Hotspot源码解析-第23章--Java Class的初始化

这一章的内容主要是对初始后的 Java Class 的需要使用（主要体现在反射）的字段计算偏移（偏移是指相对于类本身的 存储位置的起始位置）

函数调用链：

init.cpp->init_globals

javaClasses.cpp->javaClasses_init()

void javaClasses_init() {
  // 计算偏移
  JavaClasses::compute_offsets();
  // 计算完后，要验证偏移
  JavaClasses::check_offsets();
  FilteredFieldsMap::initialize();  // must be done after computing offsets.
}

计算字段偏移

void JavaClasses::compute_offsets() {
  // 计算偏移每个操作都差不多，这里抽前面 java_lang_ClassLoader 和 java_lang_Thread 两个类来讲，其他照葫芦画瓢
  java_lang_ClassLoader::compute_offsets();
  java_lang_Thread::compute_offsets();
  java_lang_ThreadGroup::compute_offsets();
  if (EnableInvokeDynamic) {
    java_lang_invoke_MethodHandle::compute_offsets();
    java_lang_invoke_DirectMethodHandle::compute_offsets();
    java_lang_invoke_MemberName::compute_offsets();
    java_lang_invoke_LambdaForm::compute_offsets();
    java_lang_invoke_MethodType::compute_offsets();
    java_lang_invoke_CallSite::compute_offsets();
  }
  java_security_AccessControlContext::compute_offsets();
  // Initialize reflection classes. The layouts of these classes
  // changed with the new reflection implementation in JDK 1.4, and
  // since the Universe doesn't know what JDK version it is until this
  // point we defer computation of these offsets until now.
  java_lang_reflect_AccessibleObject::compute_offsets();
  java_lang_reflect_Method::compute_offsets();
  java_lang_reflect_Constructor::compute_offsets();
  java_lang_reflect_Field::compute_offsets();
  if (JDK_Version::is_gte_jdk14x_version()) {
    java_nio_Buffer::compute_offsets();
  }
  if (JDK_Version::is_gte_jdk15x_version()) {
    sun_reflect_ConstantPool::compute_offsets();
    sun_reflect_UnsafeStaticFieldAccessorImpl::compute_offsets();
  }
  if (JDK_Version::is_jdk18x_version())
    java_lang_reflect_Parameter::compute_offsets();

  java_lang_ref_ReferenceQueue::compute_offsets();

  // generated interpreter code wants to know about the offsets we just computed:
  AbstractAssembler::update_delayed_values();
}

java_lang_ClassLoader::compute_offsets()

void java_lang_ClassLoader::compute_offsets() {
  assert(!offsets_computed, "offsets should be initialized only once");
  offsets_computed = true;

  // 直接看这一行，计算 ClassLoader 类的 parallelCapable 字段的偏移
  Klass* k1 = SystemDictionary::ClassLoader_klass();
  compute_optional_offset(parallelCapable_offset,
    k1, vmSymbols::parallelCapable_name(), vmSymbols::concurrenthashmap_signature());

  CLASSLOADER_INJECTED_FIELDS(INJECTED_FIELD_COMPUTE_OFFSET);
}

java_lang_Thread::compute_offsets()

void java_lang_Thread::compute_offsets() {
  assert(_group_offset == 0, "offsets should be initialized only once");

  Klass* k = SystemDictionary::Thread_klass();
  // 计算 name/group/priority...等偏移
  compute_offset(_name_offset,      k, vmSymbols::name_name(),      vmSymbols::string_signature());
  compute_offset(_group_offset,     k, vmSymbols::group_name(),     vmSymbols::threadgroup_signature());
  compute_offset(_contextClassLoader_offset, k, vmSymbols::contextClassLoader_name(), vmSymbols::classloader_signature());
  compute_offset(_inheritedAccessControlContext_offset, k, vmSymbols::inheritedAccessControlContext_name(), vmSymbols::accesscontrolcontext_signature());
  compute_offset(_priority_offset,  k, vmSymbols::priority_name(),  vmSymbols::int_signature());
  compute_offset(_daemon_offset,    k, vmSymbols::daemon_name(),    vmSymbols::bool_signature());
  compute_offset(_eetop_offset,     k, vmSymbols::eetop_name(),     vmSymbols::long_signature());
  compute_offset(_stillborn_offset, k, vmSymbols::stillborn_name(), vmSymbols::bool_signature());
  // The stackSize field is only present starting in 1.4, so don't go fatal.
  compute_optional_offset(_stackSize_offset, k, vmSymbols::stackSize_name(), vmSymbols::long_signature());
  // The tid and thread_status fields are only present starting in 1.5, so don't go fatal.
  compute_optional_offset(_tid_offset, k, vmSymbols::thread_id_name(), vmSymbols::long_signature());
  compute_optional_offset(_thread_status_offset, k, vmSymbols::thread_status_name(), vmSymbols::int_signature());
  // The parkBlocker field is only present starting in 1.6, so don't go fatal.
  compute_optional_offset(_park_blocker_offset, k, vmSymbols::park_blocker_name(), vmSymbols::object_signature());
  compute_optional_offset(_park_event_offset, k, vmSymbols::park_event_name(),
 vmSymbols::long_signature());
}

校验偏移

void JavaClasses::check_offsets() {
  bool valid = true;
  HandleMark hm;

#define CHECK_OFFSET(klass_name, cpp_klass_name, field_name, field_sig) \
  valid &= check_offset(klass_name, cpp_klass_name :: field_name ## _offset, #field_name, field_sig)

#define CHECK_LONG_OFFSET(klass_name, cpp_klass_name, field_name, field_sig) \
  valid &= check_offset(klass_name, cpp_klass_name :: long_ ## field_name ## _offset, #field_name, field_sig)

#define CHECK_STATIC_OFFSET(klass_name, cpp_klass_name, field_name, field_sig) \
  valid &= check_static_offset(klass_name, cpp_klass_name :: static_ ## field_name ## _offset, #field_name, field_sig)

#define CHECK_CONSTANT(klass_name, cpp_klass_name, field_name, field_sig) \
  valid &= check_constant(klass_name, cpp_klass_name :: field_name, #field_name, field_sig)

  // java.lang.String

  CHECK_OFFSET("java/lang/String", java_lang_String, value, "[C");
  if (java_lang_String::has_offset_field()) {
    CHECK_OFFSET("java/lang/String", java_lang_String, offset, "I");
    CHECK_OFFSET("java/lang/String", java_lang_String, count, "I");
  }
  if (java_lang_String::has_hash_field()) {
    CHECK_OFFSET("java/lang/String", java_lang_String, hash, "I");
  }

  // java.lang.Class

  // Fake fields
  // CHECK_OFFSET("java/lang/Class", java_lang_Class, klass); // %%% this needs to be checked
  // CHECK_OFFSET("java/lang/Class", java_lang_Class, array_klass); // %%% this needs to be checked

  // java.lang.Throwable

  CHECK_OFFSET("java/lang/Throwable", java_lang_Throwable, backtrace, "Ljava/lang/Object;");
  CHECK_OFFSET("java/lang/Throwable", java_lang_Throwable, detailMessage, "Ljava/lang/String;");
  CHECK_OFFSET("java/lang/Throwable", java_lang_Throwable, cause, "Ljava/lang/Throwable;");
  CHECK_OFFSET("java/lang/Throwable", java_lang_Throwable, stackTrace, "[Ljava/lang/StackTraceElement;");

  // Boxed primitive objects (java_lang_boxing_object)

  CHECK_OFFSET("java/lang/Boolean",   java_lang_boxing_object, value, "Z");
  CHECK_OFFSET("java/lang/Character", java_lang_boxing_object, value, "C");
  CHECK_OFFSET("java/lang/Float",     java_lang_boxing_object, value, "F");
  CHECK_LONG_OFFSET("java/lang/Double", java_lang_boxing_object, value, "D");
  CHECK_OFFSET("java/lang/Byte",      java_lang_boxing_object, value, "B");
  CHECK_OFFSET("java/lang/Short",     java_lang_boxing_object, value, "S");
  CHECK_OFFSET("java/lang/Integer",   java_lang_boxing_object, value, "I");
  CHECK_LONG_OFFSET("java/lang/Long", java_lang_boxing_object, value, "J");

  // java.lang.ClassLoader

  CHECK_OFFSET("java/lang/ClassLoader", java_lang_ClassLoader, parent,      "Ljava/lang/ClassLoader;");

  // java.lang.System

  CHECK_STATIC_OFFSET("java/lang/System", java_lang_System,  in, "Ljava/io/InputStream;");
  CHECK_STATIC_OFFSET("java/lang/System", java_lang_System, out, "Ljava/io/PrintStream;");
  CHECK_STATIC_OFFSET("java/lang/System", java_lang_System, err, "Ljava/io/PrintStream;");
  CHECK_STATIC_OFFSET("java/lang/System", java_lang_System, security, "Ljava/lang/SecurityManager;");

  // java.lang.StackTraceElement

  CHECK_OFFSET("java/lang/StackTraceElement", java_lang_StackTraceElement, declaringClass, "Ljava/lang/String;");
  CHECK_OFFSET("java/lang/StackTraceElement", java_lang_StackTraceElement, methodName, "Ljava/lang/String;");
  CHECK_OFFSET("java/lang/StackTraceElement", java_lang_StackTraceElement,   fileName, "Ljava/lang/String;");
  CHECK_OFFSET("java/lang/StackTraceElement", java_lang_StackTraceElement, lineNumber, "I");

  // java.lang.ref.Reference

  CHECK_OFFSET("java/lang/ref/Reference", java_lang_ref_Reference, referent, "Ljava/lang/Object;");
  CHECK_OFFSET("java/lang/ref/Reference", java_lang_ref_Reference, queue, "Ljava/lang/ref/ReferenceQueue;");
  CHECK_OFFSET("java/lang/ref/Reference", java_lang_ref_Reference, next, "Ljava/lang/ref/Reference;");
  // Fake field
  //CHECK_OFFSET("java/lang/ref/Reference", java_lang_ref_Reference, discovered, "Ljava/lang/ref/Reference;");
  CHECK_STATIC_OFFSET("java/lang/ref/Reference", java_lang_ref_Reference, lock, "Ljava/lang/ref/Reference$Lock;");
  CHECK_STATIC_OFFSET("java/lang/ref/Reference", java_lang_ref_Reference, pending, "Ljava/lang/ref/Reference;");

  // java.lang.ref.SoftReference

  CHECK_OFFSET("java/lang/ref/SoftReference", java_lang_ref_SoftReference, timestamp, "J");
  CHECK_STATIC_OFFSET("java/lang/ref/SoftReference", java_lang_ref_SoftReference, clock, "J");

  // java.lang.AssertionStatusDirectives
  //
  // The CheckAssertionStatusDirectives boolean can be removed from here and
  // globals.hpp after the AssertionStatusDirectives class has been integrated
  // into merlin "for some time."  Without it, the vm will fail with early
  // merlin builds.

  if (CheckAssertionStatusDirectives && JDK_Version::is_gte_jdk14x_version()) {
    const char* nm = "java/lang/AssertionStatusDirectives";
    const char* sig = "[Ljava/lang/String;";
    CHECK_OFFSET(nm, java_lang_AssertionStatusDirectives, classes, sig);
    CHECK_OFFSET(nm, java_lang_AssertionStatusDirectives, classEnabled, "[Z");
    CHECK_OFFSET(nm, java_lang_AssertionStatusDirectives, packages, sig);
    CHECK_OFFSET(nm, java_lang_AssertionStatusDirectives, packageEnabled, "[Z");
    CHECK_OFFSET(nm, java_lang_AssertionStatusDirectives, deflt, "Z");
  }

  if (!valid) vm_exit_during_initialization("Hard-coded field offset verification failed");
}

check_offset 检验实体字段偏移

bool JavaClasses::check_offset(const char *klass_name, int hardcoded_offset, const char *field_name, const char* field_sig) {
  EXCEPTION_MARK;
  fieldDescriptor fd;
  // 先给 klass name 创建一个符合
  TempNewSymbol klass_sym = SymbolTable::new_symbol(klass_name, CATCH);
  // 通过 klass name 符合解析出真实的 klass 对象
  Klass* k = SystemDictionary::resolve_or_fail(klass_sym, true, CATCH);
  // 增加句柄
  instanceKlassHandle h_klass (THREAD, k);
  // 再分别给 字段名 和 字段签名 创建对应的符号
  TempNewSymbol f_name = SymbolTable::new_symbol(field_name, CATCH);
  TempNewSymbol f_sig  = SymbolTable::new_symbol(field_sig, CATCH);
  // 从 klass 对象查找该字段是否存在，不存在，直接返回 false 退出
  if (!h_klass->find_local_field(f_name, f_sig, &fd)) {
    tty->print_cr("Nonstatic field %s.%s not found", klass_name, field_name);
    return false;
  }
  // 找到了 字段，但是是静态的，也返回 false 退出
  if (fd.is_static()) {
    tty->print_cr("Nonstatic field %s.%s appears to be static", klass_name, field_name);
    return false;
  }
  // 字段偏移与上面计算的偏移一致，返回 true 成功退出
  if (fd.offset() == hardcoded_offset ) {
    return true;
  } else {
    tty->print_cr("Offset of nonstatic field %s.%s is hardcoded as %d but should really be %d.",
                  klass_name, field_name, hardcoded_offset, fd.offset());
    return false;
  }
}

check_static_offset 检验静态字段偏移

bool JavaClasses::check_static_offset(const char *klass_name, int hardcoded_offset, const char *field_name, const char* field_sig) {
  EXCEPTION_MARK;
  fieldDescriptor fd;
  TempNewSymbol klass_sym = SymbolTable::new_symbol(klass_name, CATCH);
  Klass* k = SystemDictionary::resolve_or_fail(klass_sym, true, CATCH);
  instanceKlassHandle h_klass (THREAD, k);
  TempNewSymbol f_name = SymbolTable::new_symbol(field_name, CATCH);
  TempNewSymbol f_sig  = SymbolTable::new_symbol(field_sig, CATCH);
  if (!h_klass->find_local_field(f_name, f_sig, &fd)) {
    tty->print_cr("Static field %s.%s not found", klass_name, field_name);
    return false;
  }
  // 上面的代码跟 check_offset 完全一样，主要看这一行，如果不是 static，就 false 退出了
  if (!fd.is_static()) {
    tty->print_cr("Static field %s.%s appears to be nonstatic", klass_name, field_name);
    return false;
  }
  if (fd.offset() == hardcoded_offset + InstanceMirrorKlass::offset_of_static_fields()) {
    return true;
  } else {
    tty->print_cr("Offset of static field %s.%s is hardcoded as %d but should really be %d.", klass_name, field_name, hardcoded_offset, fd.offset() - InstanceMirrorKlass::offset_of_static_fields());
    return false;
  }
}