HotSpot类模型之InstanceKlass
上一篇 HotSpot源码分析之Klass 介绍了类模型的基础类Klass的重要属性及方法,这一篇介绍一下InstanceKlass及InstanceKlass的子类。
1、InstanceKlass类
每个InstanceKlass对象表示一个具体的Java类(这里的Java类不包括Java数组)。InstanceKlass类及重要属性的定义如下:
class InstanceKlass: public Klass {
...
protected:
// Annotations for this class
Annotations* _annotations;
// Array classes holding elements of this class.
Klass* _array_klasses;
// Constant pool for this class.
ConstantPool* _constants;
// The InnerClasses attribute and EnclosingMethod attribute. The
// _inner_classes is an array of shorts. If the class has InnerClasses
// attribute, then the _inner_classes array begins with 4-tuples of shorts
// [inner_class_info_index, outer_class_info_index,
// inner_name_index, inner_class_access_flags] for the InnerClasses
// attribute. If the EnclosingMethod attribute exists, it occupies the
// last two shorts [class_index, method_index] of the array. If only
// the InnerClasses attribute exists, the _inner_classes array length is
// number_of_inner_classes * 4. If the class has both InnerClasses
// and EnclosingMethod attributes the _inner_classes array length is
// number_of_inner_classes * 4 + enclosing_method_attribute_size.
Array<jushort>* _inner_classes;
// Array name derived from this class which needs unreferencing
// if this class is unloaded.
Symbol* _array_name;
// Number of heapOopSize words used by non-static fields in this klass
// (including inherited fields but after header_size()).
int _nonstatic_field_size;
int _static_field_size; // number words used by static fields (oop and non-oop) in this klass
// Constant pool index to the utf8 entry of the Generic signature,
// or 0 if none.
u2 _generic_signature_index;
// Constant pool index to the utf8 entry for the name of source file
// containing this klass, 0 if not specified.
u2 _source_file_name_index;
u2 _static_oop_field_count;// number of static oop fields in this klass
u2 _java_fields_count; // The number of declared Java fields
int _nonstatic_oop_map_size;// size in words of nonstatic oop map blocks
u2 _minor_version; // minor version number of class file
u2 _major_version; // major version number of class file
Thread* _init_thread; // Pointer to current thread doing initialization (to handle recusive initialization)
int _vtable_len; // length of Java vtable (in words)
int _itable_len; // length of Java itable (in words)
OopMapCache* volatile _oop_map_cache; // OopMapCache for all methods in the klass (allocated lazily)
JNIid* _jni_ids; // First JNI identifier for static fields in this class
jmethodID* _methods_jmethod_ids; // jmethodIDs corresponding to method_idnum, or NULL if none
nmethodBucket* _dependencies; // list of dependent nmethods
nmethod* _osr_nmethods_head; // Head of list of on-stack replacement nmethods for this class
// Class states are defined as ClassState (see above).
// Place the _init_state here to utilize the unused 2-byte after
// _idnum_allocated_count.
u1 _init_state; // state of class
u1 _reference_type; // reference type
// Method array.
Array<Method*>* _methods;
// Default Method Array, concrete methods inherited from interfaces
Array<Method*>* _default_methods;
// Interface (Klass*s) this class declares locally to implement.
Array<Klass*>* _local_interfaces;
// Interface (Klass*s) this class implements transitively.
Array<Klass*>* _transitive_interfaces;
// Int array containing the vtable_indices for default_methods
// offset matches _default_methods offset
Array<int>* _default_vtable_indices;
// Instance and static variable information, starts with 6-tuples of shorts
// [access, name index, sig index, initval index, low_offset, high_offset]
// for all fields, followed by the generic signature data at the end of
// the array. Only fields with generic signature attributes have the generic
// signature data set in the array. The fields array looks like following:
//
// f1: [access, name index, sig index, initial value index, low_offset, high_offset]
// f2: [access, name index, sig index, initial value index, low_offset, high_offset]
// ...
// fn: [access, name index, sig index, initial value index, low_offset, high_offset]
// [generic signature index]
// [generic signature index]
// ...
Array<u2>* _fields;
// embedded Java vtable follows here
// embedded Java itables follows here
// embedded static fields follows here
// embedded nonstatic oop-map blocks follows here
// embedded implementor of this interface follows here
// The embedded implementor only exists if the current klass is an
// iterface. The possible values of the implementor fall into following
// three cases:
// NULL: no implementor.
// A Klass* that's not itself: one implementor.
// Itsef: more than one implementors.
// embedded host klass follows here
// The embedded host klass only exists in an anonymous class for
// dynamic language support (JSR 292 enabled). The host class grants
// its access privileges to this class also. The host class is either
// named, or a previously loaded anonymous class. A non-anonymous class
// or an anonymous class loaded through normal classloading does not
// have this embedded field.
...
}
重要属性的介绍如下表所示。
有了InstanceKlass与Klass中定义的这些属性足够用来保存Java类元信息。在后续的类解析中会看到对相关变量的属性填充操作。除了保存类元信息外,此类还有另外一个重要的功能,即支持方法分派,主要是通过Java虚函数表和Java接口函数表来完成的,不过C++并不像Java一样,保存信息时非要在类中定义出相关属性,C++只是在分配内存时为要存储的信息分配好特定的内存,然后直接通过内存偏移来操作即可。
接下来几个属性是没有对应的属性名,只能通过指针和偏移量的方式访问:
- Java vtable:Java虚函数表,大小等于_vtable_len;
- Java itables:Java接口函数表,大小等于 _itable_len;
- 非静态oop-map blocks ,大小等于_nonstatic_oop_map_size。GC在垃圾回收时,遍历某个对象所引用的其它对象时,会结合此信息进行查找;
- 接口的实现类,只有当前类表示一个接口时存在。如果接口没有任何实现类则为NULL;如果只有一个实现类则为该实现类的Klass指针;如果有多个实现类,为当前类本身;
- host klass,只在匿名类中存在,为了支持JSR 292中的动态语言特性,会给匿名类生成一个host klass。
HotSpot在解析一个类时会调用InstanceKlass::allocate_instance_klass()方法分配内存,而分配多大的内存则是通过调用InstanceKlass::size()计算出来的,调用语句如下:
调用的size()方法的实现如下:int size = InstanceKlass::size(vtable_len,itable_len,nonstatic_oop_map_size,isinterf,is_anonymous);
方法返回值就是此次创建Klass对象所需要开辟的内存大小。由此方法的计算逻辑可以看出Klass对象的内存布局情况。static int size( int vtable_length, int itable_length, int nonstatic_oop_map_size, bool is_interface, bool is_anonymous ){ return align_object_size(header_size() + // InstanceKlass类本身占用的内存大小 align_object_offset(vtable_length) + align_object_offset(itable_length) + // [EMBEDDED nonstatic oop-map blocks] size in words = nonstatic_oop_map_size // The embedded nonstatic oop-map blocks are short pairs (offset, length) // indicating where oops are located in instances of this klass. ( (is_interface || is_anonymous) ? align_object_offset(nonstatic_oop_map_size) : nonstatic_oop_map_size ) + // [EMBEDDED implementor of the interface] only exist for interface ( is_interface ? (int)sizeof(Klass*)/HeapWordSize : 0 ) + // [EMBEDDED host klass ] only exist for an anonymous class (JSR 292 enabled) ( is_anonymous ? (int)sizeof(Klass*)/HeapWordSize : 0) ); }
图中的灰色阴影部分是可选部分。关于vtable_length和itable_length以及nonstatic_oop_map_size的值在类解析的过程中会计算好,在后续介绍类解析过程中会详细介绍。
调用的header_size()方法就是计算此类的对象所占用的内存大小,实现如下:
调用的align_object_offset()方法是进行内存对齐,这是一块非常重要的C++知识点,后面会专门进行讲解。// Sizing (in words) static int header_size(){ return align_object_offset(sizeof(InstanceKlass)/HeapWordSize); // 以HeapWordSize为单位,64位一个字为8字节,所以值为8 }
2、InstanceKlass类的子类
InstanceKlass共有3个直接子类,这3个子类用来表示一些特殊的类,下面简单介绍一下这3个子类:
(1)InstanceRefKlass
java/lang/ref/Reference的子类需要使用InstanceRefKlass类来表示,在创建这个类的实例时,_reference_type字段的值通常会说明当前的类表示的是哪种引用类型。取值已经在枚举类中定义,如下:
REF_NONE枚举常量的定义如下:
// ReferenceType is used to distinguish between java/lang/ref/Reference subclasses
enum ReferenceType {
REF_NONE, // Regular class
REF_OTHER, // Subclass of java/lang/ref/Reference, but not subclass of one of the classes below
REF_SOFT, // Subclass of java/lang/ref/SoftReference
REF_WEAK, // Subclass of java/lang/ref/WeakReference
REF_FINAL, // Subclass of java/lang/ref/FinalReference
REF_PHANTOM // Subclass of java/lang/ref/PhantomReference
};
可以看到,所有的Java类Reference及子类都会用C++类InstanceRefKlass的对象来表示。当无法判断到底是哪个Java子类时,会将_reference_type的值设置为REF_OTHER。
因为这些类需要垃圾回收器特殊处理 ,在后续讲解强引用、弱引用、虚引用以及幽灵引用时在详细介绍。
(2)InstanceMirrorKlass类
InstanceMirrorKlass对象用于表示特殊的java.lang.Class类,增加了一个静态属性_offset_of_static_fields,用来描述静态字段的起始偏移量。定义如下:
static int _offset_of_static_fields;
只所以增加这个属性,是由于java.lang.Class类比较特殊。正常情况下,HotSpot使用Klass来表示Java类,用oop来表示Java对象,而Java对象中可能定义静态或非静态字段,非静态字段值存储在oop中,而静态字段值存储在表示当前Java类的java.lang.Class对象中。java.lang.Class类用InstanceMirrorKlass对象来表示,java.lang.Class对象用oop来表示,那么Class对象的非静态字段值存储在oop中,而Class类自身也定义了静态字段,那么这些值同样存储在了Class对象中,也就是表示Class对象的oop中,这样静态与非静态字段存储在了一个oop上,通过_offset_of_static_fields属性偏移来定位静态字段的存储位置。
该属性是通过init_offset_of_static_fields方法初始化的,其初始化的过程如下:
static void init_offset_of_static_fields() {
// Cache the offset of the static fields in the Class instance
assert(_offset_of_static_fields == 0, "once");
// java.lang.Class类使用InstanceMirrorKlass对象来表示,而java.lang.Class对象通过Oop对象来表示,那么imk->size_helper()获取的就是
// Oop对象的大小,左移3位将字转换为字节。紧要着Oop对象后存储静态字段的值
InstanceMirrorKlass* imk = InstanceMirrorKlass::cast(SystemDictionary::Class_klass());
_offset_of_static_fields = imk->size_helper() << LogHeapWordSize; // LogHeapWordSize=3
}
int size_helper() const {
return layout_helper_to_size_helper(layout_helper());
}
static int layout_helper_to_size_helper(jint lh) {
assert(lh > (jint)_lh_neutral_value, "must be instance");
return lh >> LogHeapWordSize;
}
int layout_helper() const{ return _layout_helper; }
调用java.lang.Class类(通过InstanceMirrorKlass对象来表示)的size_helper()方法来获取java.lang.Class对象(通过Oop对象来表示)的大小,这个大小是java.lang.Class类中本身声明的一些属性需要占用的大小,紧随其后的就是静态存储的区域。
打开命令-XX:+PrintFieldLayout后的打印结果如下:
非静态的布局如下:
java.lang.Class: field layout
@ 12 --- instance fields start ---
@ 12 "cachedConstructor" Ljava.lang.reflect.Constructor;
@ 16 "newInstanceCallerCache" Ljava.lang.Class;
@ 20 "name" Ljava.lang.String;
@ 24 "reflectionData" Ljava.lang.ref.SoftReference;
@ 28 "genericInfo" Lsun.reflect.generics.repository.ClassRepository;
@ 32 "enumConstants" [Ljava.lang.Object;
@ 36 "enumConstantDirectory" Ljava.util.Map;
@ 40 "annotationData" Ljava.lang.Class$AnnotationData;
@ 44 "annotationType" Lsun.reflect.annotation.AnnotationType;
@ 48 "classValueMap" Ljava.lang.ClassValue$ClassValueMap;
@ 52 "protection_domain" Ljava.lang.Object;
@ 56 "init_lock" Ljava.lang.Object;
@ 60 "signers_name" Ljava.lang.Object;
@ 64 "klass" J
@ 72 "array_klass" J
@ 80 "classRedefinedCount" I
@ 84 "oop_size" I
@ 88 "static_oop_field_count" I
@ 92 --- instance fields end ---
@ 96 --- instance ends ---
这就是java.lang.Class非静态字段的布局,在类解析过程中已经计算好了各个字段的偏移量。在完成非静态字段布局后,紧接着会布局静态字段,此时的_offset_of_static_fields字段的值为96。
我们需要分清相关类的表示方法,如下图所示。
java.lang.Class对象是通过对应的Oop对象来保存类的静态属性,因此他们的实例大小不同,需要特殊的方式来计算他们的大小以及属性遍历。
Klass的属性_java_mirror就指向保存该类静态字段的Oop对象,可通过该属性访问类的静态字段。 Oop是HotSpot的对象表示模型,在后面会详细介绍。
(3)InstanceClassLoaderKlass类
没有添加新的字段,增加了新的oop遍历方法,主要用于类加载器依赖遍历使用。
3、创建类的实例
创建InstanceKlass实例会调用InstanceKlass::allocate_instance_klass()方法。在创建时,会涉及到C++对new运算符的重载,通过重载new运算符来分配对象的内存空间,然后再调用类的构造函数初始化相应的属性。方法的实现如下:
InstanceKlass* InstanceKlass::allocate_instance_klass(
ClassLoaderData* loader_data,
int vtable_len,
int itable_len,
int static_field_size,
int nonstatic_oop_map_size,
ReferenceType rt,
AccessFlags access_flags,
Symbol* name,
Klass* super_klass,
bool is_anonymous,
TRAPS
){
bool isinterf = access_flags.is_interface();
int size = InstanceKlass::size(
vtable_len,
itable_len,
nonstatic_oop_map_size,
isinterf,
is_anonymous
);
// Allocation
InstanceKlass* ik;
///////////////////////////////////////////////////////////////////////
if (rt == REF_NONE) {
if (name == vmSymbols::java_lang_Class()) { // 通过InstanceMirrorKlass对象表示java.lang.Class类
ik = new (loader_data, size, THREAD) InstanceMirrorKlass(
vtable_len,
itable_len,
static_field_size,
nonstatic_oop_map_size,
rt,
access_flags,
is_anonymous);
} else if (
name == vmSymbols::java_lang_ClassLoader() ||
(
SystemDictionary::ClassLoader_klass_loaded() &&
super_klass != NULL && // ClassLoader_klass为java_lang_ClassLoader
super_klass->is_subtype_of(SystemDictionary::ClassLoader_klass())
)
){ // 通过InstanceClassLoaderKlass对象表示java.lang.ClassLoader或相关子类
ik = new (loader_data, size, THREAD) InstanceClassLoaderKlass(
vtable_len,
itable_len,
static_field_size,
nonstatic_oop_map_size,
rt,
access_flags,
is_anonymous);
} else { // 通过InstanceKlass对象表示普通类
// normal class
ik = new (loader_data, size, THREAD) InstanceKlass(
vtable_len, itable_len,
static_field_size,
nonstatic_oop_map_size,
rt,
access_flags,
is_anonymous);
}
}
///////////////////////////////////////////////////////////////////////
else { // 通过InstanceRefKlass对象表示引用
// reference klass
ik = new (loader_data, size, THREAD) InstanceRefKlass(
vtable_len, itable_len,
static_field_size,
nonstatic_oop_map_size,
rt,
access_flags,
is_anonymous);
}
///////////////////////////////////////////////////////////////////////
// 添加所有类型到我们内部类加载器列表中,包括在根加载器中的类
// Add all classes to our internal class loader list here,
// including classes in the bootstrap (NULL) class loader.
// loader_data的类型为ClassLoaderData*,通过ClassLoaderData中的_klasses保持通过InstanceKlass._next_link属性保持的列表
loader_data->add_class(ik);
Atomic::inc(&_total_instanceKlass_count);
return ik;
}
方法的实现比较简单,当rt等于REF_NONE时,也就是为非Reference类型时,会根据类名创建对应C++类的对象。Class类创建InstanceMirrorKlass、ClassLoader类或ClassLoader的子类创建InstanceClassLoaderKlass类、普通类通过InstanceKlass来表示。当rt不为REF_NONE时,会创建InstanceRefKlass对象。
调用的size()函数在之前介绍InstanceKlass类时已经介绍过,这里不再介绍。得到size后会调用new重载运算符函数来开辟内存空间,如下:
void* Klass::operator new(size_t size, ClassLoaderData* loader_data, size_t word_size, TRAPS) throw() {
void* x = Metaspace::allocate( // 在元数据区分配内存空间
loader_data,
word_size,
false, /*read_only*/
MetaspaceObj::ClassType,
CHECK_NULL
);
return x;
}
可以看到,对于jdk1.8版本来说,Klass对象在元数据区分配内存。由于C++没有像Java一样的垃圾回收机制,所以Metaspace的内存需要自动管理和释放,这一块知识将在后面详细介绍。