HotSpot 类加载

HotSpot启动的过程中，在init_globals()函数中universe2_init()函数中已经预加载了系统运行必须的Java类。所以我们先分析预加载的系统运行必须的类，然后再看JVM启动以后我们的Hello类是如何加载的。JVM的组成如下

1. 方法区：保存Java类元数据，线程共享的运行时内存区域
2. 堆：所有类实例和数组对象分配内存的区域
3. Java栈：每个线程私有，与传统c栈类似，存储局部变量，方法栈帧等
4. pc寄存器：每个线程私有，程序运行计数指针
5. 本地方法栈：传统c栈
6. 类加载器：加载Java字节码
7. 执行引擎：字节码解释器/模板解释器

虚拟机启动以后会加载指定的初始化主类，并执行主类包含的main方法，主类运行结束以后虚拟机将会退出，一般服务器tomcat会有一个主线循环让虚拟机不退出。Java程序在虚拟机运行的第一步就是类加载。

一. HotSpot内部预加载类

hotspot/src/share/vm/memory/universe.cpp

void universe2_init() {......Universe::genesis(CATCH);
}

hotspot/src/share/vm/memory/universe.cpp

void Universe::genesis(TRAPS) {//首先是基本型的初始化.....//初始化符号表vmSymbols::initialize(CHECK);//初始化系统字典SystemDictionary::initialize(CHECK);
}

hotspot/src/share/vm/classfile/vmSymbols.cpp


//注意这里的 VM_SYMBOLS_DO 符号
#define VM_SYMBOL_BODY(name, string) string "\0"
static const char* vm_symbol_bodies = VM_SYMBOLS_DO(VM_SYMBOL_BODY, VM_ALIAS_IGNORE);void vmSymbols::initialize(TRAPS) {if (!UseSharedSpaces) {//vm_symbol_bodies声明在上面const char* string = &vm_symbol_bodies[0]; for (int index = (int)FIRST_SID; index < (int)SID_LIMIT; index++) {//为Java类创建符号Symbol* sym = SymbolTable::new_permanent_symbol(string, CHECK);//存到符号数组中_symbols[index] = sym;string += strlen(string); // skip string body//下一个string += 1;              // skip trailing null}//Java基本类型_type_signatures[T_BYTE]    = byte_signature();.....// no single signatures for T_OBJECT or T_ARRAY}
}

hotspot/src/share/vm/classfile/vmSymbols.hpp
先来看看 VM_SYMBOLS_DO，它在头文件中是个宏定义

#define VM_SYMBOLS_DO(template, do_alias)                                                         \/* commonly used class, package, module names */                                                \template(java_base,                                 JAVA_BASE_NAME)                             \template(java_lang_System,                          "java/lang/System")                         \template(java_lang_Object,                          "java/lang/Object")                         \template(java_lang_Class,                           "java/lang/Class")                          \template(java_lang_Package,                         "java/lang/Package")                        \template(java_lang_Module,                          "java/lang/Module")                         \......

hotspot/src/share/vm/classfile/symbolTable.cpp
再来看看 new_permanent_symbol，符号表维护着Java类/字符等信息

Symbol* SymbolTable::new_permanent_symbol(const char* name, TRAPS) {unsigned int hash;//从符号表中查找符号应用，SymbolTable是HashTableSymbol* result = SymbolTable::lookup_only((char*)name, (int)strlen(name), hash);if (result != NULL) {return result;}//如果不存在则创建hash索引，并放到表中SymbolTable* table = the_table();int index = table->hash_to_index(hash);return table->basic_add(index, (u1*)name, (int)strlen(name), hash, false, THREAD);
}

hotspot/src/share/vm/classfile/systemDictionary.cpp
然后是系统字典初始化，系统字典维护者HotSpot对象模型的Klass，initialize_preloaded_classes执行预加载，主要是javabase模块下的类

void SystemDictionary::initialize(TRAPS) {// Allocate arrays_dictionary          = new Dictionary(calculate_systemdictionary_size(PredictedLoadedClassCount));_placeholders        = new PlaceholderTable(_nof_buckets);_number_of_modifications = 0;_loader_constraints  = new LoaderConstraintTable(_loader_constraint_size);_resolution_errors   = new ResolutionErrorTable(_resolution_error_size);_invoke_method_table = new SymbolPropertyTable(_invoke_method_size);// Allocate private object used as system class loader lock_system_loader_lock_obj = oopFactory::new_intArray(0, CHECK);// Initialize basic classesinitialize_preloaded_classes(CHECK);
}

hotspot/src/share/vm/classfile/systemDictionary.cpp
预加载类,包括符号表中定义的类，javabase模块下的类，java基本类型等

void SystemDictionary::initialize_preloaded_classes(TRAPS) {//为Javabase模块创建ModuleEntry ClassLoader::classLoader_init2(CHECK);// 预加载类WKID scan = FIRST_WKID;....//类加载// JSR 292 classesWKID jsr292_group_start = WK_KLASS_ENUM_NAME(MethodHandle_klass);WKID jsr292_group_end   = WK_KLASS_ENUM_NAME(VolatileCallSite_klass);initialize_wk_klasses_until(jsr292_group_start, scan, CHECK);initialize_wk_klasses_through(jsr292_group_end, scan, CHECK);//其他类加载//Java基本类型int等....}

hotspot/src/share/vm/classfile/systemDictionary.cpp
取initialize_wk_klasses_until加载路径来看

void SystemDictionary::initialize_wk_klasses_until(WKID limit_id, WKID &start_id, TRAPS) {for (int id = (int)start_id; id < (int)limit_id; id++) {//initialize_wk_klass((WKID)id, opt, CHECK);}
}