查看hotspot/src/share/vm/memory/universe.cpp中的Universe::genesis()函数的实现，其中有对数组及核心类的加载逻辑。数组类没有对应的Class文件，所以在类装载阶段，数组类会被虚拟机直接创建，也不需要进行验证、准备和初始化等操作；类加载就是通过宏来定义一些需要加载的核心类，然后调用前面介绍的一些类加载器方法来加载类。数组类的创建如下：
_boolArrayKlassObj = TypeArrayKlass::create_klass(T_BOOLEAN, sizeof(jboolean), CHECK); _charArrayKlassObj = TypeArrayKlass::create_klass(T_CHAR, sizeof(jchar), CHECK); _singleArrayKlassObj = TypeArrayKlass::create_klass(T_FLOAT, sizeof(jfloat), CHECK); _doubleArrayKlassObj = TypeArrayKlass::create_klass(T_DOUBLE, sizeof(jdouble), CHECK); _byteArrayKlassObj = TypeArrayKlass::create_klass(T_BYTE, sizeof(jbyte), CHECK); _shortArrayKlassObj = TypeArrayKlass::create_klass(T_SHORT, sizeof(jshort), CHECK); _intArrayKlassObj = TypeArrayKlass::create_klass(T_INT, sizeof(jint), CHECK); _longArrayKlassObj = TypeArrayKlass::create_klass(T_LONG, sizeof(jlong), CHECK);
数组类的创建在前面介绍类模型时介绍过，这里不再介绍。
HotSpot在启动过程中会预先加载一些核心类，如Object、String等，如下：
源代码位置：/src/share/vm/classfile/systemDictionary.hpp #define WK_KLASSES_DO(do_klass) \ /* well-known classes */ \ do_klass(Object_klass, java_lang_Object , Pre) \ do_klass(String_klass, java_lang_String, Pre ) \ do_klass(Class_klass, java_lang_Class, Pre ) \ do_klass(Cloneable_klass, java_lang_Cloneable, Pre ) \ do_klass(ClassLoader_klass, java_lang_ClassLoader, Pre ) \ do_klass(Serializable_klass, java_io_Serializable, Pre) \ do_klass(System_klass, java_lang_System, Pre ) \ ...
如上通过宏定义了一些类及类的一些相关属性，这个宏在枚举类WKID中使用，如下：
enum WKID {
NO_WKID = 0,
#define WK_KLASS_ENUM(name, symbol, ignore_o) WK_KLASS_ENUM_NAME(name), WK_KLASS_ENUM_NAME(symbol) = WK_KLASS_ENUM_NAME(name),
WK_KLASSES_DO(WK_KLASS_ENUM)
#undef WK_KLASS_ENUM
WKID_LIMIT,
FIRST_WKID = NO_WKID + 1
};
经过宏扩展后，变为了如下的形式：
enum WKID {
NO_WKID = 0,
Object_klass_knum, java_lang_Object_knum = Object_klass_knum, \
String_klass_knum, java_lang_String_knum = String_klass_knum, \
Class_klass_knum, java_lang_Class_knum = Class_klass_knum, \
Cloneable_klass_knum, java_lang_Cloneable_knum = Cloneable_klass_knum, \
ClassLoader_klass_knum, java_lang_ClassLoader_knum = ClassLoader_klass_knum, \
Serializable_klass_knum, java_io_Serializable_knum = Serializable_klass_knum, \
System_klass_knum, java_lang_System_knum = System_klass_knum, \
...
WKID_LIMIT, // 70
FIRST_WKID = NO_WKID + 1 // 1
};
这些类在HotSpot启动时就会进行预加载，调用链路如下：
Universe::genesis() universe.cpp SystemDictionary::initialize() systemDictionary.cpp SystemDictionary::initialize_preloaded_classes() systemDictionary.cpp SystemDictionary::initialize_wk_klasses_through() systemDictionary.hpp SystemDictionary::initialize_wk_klasses_until() systemDictionary.cpp
SystemDictionary::initialize_preloaded_classes 是分批次预加载类的。首先会调用SystemDictionary::initialize_wk_klasses_until()函数，在这个函数中遍历WK_KLASSES_DO宏中定义的所有需要预加载的类，函数的具体实现如下：
void SystemDictionary::initialize_wk_klasses_until(WKID limit_id, WKID &start_id, TRAPS) {
assert((int)start_id <= (int)limit_id, "IDs are out of order!");
for (int id = (int)start_id; id < (int)limit_id; id++) {
assert(id >= (int)FIRST_WKID && id < (int)WKID_LIMIT, "oob");
int info = wk_init_info[id - FIRST_WKID];
int sid = (info >> CEIL_LG_OPTION_LIMIT);
// right_n_bits的宏扩展为((CEIL_LG_OPTION_LIMIT >= BitsPerWord ? 0 : OneBit << (CEIL_LG_OPTION_LIMIT)) - 1)
int opt = (info & right_n_bits(CEIL_LG_OPTION_LIMIT));
initialize_wk_klass((WKID)id, opt, CHECK);
}
// move the starting value forward to the limit:
start_id = limit_id;
}
其中wk_init_info数组的定义如下：
static const short wk_init_info[] = {
#define WK_KLASS_INIT_INFO(name, symbol, option) \
( ((int)vmSymbols::VM_SYMBOL_ENUM_NAME(symbol) << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::option ),
WK_KLASSES_DO(WK_KLASS_INIT_INFO)
#undef WK_KLASS_INIT_INFO
0
};
最终的wk_init_info数组经过宏扩展后如下：
static const short wk_init_info[] = {
( ((int)vmSymbols::java_lang_Object_enum << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::Pre ), \
( ((int)vmSymbols::java_lang_String_enum << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::Pre ), \
( ((int)vmSymbols::java_lang_Class_enum << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::Pre ), \
( ((int)vmSymbols::java_lang_Cloneable_enum << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::Pre ), \
( ((int)vmSymbols::java_lang_ClassLoader_enum << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::Pre ), \
( ((int)vmSymbols::java_io_Serializable_enum << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::Pre ), \
( ((int)vmSymbols::java_lang_System_enum << SystemDictionary::CEIL_LG_OPTION_LIMIT) | (int)SystemDictionary::Pre ), \
...
0
};
在SystemDictionary::initialize_wk_klasses_until()函数或如上wk_init_info数组中用到的CEIL_LG_OPTION_LIMIT是枚举变量，定义在InitOption枚举类中，如下：
enum InitOption {
Pre, // preloaded; error if not present
Pre_JSR292, // preloaded if EnableInvokeDynamic
// Order is significant. Options before this point require resolve_or_fail.
// Options after this point will use resolve_or_null instead.
Opt, // preload tried; NULL if not present
Opt_Only_JDK14NewRef, // preload tried; use only with NewReflection
Opt_Only_JDK15, // preload tried; use only with JDK1.5+
OPTION_LIMIT,
CEIL_LG_OPTION_LIMIT = 4 // OPTION_LIMIT <= (1<
在initialize_wk_klasses_until()函数中调用的initialize_wk_klasses()函数的实现如下：
bool SystemDictionary::initialize_wk_klass(WKID id, int init_opt, TRAPS) {
assert(id >= (int)FIRST_WKID && id < (int)WKID_LIMIT, "oob");
int info = wk_init_info[id - FIRST_WKID];
int sid = (info >> CEIL_LG_OPTION_LIMIT);
Symbol* symbol = vmSymbols::symbol_at((vmSymbols::SID)sid);
Klass** klassp = &_well_known_klasses[id];
bool must_load = (init_opt < SystemDictionary::Opt);
if ((*klassp) == NULL) {
if (must_load) {
(*klassp) = resolve_or_fail(symbol, true, CHECK_0); // load required class
} else {
(*klassp) = resolve_or_null(symbol, CHECK_0); // load optional klass
}
}
return ((*klassp) != NULL);
}
调用resolve_or_fail()或resolve_or_null()函数进行类的加载，由名称也可以区分出这两个方法的区别。resolve_or_fail()方法表示加载的类一定要成功加载，也就是在wk_init_info数组中标注有Pre与Pre_JSR292的类；resolve_or_null()方法表示加载的类如果有就加载，没有返回NULL即可。
最终会调用到SystemDictionary::load_instance_class()函数，如在加载核心类时，调用链路如下：
SystemDictionary::resolve_or_fail() systemDictionary.cpp SystemDictionary::resolve_or_fail() systemDictionary.cpp SystemDictionary::resolve_or_null() systemDictionary.cpp SystemDictionary::resolve_instance_class_or_null() systemDictionary.cpp SystemDictionary::load_instance_class() systemDictionary.cpp
resolve_or_fail()方法的实现如下：
Klass* SystemDictionary::resolve_or_fail(Symbol* class_name, Handle class_loader, Handle protection_domain, bool throw_error, TRAPS) {
Klass* klass = resolve_or_null(class_name, class_loader, protection_domain, THREAD);
if (HAS_PENDING_EXCEPTION || klass == NULL) {
KlassHandle k_h(THREAD, klass);
// can return a null klass
klass = handle_resolution_exception(class_name, class_loader, protection_domain, throw_error, k_h, THREAD);
}
return klass;
}
调用resolve_or_null()方法来加载类，不过klass一定不能为空，如果为空则出异常。调用的resolve_or_null()方法的实现如下：
Klass* SystemDictionary::resolve_or_null(Symbol* class_name, Handle class_loader, Handle protection_domain, TRAPS) {
// 数组，通过签名的格式来判断
if (FieldType::is_array(class_name)) {
return resolve_array_class_or_null(class_name, class_loader, protection_domain, CHECK_NULL);
}
// 普通类，通过签名的格式来判断
else if (FieldType::is_obj(class_name)) {
ResourceMark rm(THREAD);
// Ignore wrapping L and ;.
TempNewSymbol name = SymbolTable::new_symbol(class_name->as_C_string() + 1,
class_name->utf8_length() - 2,
CHECK_NULL);
return resolve_instance_class_or_null(name, class_loader, protection_domain, CHECK_NULL);
}
else {
return resolve_instance_class_or_null(class_name, class_loader, protection_domain, CHECK_NULL);
}
}
调用resolve_array_class_or_null()方法来加载数组，调用resolve_instance_class_or_null()方法加载类。数组类没有对应的Class文件，所以在类装载阶段，数组类会被虚拟机直接创建，并且数组类在装载完成后的状态为generated，即不需要进行验证、准备和初始化等操作。方法已经在前一篇介绍过，这里不再介绍。
相关文章的链接如下：
1、在Ubuntu 16.04上编译OpenJDK8的源代码
2、调试HotSpot源代码
3、HotSpot项目结构
4、HotSpot的启动过程
5、HotSpot二分模型（1）
6、HotSpot的类模型（2）
7、HotSpot的类模型（3）
8、HotSpot的类模型（4）
9、HotSpot的对象模型（5）
10、HotSpot的对象模型（6）
11、操作句柄Handle（7）
12、句柄Handle的释放（8）
13、类加载器
14、类的双亲委派机制
作者持续维护的个人博客classloading.com。
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