假设有一个类Apple(Apple的colne只是简单的调用super.clone), 当我clone一个Apple对象时,首先可以通过在Apple的构造函数中加上输出证明Apple的构造器是不会被调用到的。但是如何证明Object的构造器也不会被调用到呢? 有没有什么工具或方法去证明clone是不会调用Object的clone方法的
我跟踪了colne这个native方法, 并下载了hotspot的源代码, 在jvm.cpp中发现了clone方法的实现。
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