1、JVM中OOP-KLASS模型
在JVM中，使用了OOP-KLASS模型来表示java对象，即：
1.jvm在加载class时，创建instanceKlass，表示其元数据，包括常量池、字段、方法等，存放在方法区；instanceKlass是jvm中的数据结构；
2.在new一个对象时，jvm创建instanceOopDesc，来表示这个对象，存放在堆区，其引用，存放在栈区；它用来表示对象的实例信息，看起来像个指针实际上是藏在指针里的对象；instanceOopDesc对应java中的对象实例；
3.HotSpot并不把instanceKlass暴露给Java，而会另外创建对应的instanceOopDesc来表示java.lang.Class对象，并将后者称为前者的“Java镜像”，klass持有指向oop引用(_java_mirror便是该instanceKlass对Class对象的引用)；
4.要注意，new操作返回的instanceOopDesc类型指针指向instanceKlass，而instanceKlass指向了对应的类型的Class实例的instanceOopDesc；有点绕，简单说，就是Person实例——>Person的instanceKlass——>Person的Class。
instanceOopDesc，只包含数据信息，它包含三部分：
1. 对象头，也叫Mark Word，主要存储对象运行时记录信息，如hashcode, GC分代年龄，锁状态标志，线程ID，时间戳等;
2. 元数据指针，即指向方法区的instanceKlass实例
3. 实例数据;
4. 另外，如果是数组对象，还多了一个数组长度
2、方法区变迁
JVM规范虽说编译后代码在方法区，但是不做强制要求，具体要看JVM实现，hotspot将JIT编译生成的代码存放在native memory的CodeCache区域
jdk6
Klass元数据信息
每个类的运行时常量池(字段、方法、类、接口等符号引用)、编译后的代码
静态字段(无论是否有final)在instanceKlass末尾（位于PermGen内）
oop 其实就是Class对象实例
全局字符串常量池StringTable，本质上就是个Hashtable
符号引用（类型指针是SymbolKlass）
jdk7
Klass元数据信息
每个类的运行时常量池(字段、方法、类、接口等符号引用)、编译后的代码
静态字段从instanceKlass末尾移动到了java.lang.Class对象（oop）的末尾（位于普通Java heap内）
oop与全局字符串常量池移到java heap上
符号引用被移动到native heap中
jdk8
移除永久代：
Klass元数据信息、每个类的运行时常量池、编译后的代码移到了另一块与堆不相连的本地内存————元空间（Metaspace）
参数控制-XX:MetaspaceSize与-XX:MaxMetaspaceSize。
关于openjdk移除永久代的相关信息：http://openjdk.java.net/jeps/122
3、Class实例在堆中还是方法区中？
JDK6
查看openjdkhotspotsrcsharevmclassfileclassFileParser.cpp文件的instanceKlassHandle ClassFileParser::parseClassFile方法，在3522行使用oopFactory::new_instanceKlass来创建klassOop；
klassOop相当于Java中的class,一个klassOop对象包含header、klass_field和Klass
instanceKlassHandle ClassFileParser::parseClassFile(symbolHandle name, Handle class_loader, Handle protection_domain, KlassHandle host_klass, GrowableArray<Handle>* cp_patches, symbolHandle& parsed_name, bool verify, TRAPS) { // 3522 // We can now create the basic klassOop for this klass klassOop ik = oopFactory::new_instanceKlass(vtable_size, itable_size, static_field_size, total_oop_map_count, rt, CHECK_(nullHandle)); instanceKlassHandle this_klass (THREAD, ik); // Create new handle outside HandleMark instanceKlassHandle this_klass (THREAD, preserve_this_klass); debug_only(this_klass->as_klassOop()->verify();) return this_klass; }
在openjdkhotspotsrcsharevmmemoryoopFactory.cpp中找到这个方法，发现调用了instanceKlassKlass::allocate_instance_klass方法来创建klassOop
klassOop oopFactory::new_instanceKlass(Symbol* name, int vtable_len, int itable_len, int static_field_size, unsigned int nonstatic_oop_map_count, ReferenceType rt, TRAPS) { instanceKlassKlass* ikk = instanceKlassKlass::cast(Universe::instanceKlassKlassObj()); return ikk->allocate_instance_klass(name, vtable_len, itable_len, static_field_size, nonstatic_oop_map_count, rt, CHECK_NULL); }
在openjdkhotspotsrcsharevmoopsinstanceKlassKlass.cpp中找到allocate_instance_klass方法，发现调用base_create_klass
klassOop instanceKlassKlass::allocate_instance_klass(Symbol* name, int vtable_len, int itable_len, int static_field_size, unsigned nonstatic_oop_map_count, ReferenceType rt, TRAPS) { const int nonstatic_oop_map_size = instanceKlass::nonstatic_oop_map_size(nonstatic_oop_map_count); int size = instanceKlass::object_size(align_object_offset(vtable_len) + align_object_offset(itable_len) + nonstatic_oop_map_size); // Allocation KlassHandle h_this_klass(THREAD, as_klassOop()); KlassHandle k; if (rt == REF_NONE) { if (name != vmSymbols::java_lang_Class()) { // regular klass instanceKlass o; k = base_create_klass(h_this_klass, size, o.vtbl_value(), CHECK_NULL); } else { // Class instanceMirrorKlass o; k = base_create_klass(h_this_klass, size, o.vtbl_value(), CHECK_NULL); } } else { // reference klass instanceRefKlass o; k = base_create_klass(h_this_klass, size, o.vtbl_value(), CHECK_NULL); } ..... k()->set_partially_loaded(); } return k(); }
在openjdkhotspotsrcsharevmoopsklass.cpp中base_create_klass方法内部直接调用base_create_klass_oop来创建klassOop
KlassHandle Klass::base_create_klass(KlassHandle& klass, int size, const Klass_vtbl& vtbl, TRAPS) { klassOop ek = base_create_klass_oop(klass, size, vtbl, THREAD); return KlassHandle(THREAD, ek); }
allocate_permanent方法默认在PermGen分配内存，instanceKlass对象保存在永久代区域
klassOop Klass::base_create_klass_oop(KlassHandle& klass, int size, const Klass_vtbl& vtbl, TRAPS) { size = align_object_size(size); // allocate and initialize vtable Klass* kl = (Klass*) vtbl.allocate_permanent(klass, size, CHECK_NULL);
// as_klassOop在klass.hpp文件中 klassOop k = kl->as_klassOop(); ......
return k; }
总结，从klassOop构建流程看出，JDK1.6中Class实例在方法区，而且和JDK7创建流程有了很大差异
JDK7
对于引用类型和数组类型对应的Class实例，openjdkhotspotsrcsharevmclassfileclassFileParser.cpp的instanceKlassHandle ClassFileParser::parseClassFile方法的3615行通过java_lang_Class::create_mirror来创建
instanceKlassHandle ClassFileParser::parseClassFile(Symbol* name, Handle class_loader, Handle protection_domain, KlassHandle host_klass, GrowableArray<Handle>* cp_patches, TempNewSymbol& parsed_name, bool verify, TRAPS) { // 3615 // Allocate mirror and initialize static fields java_lang_Class::create_mirror(this_klass, CHECK_(nullHandle)); return this_klass; }
在openjdkhotspotsrcsharevmclassfilejavaClasses.cpp中找到create_mirror方法,看如下发现InstanceMirrorKlass::allocate_instance创建了对象实例，而且实例不是数组时会初始化静态字段
oop java_lang_Class::create_mirror(KlassHandle k, TRAPS) { assert(k->java_mirror() == NULL, "should only assign mirror once"); // Use this moment of initialization to cache modifier_flags also, // to support Class.getModifiers(). Instance classes recalculate // the cached flags after the class file is parsed, but before the // class is put into the system dictionary. int computed_modifiers = k->compute_modifier_flags(CHECK_0); k->set_modifier_flags(computed_modifiers); if (SystemDictionary::Class_klass_loaded() && (k->oop_is_instance() || k->oop_is_javaArray())) { // Allocate mirror (java.lang.Class instance) Handle mirror = instanceMirrorKlass::cast(SystemDictionary::Class_klass())->allocate_instance(k, CHECK_0); // Setup indirections mirror->obj_field_put(klass_offset, k()); k->set_java_mirror(mirror()); instanceMirrorKlass* mk = instanceMirrorKlass::cast(mirror->klass()); java_lang_Class::set_oop_size(mirror(), mk->instance_size(k)); java_lang_Class::set_static_oop_field_count(mirror(), mk->compute_static_oop_field_count(mirror())); // It might also have a component mirror. This mirror must already exist. if (k->oop_is_javaArray()) { Handle comp_mirror; if (k->oop_is_typeArray()) { BasicType type = typeArrayKlass::cast(k->as_klassOop())->element_type(); comp_mirror = Universe::java_mirror(type); assert(comp_mirror.not_null(), "must have primitive mirror"); } else if (k->oop_is_objArray()) { klassOop element_klass = objArrayKlass::cast(k->as_klassOop())->element_klass(); if (element_klass != NULL && (Klass::cast(element_klass)->oop_is_instance() || Klass::cast(element_klass)->oop_is_javaArray())) { comp_mirror = Klass::cast(element_klass)->java_mirror(); assert(comp_mirror.not_null(), "must have element mirror"); } // else some object array internal to the VM, like systemObjArrayKlassObj } if (comp_mirror.not_null()) { // Two-way link between the array klass and its component mirror: arrayKlass::cast(k->as_klassOop())->set_component_mirror(comp_mirror()); set_array_klass(comp_mirror(), k->as_klassOop()); } } else if (k->oop_is_instance()) { // Initialize static fields instanceKlass::cast(k())->do_local_static_fields(&initialize_static_field, CHECK_NULL); } return mirror(); } else { return NULL; } }
对于基本数据类型，是JVM内置的Class类型，openjdkhotspotsrcsharevmmemoryuniverse.cpp中initialize_basic_type_mirrors方法便是初始化基本类型的类型实例的，通过调用java_lang_Class::create_basic_type_mirror
void Universe::initialize_basic_type_mirrors(TRAPS) { if (UseSharedSpaces) { assert(_int_mirror != NULL, "already loaded"); assert(_void_mirror == _mirrors[T_VOID], "consistently loaded"); } else { assert(_int_mirror==NULL, "basic type mirrors already initialized"); _int_mirror = java_lang_Class::create_basic_type_mirror("int", T_INT, CHECK); _float_mirror = java_lang_Class::create_basic_type_mirror("float", T_FLOAT, CHECK); _double_mirror = java_lang_Class::create_basic_type_mirror("double", T_DOUBLE, CHECK); _byte_mirror = java_lang_Class::create_basic_type_mirror("byte", T_BYTE, CHECK); _bool_mirror = java_lang_Class::create_basic_type_mirror("boolean",T_BOOLEAN, CHECK); _char_mirror = java_lang_Class::create_basic_type_mirror("char", T_CHAR, CHECK); _long_mirror = java_lang_Class::create_basic_type_mirror("long", T_LONG, CHECK); _short_mirror = java_lang_Class::create_basic_type_mirror("short", T_SHORT, CHECK); _void_mirror = java_lang_Class::create_basic_type_mirror("void", T_VOID, CHECK); _mirrors[T_INT] = _int_mirror; _mirrors[T_FLOAT] = _float_mirror; _mirrors[T_DOUBLE] = _double_mirror; _mirrors[T_BYTE] = _byte_mirror; _mirrors[T_BOOLEAN] = _bool_mirror; _mirrors[T_CHAR] = _char_mirror; _mirrors[T_LONG] = _long_mirror; _mirrors[T_SHORT] = _short_mirror; _mirrors[T_VOID] = _void_mirror; //_mirrors[T_OBJECT] = instanceKlass::cast(_object_klass)->java_mirror(); //_mirrors[T_ARRAY] = instanceKlass::cast(_object_klass)->java_mirror(); } }
openjdkhotspotsrcsharevmclassfilejavaClasses.cpp中的create_basic_type_mirror方法
oop java_lang_Class::create_basic_type_mirror(const char* basic_type_name, BasicType type, TRAPS) { // This should be improved by adding a field at the Java level or by // introducing a new VM klass (see comment in ClassFileParser) oop java_class = instanceMirrorKlass::cast(SystemDictionary::Class_klass())->allocate_instance((oop)NULL, CHECK_0); if (type != T_VOID) { klassOop aklass = Universe::typeArrayKlassObj(type); assert(aklass != NULL, "correct bootstrap"); set_array_klass(java_class, aklass); } instanceMirrorKlass* mk = instanceMirrorKlass::cast(SystemDictionary::Class_klass()); java_lang_Class::set_oop_size(java_class, mk->instance_size(oop(NULL))); java_lang_Class::set_static_oop_field_count(java_class, 0); return java_class; }
对比引用类型和基本数据类型的Class实例创建，发现都是通过instanceOop instanceMirrorKlass::allocate_instance方法；在openjdkhotspotsrcsharevmoopsinstanceMirrorKlass.cpp中可以找到；以下代码就是核心了，通过对JavaObjectsInPerm
参数的判断来决定Class实例存在方法区还是在堆中。
instanceOop instanceMirrorKlass::allocate_instance(KlassHandle k, TRAPS) { // Query before forming handle. int size = instance_size(k); KlassHandle h_k(THREAD, as_klassOop()); instanceOop i; if (JavaObjectsInPerm) { i = (instanceOop) CollectedHeap::permanent_obj_allocate(h_k, size, CHECK_NULL); } else { assert(ScavengeRootsInCode > 0, "must be"); i = (instanceOop) CollectedHeap::obj_allocate(h_k, size, CHECK_NULL); } return i; }
接下来先看JavaObjectsInPerm为true的情况，CollectedHeap::permanent_obj_allocate方法，在openjdkhotspotsrcsharevmgc_interfacecollectedHeap.inline.hpp中；
oop CollectedHeap::permanent_obj_allocate(KlassHandle klass, int size, TRAPS) { oop obj = permanent_obj_allocate_no_klass_install(klass, size, CHECK_NULL); post_allocation_install_obj_klass(klass, obj, size); NOT_PRODUCT(Universe::heap()->check_for_bad_heap_word_value((HeapWord*) obj, size)); return obj; } oop CollectedHeap::permanent_obj_allocate_no_klass_install(KlassHandle klass, int size, TRAPS) { debug_only(check_for_valid_allocation_state()); assert(!Universe::heap()->is_gc_active(), "Allocation during gc not allowed"); assert(size >= 0, "int won't convert to size_t"); HeapWord* obj = common_permanent_mem_allocate_init(size, CHECK_NULL); post_allocation_setup_no_klass_install(klass, obj, size); NOT_PRODUCT(Universe::heap()->check_for_bad_heap_word_value(obj, size)); return (oop)obj; }
最终调用的是common_permanent_mem_allocate_init方法中的common_permanent_mem_allocate_noinit方法来创建实例
HeapWord* CollectedHeap::common_permanent_mem_allocate_init(size_t size, TRAPS) {
// 内存空间的分配
HeapWord* obj = common_permanent_mem_allocate_noinit(size, CHECK_NULL);
// 对象的初始化
init_obj(obj, size);
return obj;
}
从common_permanent_mem_allocate_noinit方法的名字，和内存溢出的异常"PermGen space"可以看出最后创建oop实例是在方法区中
// Need to investigate, do we really want to throw OOM exception here?
HeapWord* CollectedHeap::common_permanent_mem_allocate_noinit(size_t size, TRAPS) {
if (HAS_PENDING_EXCEPTION) {
NOT_PRODUCT(guarantee(false, "Should not allocate with exception pending"));
return NULL; // caller does a CHECK_NULL too
}
#ifdef ASSERT
if (CIFireOOMAt > 0 && THREAD->is_Compiler_thread() &&
++_fire_out_of_memory_count >= CIFireOOMAt) {
// For testing of OOM handling in the CI throw an OOM and see how
// it does. Historically improper handling of these has resulted
// in crashes which we really don't want to have in the CI.
THROW_OOP_0(Universe::out_of_memory_error_perm_gen());
}
#endif
HeapWord* result = Universe::heap()->permanent_mem_allocate(size);
if (result != NULL) {
NOT_PRODUCT(Universe::heap()->
check_for_non_bad_heap_word_value(result, size));
assert(!HAS_PENDING_EXCEPTION,
"Unexpected exception, will result in uninitialized storage");
return result;
}
// -XX:+HeapDumpOnOutOfMemoryError and -XX:OnOutOfMemoryError support
report_java_out_of_memory("PermGen space");
if (JvmtiExport::should_post_resource_exhausted()) {
JvmtiExport::post_resource_exhausted(
JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR,
"PermGen space");
}
THROW_OOP_0(Universe::out_of_memory_error_perm_gen());
}
再看JavaObjectsInPerm参数为false的情况，仍然是在collectedHeap.inline.hpp文件中，发现调用了common_mem_allocate_init，接着又调用了common_mem_allocate_noinit
oop CollectedHeap::obj_allocate(KlassHandle klass, int size, TRAPS) { debug_only(check_for_valid_allocation_state()); assert(!Universe::heap()->is_gc_active(), "Allocation during gc not allowed"); assert(size >= 0, "int won't convert to size_t"); HeapWord* obj = common_mem_allocate_init(size, false, CHECK_NULL); post_allocation_setup_obj(klass, obj, size); NOT_PRODUCT(Universe::heap()->check_for_bad_heap_word_value(obj, size)); return (oop)obj; }
HeapWord* CollectedHeap::common_mem_allocate_init(size_t size, bool is_noref, TRAPS) {
// 内存空间的分配 HeapWord* obj = common_mem_allocate_noinit(size, is_noref, CHECK_NULL);
// 对象的初始化 init_obj(obj, size); return obj; }
从common_mem_allocate_noinit方法的名字，和内存溢出的异常"Java heap space"可以创建oop实例是在java heap中
HeapWord* CollectedHeap::common_mem_allocate_noinit(size_t size, bool is_noref, TRAPS) { // Clear unhandled oops for memory allocation. Memory allocation might // not take out a lock if from tlab, so clear here. CHECK_UNHANDLED_OOPS_ONLY(THREAD->clear_unhandled_oops();) if (HAS_PENDING_EXCEPTION) { NOT_PRODUCT(guarantee(false, "Should not allocate with exception pending")); return NULL; // caller does a CHECK_0 too } // We may want to update this, is_noref objects might not be allocated in TLABs. HeapWord* result = NULL; if (UseTLAB) { result = CollectedHeap::allocate_from_tlab(THREAD, size); if (result != NULL) { assert(!HAS_PENDING_EXCEPTION, "Unexpected exception, will result in uninitialized storage"); return result; } } bool gc_overhead_limit_was_exceeded = false; result = Universe::heap()->mem_allocate(size, is_noref, false, &gc_overhead_limit_was_exceeded); if (result != NULL) { NOT_PRODUCT(Universe::heap()-> check_for_non_bad_heap_word_value(result, size)); assert(!HAS_PENDING_EXCEPTION, "Unexpected exception, will result in uninitialized storage"); THREAD->incr_allocated_bytes(size * HeapWordSize); return result; } if (!gc_overhead_limit_was_exceeded) { // -XX:+HeapDumpOnOutOfMemoryError and -XX:OnOutOfMemoryError support report_java_out_of_memory("Java heap space"); if (JvmtiExport::should_post_resource_exhausted()) { JvmtiExport::post_resource_exhausted( JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_JAVA_HEAP, "Java heap space"); } THROW_OOP_0(Universe::out_of_memory_error_java_heap()); } else { // -XX:+HeapDumpOnOutOfMemoryError and -XX:OnOutOfMemoryError support report_java_out_of_memory("GC overhead limit exceeded"); if (JvmtiExport::should_post_resource_exhausted()) { JvmtiExport::post_resource_exhausted( JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_JAVA_HEAP, "GC overhead limit exceeded"); } THROW_OOP_0(Universe::out_of_memory_error_gc_overhead_limit()); } }
最后查看openjdkhotspotsrcsharevm untimeglobals.hpp文件，发现JavaObjectsInPerm的值是定义为false的
develop(bool, JavaObjectsInPerm, false, "controls whether Classes and interned Strings are allocated" "in perm. This purely intended to allow debugging issues" "in production.")
总结：JDK7创建Class实例存在堆中；因为JDK7中JavaObjectsInPerm参数值固定为false。
JDK8
JDK8和JDK7创建Class实例的代码大同小异，从openjdkhotspotsrcsharevmoopsinstanceMirrorKlass.cpp中的instanceOop InstanceMirrorKlass::allocate_instance方法开始才有了区别;
不在判断JavaObjectsInPerm参数，因为jdk8已经去除了永久代，取而代之的值MetaSpace。这里直接调用CollectedHeap::Class_obj_allocate
instanceOop InstanceMirrorKlass::allocate_instance(KlassHandle k, TRAPS) { // Query before forming handle. int size = instance_size(k); KlassHandle h_k(THREAD, this); instanceOop i = (instanceOop) CollectedHeap::Class_obj_allocate(h_k, size, k, CHECK_NULL); return i; }
openjdkhotspotsrcsharevmgc_interfacecollectedHeap.cpp中找到CollectedHeap::Class_obj_allocate
oop CollectedHeap::Class_obj_allocate(KlassHandle klass, int size, KlassHandle real_klass, TRAPS) { HeapWord* obj; obj = common_mem_allocate_init(real_klass, size, CHECK_NULL); oop mirror = (oop)obj; java_lang_Class::set_oop_size(mirror, size); // Setup indirections if (!real_klass.is_null()) { java_lang_Class::set_klass(mirror, real_klass()); real_klass->set_java_mirror(mirror); } InstanceMirrorKlass* mk = InstanceMirrorKlass::cast(mirror->klass()); assert(size == mk->instance_size(real_klass), "should have been set"); // notify jvmti and dtrace post_allocation_notify(klass, (oop)obj); return mirror; }
在openjdkhotspotsrcsharevmgc_interfacecollectedHeap.inline.hpp中找到common_mem_allocate_init发现调用了common_mem_allocate_noinit
HeapWord* CollectedHeap::common_mem_allocate_init(KlassHandle klass, size_t size, TRAPS) { HeapWord* obj = common_mem_allocate_noinit(klass, size, CHECK_NULL); init_obj(obj, size); return obj; }
内存溢出的异常"Java heap space"可以看出JDK8创建的Class实例最终在堆中
HeapWord* CollectedHeap::common_mem_allocate_noinit(KlassHandle klass, size_t size, TRAPS) { // Clear unhandled oops for memory allocation. Memory allocation might // not take out a lock if from tlab, so clear here. CHECK_UNHANDLED_OOPS_ONLY(THREAD->clear_unhandled_oops();) if (HAS_PENDING_EXCEPTION) { NOT_PRODUCT(guarantee(false, "Should not allocate with exception pending")); return NULL; // caller does a CHECK_0 too } HeapWord* result = NULL; if (UseTLAB) { result = allocate_from_tlab(klass, THREAD, size); if (result != NULL) { assert(!HAS_PENDING_EXCEPTION, "Unexpected exception, will result in uninitialized storage"); return result; } } bool gc_overhead_limit_was_exceeded = false; result = Universe::heap()->mem_allocate(size, &gc_overhead_limit_was_exceeded); if (result != NULL) { NOT_PRODUCT(Universe::heap()-> check_for_non_bad_heap_word_value(result, size)); assert(!HAS_PENDING_EXCEPTION, "Unexpected exception, will result in uninitialized storage"); THREAD->incr_allocated_bytes(size * HeapWordSize); AllocTracer::send_allocation_outside_tlab_event(klass, size * HeapWordSize); return result; } if (!gc_overhead_limit_was_exceeded) { // -XX:+HeapDumpOnOutOfMemoryError and -XX:OnOutOfMemoryError support report_java_out_of_memory("Java heap space"); if (JvmtiExport::should_post_resource_exhausted()) { JvmtiExport::post_resource_exhausted( JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_JAVA_HEAP, "Java heap space"); } THROW_OOP_0(Universe::out_of_memory_error_java_heap()); } else { // -XX:+HeapDumpOnOutOfMemoryError and -XX:OnOutOfMemoryError support report_java_out_of_memory("GC overhead limit exceeded"); if (JvmtiExport::should_post_resource_exhausted()) { JvmtiExport::post_resource_exhausted( JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_JAVA_HEAP, "GC overhead limit exceeded"); } THROW_OOP_0(Universe::out_of_memory_error_gc_overhead_limit()); } }
总结：JDK8移除了永久代，转而使用元空间来实现方法区，创建的Class实例在java heap中