the timing of String Literal loaded into StringTable in Java HotSpot vm

Question

The Question came out when i was learning java.lang.String Java API.

I found an article in Chinese. Java 中new String("字面量") 中 "字面量" 是何时进入字符串常量池的?

it said,CONSTANT_String is lazy resolve in HotSpot VM, so String Literal is loaded into StringTable util it is used.

And i found some relavant saying.

For example, a Java Virtual Machine implementation may choose to resolve each symbolic reference in a class or interface individually when it is used ("lazy" or "late" resolution), or to resolve them all at once when the class is being verified ("eager" or "static" resolution).

I found some openjdk code about ldc

IRT_ENTRY(void, InterpreterRuntime::ldc(JavaThread* thread, bool wide))  
  // access constant pool  
  constantPoolOop pool = method(thread)->constants();  
  int index = wide ? get_index_u2(thread, Bytecodes::_ldc_w) :get_index_u1(thread, Bytecodes::_ldc);  
  constantTag tag = pool->tag_at(index);  

  if (tag.is_unresolved_klass() || tag.is_klass()) {  
    klassOop klass = pool->klass_at(index, CHECK);  
    oop java_class = klass->java_mirror();  
    thread->set_vm_result(java_class);  
  } else {  
#ifdef ASSERT  
    // If we entered this runtime routine, we believed the tag contained  
    // an unresolved string, an unresolved class or a resolved class.  
    // However, another thread could have resolved the unresolved string  
    // or class by the time we go there.  
    assert(tag.is_unresolved_string()|| tag.is_string(), "expected string");  
#endif  
    oop s_oop = pool->string_at(index, CHECK);  
    thread->set_vm_result(s_oop);  
  }  
IRT_END

and code about pool->string_at(index, CHECK)

oop constantPoolOopDesc::string_at_impl(constantPoolHandle this_oop, int which, TRAPS) {  
  oop str = NULL;  
  CPSlot entry = this_oop->slot_at(which);  
  if (entry.is_metadata()) {  
    ObjectLocker ol(this_oop, THREAD);  
    if (this_oop->tag_at(which).is_unresolved_string()) {  
      // Intern string  
      Symbol* sym = this_oop->unresolved_string_at(which);  
      str = StringTable::intern(sym, CHECK_(constantPoolOop(NULL)));  
      this_oop->string_at_put(which, str);  
   } else {  
      // Another thread beat us and interned string, read string from constant pool  
     str = this_oop->resolved_string_at(which);  
    }  
  } else {  
    str = entry.get_oop();  
  }  
  assert(java_lang_String::is_instance(str), "must be string");  
  return str;  
}

But

those code only could prove String Literal maybe loaded into StringTable util ldc, but can not prove lazy resolve in HotSpot VM.

Could someone explicate it explicitly.

FYI, i know little c but not c++.

Thanks.!

the timing of String Literal loaded into StringTable in Java HotSpot vm

1 Answers