Java如何初始化String文字 [英] How does Java initialize String literal

问题描述

Javadoc表示:

Javadoc said that:

String类表示字符串. Java程序中的所有字符串文字(例如"abc")都实现为此类的实例.

The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.

我们知道String类具有两个属性，即:value[]和hash，并且String文字存储在String池中.

We know that String class has two properties namely: value[] and hash, and String literal are stored in a String pool.

但是在放入该池之前，我无法弄清如何初始化String文字.好像稍后再调试字符串文字一样，我可以看到value[]和hash已填充.

But I am not able to figure out how that String literal is initialized before putting to that pool. As if I debug the string literal later, I can see the value[] and hash is somehow populated.

JVM是否调用特殊指令?

Does JVM invoke a special instruction?

推荐答案

JVM在

JVM creates a new string literal object during constant pool resolution, if the same string has not been put to the string table before by String.intern call.

未指定JVM如何创建和初始化此类字符串，因此JVM可以执行任何所需的操作，只要结果对象是可以从应用程序代码访问的常规java.lang.String实例即可.

It is not specified how JVM creates and initializes such strings, so JVM may do whatever it wants as long as the result object is a regular java.lang.String instance that can be accessed from application code.

对于HotSpot JVM，它不调用任何String构造函数，它只是在Heap中分配一个新对象并填写C ++代码中的字段，请参见

As to HotSpot JVM, it does not call any of String constructors, it just allocates a new object in Heap and fills in the fields in C++ code, see java_lang_String::basic_create:

Handle java_lang_String::basic_create(int length, TRAPS) {
  assert(initialized, "Must be initialized");
  // Create the String object first, so there's a chance that the String
  // and the char array it points to end up in the same cache line.
  oop obj;
  obj = InstanceKlass::cast(SystemDictionary::String_klass())->allocate_instance(CHECK_NH);

  // Create the char array.  The String object must be handlized here
  // because GC can happen as a result of the allocation attempt.
  Handle h_obj(THREAD, obj);
  typeArrayOop buffer;
    buffer = oopFactory::new_charArray(length, CHECK_NH);

  // Point the String at the char array
  obj = h_obj();
  set_value(obj, buffer);
  // No need to zero the offset, allocation zero'ed the entire String object
  assert(offset(obj) == 0, "initial String offset should be zero");
//set_offset(obj, 0);
  set_count(obj, length);

  return h_obj;
}

这样的新对象的

hash字段被初始化为零.正确的哈希码将在第一次调用String.hashCode时计算出来.

hash field of such new object is initialized to zero. The right hash code will be calculated on the first call to String.hashCode.

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