Question

I have found the clone() method of Object in Java:

 protected native Object clone() throws CloneNotSupportedException;

Is the source of this method available? Maybe in OpenJDK?

Answer 1

From jdk/src/share/native/java/lang/Object.c

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
};

Meaning its a function pointer(probably done so they could implement platform-specific native code)

doing a grep for JVM_Clone produces, among other things:

(from hotspot/src/share/vm/prims/jvm.cpp)

JVM_ENTRY(jobject, JVM_Clone(JNIEnv* env, jobject handle))
  JVMWrapper("JVM_Clone");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  const KlassHandle klass (THREAD, obj->klass());
  JvmtiVMObjectAllocEventCollector oam;

#ifdef ASSERT
  // Just checking that the cloneable flag is set correct
  if (obj->is_javaArray()) {
    guarantee(klass->is_cloneable(), "all arrays are cloneable");
  } else {
    guarantee(obj->is_instance(), "should be instanceOop");
    bool cloneable = klass->is_subtype_of(SystemDictionary::Cloneable_klass());
    guarantee(cloneable == klass->is_cloneable(), "incorrect cloneable flag");
  }
#endif

  // Check if class of obj supports the Cloneable interface.
  // All arrays are considered to be cloneable (See JLS 20.1.5)
  if (!klass->is_cloneable()) {
    ResourceMark rm(THREAD);
    THROW_MSG_0(vmSymbols::java_lang_CloneNotSupportedException(), klass->external_name());
  }

  // Make shallow object copy
  const int size = obj->size();
  oop new_obj = NULL;
  if (obj->is_javaArray()) {
    const int length = ((arrayOop)obj())->length();
    new_obj = CollectedHeap::array_allocate(klass, size, length, CHECK_NULL);
  } else {
    new_obj = CollectedHeap::obj_allocate(klass, size, CHECK_NULL);
  }
  // 4839641 (4840070): We must do an oop-atomic copy, because if another thread
  // is modifying a reference field in the clonee, a non-oop-atomic copy might
  // be suspended in the middle of copying the pointer and end up with parts
  // of two different pointers in the field.  Subsequent dereferences will crash.
  // 4846409: an oop-copy of objects with long or double fields or arrays of same
  // won't copy the longs/doubles atomically in 32-bit vm's, so we copy jlongs instead
  // of oops.  We know objects are aligned on a minimum of an jlong boundary.
  // The same is true of StubRoutines::object_copy and the various oop_copy
  // variants, and of the code generated by the inline_native_clone intrinsic.
  assert(MinObjAlignmentInBytes >= BytesPerLong, "objects misaligned");
  Copy::conjoint_jlongs_atomic((jlong*)obj(), (jlong*)new_obj,
                               (size_t)align_object_size(size) / HeapWordsPerLong);
  // Clear the header
  new_obj->init_mark();

  // Store check (mark entire object and let gc sort it out)
  BarrierSet* bs = Universe::heap()->barrier_set();
  assert(bs->has_write_region_opt(), "Barrier set does not have write_region");
  bs->write_region(MemRegion((HeapWord*)new_obj, size));

  // Caution: this involves a java upcall, so the clone should be
  // "gc-robust" by this stage.
  if (klass->has_finalizer()) {
    assert(obj->is_instance(), "should be instanceOop");
    new_obj = instanceKlass::register_finalizer(instanceOop(new_obj), CHECK_NULL);
  }

  return JNIHandles::make_local(env, oop(new_obj));
JVM_END