眾所周知杨何，Java語言中占遥，Object對象有個特殊的方法：hashcode(), hashcode()表示的是JVM虛擬機為這個Object對象分配的一個int類型的數(shù)值堕伪，JVM會使用對象的hashcode值來提高對HashMap纺弊、Hashtable哈希表存取對象的使用效率阿逃。而在很多網(wǎng)上的教程中告訴我們的是hashcode返回的是對象的內(nèi)存地址。然而實際使用中會發(fā)現(xiàn)哈希值存在負(fù)數(shù)的情況时肿，而內(nèi)存地址肯定不會用負(fù)數(shù)表示庇茫，因此有了本文對其底層實現(xiàn)進一步進行探索。須知的是螃成，不同的JRE對這些方法的實現(xiàn)各有不同旦签，只是遵循著Java對這些方法所訂立的規(guī)范進行設(shè)計的，本文基于的是openJDK的源碼寸宏。
首先我們確認(rèn)一下hashcode()方法返回值到底是不是內(nèi)存地址:
import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class HashcodeTest {
private static Unsafe unsafe;
static {
try {
Field field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
unsafe = (Unsafe) field.get(null);
} catch (Exception e) {
e.printStackTrace();
}
}
public static long addressOf(Object o) throws Exception {
Object[] array = new Object[] { o };
long baseOffset = unsafe.arrayBaseOffset(Object[].class);
int addressSize = unsafe.addressSize();
long objectAddress;
switch (addressSize) {
case 4:
objectAddress = unsafe.getInt(array, baseOffset);
break;
case 8:
objectAddress = unsafe.getLong(array, baseOffset);
break;
default:
throw new Error("unsupported address size: " + addressSize);
}
return (objectAddress);
}
public static void main(String... args) throws Exception {
Object object = "object";
long address_1 = addressOf(object);
int address_2 = object.hashCode();
System.out.println("Addess: " + address_1+" "+address_2);
}
}
代碼中用到了大名鼎鼎的Unsafe類宁炫，它提供了繞開"安全"的JVM機制直接調(diào)用"native"的底層方法的功能，在java.concurrent包中大量使用了其提供的CAS功能氮凝。
Screen Shot 2019-02-17 at 10.14.18 PM.png
由結(jié)果中可以看出兩者是截然不同的兩個整型數(shù)羔巢。
那JDK到底是通過什么策略來生成哈希值的呢？
查看openJDK 關(guān)于 java.lang.Object類及其hashcode()方法的定義
public native int hashCode();
即該方法是一個本地方法罩阵，Java將調(diào)用本地方法庫對此方法的實現(xiàn)竿秆。由于Object類中有JNI方法調(diào)用，按照J(rèn)NI的規(guī)則稿壁，應(yīng)當(dāng)生成JNI 的頭文件袍辞，在此目錄下執(zhí)行javah -jni java.lang.Object 指令，將生成一個java_lang_Object.h頭文件常摧，該頭文件將在后面用到它:
/*
* Class: java_lang_Object
* Method: hashCode
* Signature: ()I
*/
JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode
(JNIEnv *, jobject);
Object對象的hashCode()方法在C語言文件Object.c中實現(xiàn)
#include <stdio.h>
#include <signal.h>
#include <limits.h>
#include "jni.h"
#include "jni_util.h"
#include "jvm.h"
#include "java_lang_Object.h"
static JNINativeMethod methods[] = {
{"hashCode", "()I", (void *)&JVM_IHashCode},//hashcode的方法指針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},
};
JNIEXPORT void JNICALL
Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls)
{
(*env)->RegisterNatives(env, cls,
methods, sizeof(methods)/sizeof(methods[0]));
}
JNIEXPORT jclass JNICALL
Java_java_lang_Object_getClass(JNIEnv *env, jobject this)
{
if (this == NULL) {
JNU_ThrowNullPointerException(env, NULL);
return 0;
} else {
return (*env)->GetObjectClass(env, this);
}
}
JVM_IHashCode方法指針在 openjdk\hotspot\src\share\vm\prims\jvm.cpp中定義
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
ObjectSynchronizer::fashHashCode方法的實現(xiàn)
// 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) ;// 直接用地址
} 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;
}
// ObjectSynchronizer::FastHashCode方法的實現(xiàn)，該方法最終會返回我們期望已久的hashcode
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;
}
從上面的追根溯源可以發(fā)現(xiàn)威创，在openJDK中實現(xiàn)了多種不同的hash值計算策略落午，只有一種是直接返回對象的內(nèi)存地址。
