Java中的hashCode 真的是地址吗?
在知乎上写的,直接搬过来。
java中的hashCode是怎么来的?
问题
1.在java中hashCode获取是如何实现的?
2.hashCode的值是否是可预测的?
(注:hashCode(散列值)——将对象映射为一个整型值,不同的对象返回不同的数值)
正文
在Object.java#hashCode 的注解中找到怎么一句话:
(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.)
意思是:hash值来源于这个对象的内部地址转换成的整型值。
我就很好奇了,这里的内部地址到底指的是什么地址?莫非类似下面这样
int main()
{
char var = 1;
printf("%p\n", &var);
}
console:
0028FF3F
在C当中上述代码输出的是var变量的内存地址。
为了解决这个谜团,还是得看看#Object.java#hashCode的具体实现方法了。native方法本身非java实现,如果想要看源码,只有下载完整的jdk呗(openJdk1.8)。找到Object.c文件,查看上面的方法映射表发现,hashCode被映射到了一个叫JVM_IHashCode上去了。
static JNINativeMethod methods[] = {
{"hashCode", "()I", (void *)&JVM_IHashCode},
{"wait", "(J)V", (void *)&JVM_MonitorWait},
{"notify", "()V", (void *)&JVM_MonitorNotify},
{"notifyAll", "()V", (void *)&JVM_MonitorNotifyAll},
{"clone", "()Ljava/lang/Object;", (void *)&JVM_Clone},
};
顺藤摸瓜去看看JVM_IHashCode到底干了什么?熟悉的味道,我猜在jvm.h里面有方法声明,那实现一定在jvm.cpp里面。
果然处处有惊喜,和猜想的没错,不过jvm.cpp对于JVM_IHashCode的实现调用的是ObjectSynchronizer::FastHashCode的方法。看来革命尚未成功啊!
JVM_ENTRY(jint, JVM_IHashCode(JNIEnv* env, jobject handle))
JVMWrapper("JVM_IHashCode");
// as implemented in the classic virtual machine; return 0 if object is NULL
return handle == NULL ? 0 : ObjectSynchronizer::FastHashCode (THREAD, JNIHandles::resolve_non_null(handle)) ;
JVM_END
找了一会儿,没找到,这就尴尬了。后面百度了一下,发现声明在synchronizer.hpp 实现在这里synchronizer.cpp。感谢前辈们走出的路啊!
// 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 global Park-Miller 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 = cast_from_oop<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 = cast_from_oop<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;
}
intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
if (UseBiasedLocking) {
// NOTE: many places throughout the JVM do not expect a safepoint
// to be taken here, in particular most operations on perm gen
// objects. However, we only ever bias Java instances and all of
// the call sites of identity_hash that might revoke biases have
// been checked to make sure they can handle a safepoint. The
// added check of the bias pattern is to avoid useless calls to
// thread-local storage.
if (obj->mark()->has_bias_pattern()) {
// Handle for oop obj in case of STW safepoint
Handle hobj(Self, obj);
// Relaxing assertion for bug 6320749.
assert(Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(),
"biases should not be seen by VM thread here");
BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
obj = hobj();
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
}
// hashCode() is a heap mutator ...
// Relaxing assertion for bug 6320749.
assert(Universe::verify_in_progress() || DumpSharedSpaces ||
!SafepointSynchronize::is_at_safepoint(), "invariant");
assert(Universe::verify_in_progress() || DumpSharedSpaces ||
Self->is_Java_thread() , "invariant");
assert(Universe::verify_in_progress() || DumpSharedSpaces ||
((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
ObjectMonitor* monitor = NULL;
markOop temp, test;
intptr_t hash;
markOop mark = ReadStableMark(obj);
// object should remain ineligible for biased locking
assert(!mark->has_bias_pattern(), "invariant");
if (mark->is_neutral()) {
hash = mark->hash(); // this is a normal header
if (hash) { // if it has hash, just return it
return hash;
}
hash = get_next_hash(Self, obj); // allocate a new hash code
temp = mark->copy_set_hash(hash); // merge the hash code into header
// use (machine word version) atomic operation to install the hash
test = obj->cas_set_mark(temp, mark);
if (test == mark) {
return hash;
}
// If atomic operation failed, we must inflate the header
// into heavy weight monitor. We could add more code here
// for fast path, but it does not worth the complexity.
} else if (mark->has_monitor()) {
monitor = mark->monitor();
temp = monitor->header();
assert(temp->is_neutral(), "invariant");
hash = temp->hash();
if (hash) {
return hash;
}
// Skip to the following code to reduce code size
} else if (Self->is_lock_owned((address)mark->locker())) {
temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
assert(temp->is_neutral(), "invariant");
hash = temp->hash(); // by current thread, check if the displaced
if (hash) { // header contains hash code
return hash;
}
// WARNING:
// The displaced header is strictly immutable.
// It can NOT be changed in ANY cases. So we have
// to inflate the header into heavyweight monitor
// even the current thread owns the lock. The reason
// is the BasicLock (stack slot) will be asynchronously
// read by other threads during the inflate() function.
// Any change to stack may not propagate to other threads
// correctly.
}
// Inflate the monitor to set hash code
monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code);
// Load displaced header and check it has hash code
mark = monitor->header();
assert(mark->is_neutral(), "invariant");
hash = mark->hash();
if (hash == 0) {
hash = get_next_hash(Self, obj);
temp = mark->copy_set_hash(hash); // merge hash code into header
assert(temp->is_neutral(), "invariant");
test = Atomic::cmpxchg(temp, monitor->header_addr(), mark);
if (test != mark) {
// The only update to the header in the monitor (outside GC)
// is install the hash code. If someone add new usage of
// displaced header, please update this code
hash = test->hash();
assert(test->is_neutral(), "invariant");
assert(hash != 0, "Trivial unexpected object/monitor header usage.");
}
}
// We finally get the hash
return hash;
}
没想到代码这么长,确实比
int var;
return &var;
长太多了。接下来看看这段代码到底干了些什么?
可以看到在get_next_hash函数中,有五种不同的hashCode生成策略。
第一种:是使用全局的os::random()随机数生成策略。os::random()的实现方式在os.cpp中,代码如下
void os::init_random(unsigned int initval) {
_rand_seed = initval;
}
static int random_helper(unsigned int rand_seed) {
/* standard, well-known linear congruential random generator with
* next_rand = (16807*seed) mod (2**31-1)
* see
* (1) "Random Number Generators: Good Ones Are Hard to Find",
* S.K. Park and K.W. Miller, Communications of the ACM 31:10 (Oct 1988),
* (2) "Two Fast Implementations of the 'Minimal Standard' Random
* Number Generator", David G. Carta, Comm. ACM 33, 1 (Jan 1990), pp. 87-88.
*/
const unsigned int a = 16807;
const unsigned int m = 2147483647;
const int q = m / a; assert(q == 127773, "weird math");
const int r = m % a; assert(r == 2836, "weird math");
// compute az=2^31p+q
unsigned int lo = a * (rand_seed & 0xFFFF);
unsigned int hi = a * (rand_seed >> 16);
lo += (hi & 0x7FFF) << 16;
// if q overflowed, ignore the overflow and increment q
if (lo > m) {
lo &= m;
++lo;
}
lo += hi >> 15;
// if (p+q) overflowed, ignore the overflow and increment (p+q)
if (lo > m) {
lo &= m;
++lo;
}
return lo;
}
int os::random() {
// Make updating the random seed thread safe.
while (true) {
unsigned int seed = _rand_seed;
unsigned int rand = random_helper(seed);
if (Atomic::cmpxchg(rand, &_rand_seed, seed) == seed) {
return static_cast<int>(rand);
}
}
}
根据代码注解的提示,随机数的生成策略是一种线性取余方式生成的。具体原理,看wiki吧(以后更新,或者大佬们不嫌弃分享一下呗)。
第二种:addrBits ^ (addrBits >> 5) ^ GVars.stwRandom。这里是第一次 看到和地址相关的变量,addrBits通过调用cast_from_oop方法得到。cast_from_oop实现在oopsHierarchy.cpp。具体代码如下
template <class T> inline oop cast_to_oop(T value) {
return (oop)(CHECK_UNHANDLED_OOPS_ONLY((void *))(value));
}
//以下部分内容来源于 oopsHierachy.hpp
template <class T> inline T cast_from_oop(oop o) {
return (T)(CHECK_UNHANDLED_OOPS_ONLY((void*))o);
}
很遗憾的是我还是没有看到 cast_to_oop具体是怎么实现的,后面会更新的
第三种:敏感测试
value = 1;
第四种:自增序列
value = ++GVars.hcSequence;
第五种:官方将会默认。利用位移生成随机数
// 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;
最后来回答 一开始的问题。
1.hashCode 是怎么来的?——原来有很多,自增序列,随机数,内存地址。这里又有个新问题产生了,为什么不用时间戳了?
2.可以预测值?——这很难说啊!