAD);
    THROW_MSG(throwError ? vmSymbols::java_lang_InstantiationError()
              : vmSymbols::java_lang_InstantiationException(), external_name());
  }
  if (this == SystemDictionary::Class_klass()) {
    ResourceMark rm(THREAD);
    THROW_MSG(throwError ? vmSymbols::java_lang_IllegalAccessError()
              : vmSymbols::java_lang_IllegalAccessException(), external_name());
  }
}

Such instance would not be valid anyway, so it does not make sense.