Java中Object类hashCode的底层实现
openjdkjdksrcshare
ativejavalangObject.c
42
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},
};
本地方法名称对应 JVM_IHashCodejvm.h 96 /************************************************************************* PART 1: Functions for Native Libraries ************************************************************************/ /* * java.lang.Object */ JNIEXPORT jint JNICALL JVM_IHashCode(JNIEnv *env, jobject obj);
JVM_IHashCode实现：
jvm.cpp
533
// java.lang.Object ///////////////////////////////////////////////
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
FastHashCode声明
synchronizer.hpp
37
class ObjectSynchronizer : AllStatic {
100
static intptr_t FastHashCode (Thread * Self, oop obj) ;
synchronizer.cpp
471
static markOop ReadStableMark (oop obj) {
markOop mark = obj->mark() ;
if (!mark->is_being_inflated()) {
return mark ; // normal fast-path return
}
557
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;
}
603
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()) {
// Box and unbox the raw reference just in case we cause a 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() ||
!SafepointSynchronize::is_at_safepoint(), "invariant") ;
assert (Universe::verify_in_progress() ||
Self->is_Java_thread() , "invariant") ;
assert (Universe::verify_in_progress() ||
((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 = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), 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);
// 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 = (markOop) Atomic::cmpxchg_ptr(temp, monitor, 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;
}
我们看到最终还是 尝试用 mark->hash()获取hash的
必要信息：
globalDefinitions.hpp
58
#ifdef _LP64
const int LogBytesPerWord = 3;
#else
const int LogBytesPerWord = 2;
#endif
70
const int LogBitsPerByte = 3;
73
const int LogBitsPerWord = LogBitsPerByte + LogBytesPerWord;
79
const int BitsPerWord = 1 << LogBitsPerWord;
243
typedef uintptr_t address_word; // unsigned integer which will hold a pointer
// except for some implementations of a C++
// linkage pointer to function. Should never
// need one of those to be placed in this
// type anyway.
955
const intptr_t OneBit = 1; // only right_most bit set in a word
// get a word with the n.th or the right-most or left-most n bits set
// (note: #define used only so that they can be used in enum constant definitions)
#define nth_bit(n) (n >= BitsPerWord ? 0 : OneBit << (n))
#define right_n_bits(n) (nth_bit(n) - 1)
#define left_n_bits(n) (right_n_bits(n) << (n >= BitsPerWord ? 0 : (BitsPerWord - n)))
960
#define right_n_bits(n) (nth_bit(n) - 1)
966
inline intptr_t mask_bits (intptr_t x, intptr_t m) { return x & m; }
globalDefinitions_gcc.hpp 155 typedef int intptr_t; typedef unsigned int uintptr_t;
oop.hpp
61
class oopDesc.....整个都是
79
markOop mark() const { return _mark; }
markOop.hpp markOopDesc继承自oopDesc
28
#include "oops/oop.hpp"
// The markOop describes the header of an object.
//
// Note that the mark is not a real oop but just a word.
// It is placed in the oop hierarchy for historical reasons.
//
// Bit-format of an object header (most significant first, big endian layout below):
//
// 32 bits:
// --------
// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
// size:32 ------------------------------------------>| (CMS free block)
// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
// 64 bits:
// --------
// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
// size:64 ----------------------------------------------------->| (CMS free block)
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
//
// - hash contains the identity hash value: largest value is
// 31 bits, see os::random(). Also, 64-bit vm's require
// a hash value no bigger than 32 bits because they will not
// properly generate a mask larger than that: see library_call.cpp
// and c1_CodePatterns_sparc.cpp.
//
// - the biased lock pattern is used to bias a lock toward a given
// thread. When this pattern is set in the low three bits, the lock
// is either biased toward a given thread or "anonymously" biased,
// indicating that it is possible for it to be biased. When the
// lock is biased toward a given thread, locking and unlocking can
// be performed by that thread without using atomic operations.
// When a lock's bias is revoked, it reverts back to the normal
// locking scheme described below.
//
// Note that we are overloading the meaning of the "unlocked" state
// of the header. Because we steal a bit from the age we can
// guarantee that the bias pattern will never be seen for a truly
// unlocked object.
//
// Note also that the biased state contains the age bits normally
// contained in the object header. Large increases in scavenge
// times were seen when these bits were absent and an arbitrary age
// assigned to all biased objects, because they tended to consume a
// significant fraction of the eden semispaces and were not
// promoted promptly, causing an increase in the amount of copying
// performed. The runtime system aligns all JavaThread* pointers to
// a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))
// to make room for the age bits & the epoch bits (used in support of
// biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs).
//
// [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread
// [0 | epoch | age | 1 | 01] lock is anonymously biased
//
// - the two lock bits are used to describe three states: locked/unlocked and monitor.
//
// [ptr | 00] locked ptr points to real header on stack
// [header | 0 | 01] unlocked regular object header
// [ptr | 10] monitor inflated lock (header is wapped out)
// [ptr | 11] marked used by markSweep to mark an object
// not valid at any other time
//
// We assume that stack/thread pointers have the lowest two bits cleared.
104
class markOopDesc: public oopDesc {
private:
// Conversion
uintptr_t value() const { return (uintptr_t) this; }
....................
111
enum { age_bits = 4,
lock_bits = 2,
biased_lock_bits = 1,
max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits,
hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits,
cms_bits = LP64_ONLY(1) NOT_LP64(0),
epoch_bits = 2
};
120
// The biased locking code currently requires that the age bits be
// contiguous to the lock bits.
enum { lock_shift = 0,
biased_lock_shift = lock_bits,
age_shift = lock_bits + biased_lock_bits,
cms_shift = age_shift + age_bits,
hash_shift = cms_shift + cms_bits,
epoch_shift = hash_shift
};
141
#ifndef _WIN64
,hash_mask = right_n_bits(hash_bits),
hash_mask_in_place = (address_word)hash_mask << hash_shift
#endif
.................
227
bool is_being_inflated() const { return (value() == 0); }
302
markOop copy_set_hash(intptr_t hash) const {
intptr_t tmp = value() & (~hash_mask_in_place);
tmp |= ((hash & hash_mask) << hash_shift);
return (markOop)tmp;
}
342
// hash operations
intptr_t hash() const {
return mask_bits(value() >> hash_shift, hash_mask);//二者进行相与 value() >> hash_shift ，hash_mask
//注：为了方便看：
//hash_shift = cms_shift(其实=7) + cms_bits(1)
//cms_bits = LP64_ONLY(1) NOT_LP64(0),
//hash_mask = ( hash_bits >= BitsPerWord ? 0 : OneBit << (hash_bits) ) -1
//hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits
//max_hash_bits = BitsPerWord - 4 - 2 - 1
//BitsPerWord = 1 << LogBitsPerWord
//LogBitsPerWord = 3 + 3;
}
macros.hpp 112 #ifdef _LP64 #define LP64_ONLY(code) code #define NOT_LP64(code) #else // !_LP64 #define LP64_ONLY(code) #define NOT_LP64(code) code #endif // _LP64
oopsHierarchy.hpp
40
#ifndef CHECK_UNHANDLED_OOPS
....
typedef class oopDesc* oop;
74
typedef class markOopDesc* markOop;
....
#else
...
class oop {
oopDesc* _o;
.....
}