java - Java 中的 "internal address"是什么？

Question

在 Object.hashCode() 的 Javadoc 中指出

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java™ programming language.)

这是一个常见的误解，这与内存地址有关，但它不会在没有通知的情况下更改，并且 hashCode() 不会也不能更改对象。

@Neet 提供了一个很好的答案 https://stackoverflow.com/a/565416/57695 的链接，但我正在寻找更多详细信息。

---

这里有一个例子来说明我的担心

Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
Unsafe unsafe = (Unsafe) theUnsafe.get(null);

for (int t = 0; t < 10; t++) {
    System.gc();
    Object[] objects = new Object[10];
    for (int i = 0; i < objects.length; i++)
        objects[i] = new Object();

    for (int i = 0; i < objects.length; i++) {
        if (i > 0) System.out.print(", ");
        int location = unsafe.getInt(objects, Unsafe.ARRAY_OBJECT_BASE_OFFSET + Unsafe.ARRAY_OBJECT_INDEX_SCALE * i);
        System.out.printf("%08x: hc= %08x", location, objects[i].hashCode());
    }
    System.out.println();
}

打印

eac00038: hc= 4f47e0ba, eac00048: hc= 2342d884, eac00058: hc= 7994d431, eac00068: hc= 19f71b53, eac00078: hc= 2e22f376, eac00088: hc= 789ddfa3, eac00098: hc= 44c58432, eac000a8: hc= 036a11e4, eac000b8: hc= 28bc917c, eac000c8: hc= 73f378c8
eac00038: hc= 30813486, eac00048: hc= 729f624a, eac00058: hc= 3dee2310, eac00068: hc= 5d400f33, eac00078: hc= 18a60d19, eac00088: hc= 3da5f0f3, eac00098: hc= 596e0123, eac000a8: hc= 450cceb3, eac000b8: hc= 4bd66d2f, eac000c8: hc= 6a9a4f8e
eac00038: hc= 711dc088, eac00048: hc= 584b5abc, eac00058: hc= 3b3219ed, eac00068: hc= 564434f7, eac00078: hc= 17f17060, eac00088: hc= 6c08bae7, eac00098: hc= 3126cb1a, eac000a8: hc= 69e0312b, eac000b8: hc= 7dbc345a, eac000c8: hc= 4f114133
eac00038: hc= 50c8c3b8, eac00048: hc= 2ca98e77, eac00058: hc= 2fc83d89, eac00068: hc= 034005e1, eac00078: hc= 6041f871, eac00088: hc= 0b1df416, eac00098: hc= 5b83d60d, eac000a8: hc= 2c5a1e6b, eac000b8: hc= 5083198c, eac000c8: hc= 4f025f9f
eac00038: hc= 00c5eb8a, eac00048: hc= 41eab16b, eac00058: hc= 1726099c, eac00068: hc= 4240eca3, eac00078: hc= 346fe350, eac00088: hc= 1db4b415, eac00098: hc= 429addef, eac000a8: hc= 45609812, eac000b8: hc= 489fe953, eac000c8: hc= 7a8f6d64
eac00038: hc= 7e628e42, eac00048: hc= 7869cfe0, eac00058: hc= 6aceb8e2, eac00068: hc= 29cc3436, eac00078: hc= 1d77daaa, eac00088: hc= 27b4de03, eac00098: hc= 535bab52, eac000a8: hc= 274cbf3f, eac000b8: hc= 1f9fd541, eac000c8: hc= 3669ae9f
eac00038: hc= 772a3766, eac00048: hc= 749b46a8, eac00058: hc= 7e3bfb66, eac00068: hc= 13f62649, eac00078: hc= 054b8cdc, eac00088: hc= 230cc23b, eac00098: hc= 1aa3c177, eac000a8: hc= 74f2794a, eac000b8: hc= 5af92541, eac000c8: hc= 1afcfd10
eac00038: hc= 396e1dd8, eac00048: hc= 6c696d5c, eac00058: hc= 7d8aea9e, eac00068: hc= 2b316b76, eac00078: hc= 39862621, eac00088: hc= 16315e08, eac00098: hc= 03146a9a, eac000a8: hc= 3162a60a, eac000b8: hc= 4382f3da, eac000c8: hc= 4a578fd6
eac00038: hc= 225765b0, eac00048: hc= 17d5176d, eac00058: hc= 26f50154, eac00068: hc= 1f2a45c7, eac00078: hc= 104b1bcd, eac00088: hc= 330e3816, eac00098: hc= 6a844689, eac000a8: hc= 12330301, eac000b8: hc= 530a3ffc, eac000c8: hc= 45eee3fb
eac00038: hc= 3f9432e0, eac00048: hc= 1a9830bc, eac00058: hc= 7da79447, eac00068: hc= 04f801c4, eac00078: hc= 363bed68, eac00088: hc= 185f62a9, eac00098: hc= 1e4651bf, eac000a8: hc= 1aa0e220, eac000b8: hc= 385db088, eac000c8: hc= 0ef0cda1

---

作为旁注；如果你看这段代码

if (value == 0) value = 0xBAD ;

0xBAD 似乎是任何 hashCode 的两倍，因为 0 映射到该值。如果你运行足够长的时间，你会看到

long count = 0, countBAD = 0;
while (true) {
    for (int i = 0; i < 200000000; i++) {
        int hc = new Object().hashCode();
        if (hc == 0xBAD)
            countBAD++;
        count++;
    }
    System.out.println("0xBAD ratio is " + (double) (countBAD << 32) / count + " times expected.");
}

打印

0xBAD ratio is 2.0183116992481205 times expected.

Answer 1

这显然是特定于实现的。

下面我包括 OpenJDK 7 中使用的 Object.hashCode() 实现。

该函数支持六种不同的计算方法，其中只有两种注意对象的地址(“地址”是 C++ oop 转换为 intptr_t)。这两种方法中的一种按原样使用地址，而另一种则进行一些操作，然后将结果与不经常更新的随机数混合。

其余方法中，一种返回常量(可能用于测试)，一种返回序列号，其余的基于伪随机序列。

貌似可以在运行时选择方法，默认好像是方法0，也就是os::random()。后者是 linear congruential generator , 带有所谓的竞争条件。:-) 竞争条件是可以接受的，因为在最坏的情况下它会导致两个对象共享相同的哈希码；这不会破坏任何不变量。

第一次需要哈希码时执行计算。为了保持一致性，结果将存储在对象的 header 中，并在后续调用 hashCode() 时返回。缓存是在这个函数之外完成的。

总而言之，Object.hashCode() 基于对象地址的概念在很大程度上是一个历史产物，已被现代垃圾收集器的属性所淘汰。

// hotspot/src/share/vm/runtime/synchronizer.hpp

// hashCode() generation :
//
// Possibilities:
// * MD5Digest of {obj,stwRandom}
// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
// * A DES- or AES-style SBox[] mechanism
// * One of the Phi-based schemes, such as:
//   2654435761 = 2^32 * Phi (golden ratio)
//   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
// * A variation of Marsaglia's shift-xor RNG scheme.
// * (obj ^ stwRandom) is appealing, but can result
//   in undesirable regularity in the hashCode values of adjacent objects
//   (objects allocated back-to-back, in particular).  This could potentially
//   result in hashtable collisions and reduced hashtable efficiency.
//   There are simple ways to "diffuse" the middle address bits over the
//   generated hashCode values:
//

static inline intptr_t get_next_hash(Thread * Self, oop obj) {
  intptr_t value = 0 ;
  if (hashCode == 0) {
     // This form uses an unguarded global Park-Miller RNG,
     // so it's possible for two threads to race and generate the same RNG.
     // On MP system we'll have lots of RW access to a global, so the
     // mechanism induces lots of coherency traffic.
     value = os::random() ;
  } else
  if (hashCode == 1) {
     // This variation has the property of being stable (idempotent)
     // between STW operations.  This can be useful in some of the 1-0
     // synchronization schemes.
     intptr_t addrBits = intptr_t(obj) >> 3 ;
     value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
  } else
  if (hashCode == 2) {
     value = 1 ;            // for sensitivity testing
  } else
  if (hashCode == 3) {
     value = ++GVars.hcSequence ;
  } else
  if (hashCode == 4) {
     value = intptr_t(obj) ;
  } else {
     // Marsaglia's xor-shift scheme with thread-specific state
     // This is probably the best overall implementation -- we'll
     // likely make this the default in future releases.
     unsigned t = Self->_hashStateX ;
     t ^= (t << 11) ;
     Self->_hashStateX = Self->_hashStateY ;
     Self->_hashStateY = Self->_hashStateZ ;
     Self->_hashStateZ = Self->_hashStateW ;
     unsigned v = Self->_hashStateW ;
     v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
     Self->_hashStateW = v ;
     value = v ;
  }

  value &= markOopDesc::hash_mask;
  if (value == 0) value = 0xBAD ;
  assert (value != markOopDesc::no_hash, "invariant") ;
  TEVENT (hashCode: GENERATE) ;
  return value;
}