Java instanceof 關鍵字是如何實現的？

// obj instanceof T
boolean result;
try {
    T temp = (T) obj; // checkcast
    result = true;
} catch (ClassCastException e) {
    result = false;
}

obj->vtable->class

ClassBlock* cb = CLASS_CB(c);

Object f = new Apple();
boolean result = f instanceof Plant;

S.is_subtype_of(T) := {
  int off = T.offset;
  if (S == T) return true;
  if (T == S[off]) return true;
  if (off != &cache) return false;
  if ( S.scan_secondary_subtype_array(T) ) {
    S.cache = T;
    return true;
  }
  return false;
}

inline bool oopDesc::is_a(klassOop k)        const { return blueprint()->is_subtype_of(k); }

  bool is_subtype_of(klassOop k) const {
    juint    off = k->klass_part()->super_check_offset();
    klassOop sup = *(klassOop*)( (address)as_klassOop() + off );
    const juint secondary_offset = in_bytes(secondary_super_cache_offset());
    if (sup == k) {
      return true;
    } else if (off != secondary_offset) {
      return false;
    } else {
      return search_secondary_supers(k);
    }
  }
  bool search_secondary_supers(klassOop k) const;

bool Klass::search_secondary_supers(klassOop k) const {
  // Put some extra logic here out-of-line, before the search proper.
  // This cuts down the size of the inline method.

  // This is necessary, since I am never in my own secondary_super list.
  if (this->as_klassOop() == k)
    return true;
  // Scan the array-of-objects for a match
  int cnt = secondary_supers()->length();
  for (int i = 0; i < cnt; i++) {
    if (secondary_supers()->obj_at(i) == k) {
      ((Klass*)this)->set_secondary_super_cache(k);
      return true;
    }
  }
  return false;
}

if (obj.getClass() == A.class) {
  boolean isInst = obj instanceof A;
}

if (obj.getClass() == A.class) {
  boolean isInst = true;
}