眾所周知，Java語(yǔ)言中，Object對(duì)象有個(gè)特殊的方法：hashcode(), hashcode()表示的是JVM虛擬機(jī)為這個(gè)Object對(duì)象分配的一個(gè)int類型的數(shù)值，JVM會(huì)使用對(duì)象的hashcode值來(lái)提高對(duì)HashMap、Hashtable哈希表存取對(duì)象的使用效率。而在很多網(wǎng)上的教程中告訴我們的是hashcode返回的是對(duì)象的內(nèi)存地址。然而實(shí)際使用中會(huì)發(fā)現(xiàn)哈希值存在負(fù)數(shù)的情況，而內(nèi)存地址肯定不會(huì)用負(fù)數(shù)表示，因此有了本文對(duì)其底層實(shí)現(xiàn)進(jìn)一步進(jìn)行探索。須知的是，不同的JRE對(duì)這些方法的實(shí)現(xiàn)各有不同，只是遵循著Java對(duì)這些方法所訂立的規(guī)范進(jìn)行設(shè)計(jì)的，本文基于的是openJDK的源碼。
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，它提供了繞開(kāi)"安全"的JVM機(jī)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。

Screen Shot 2019-02-17 at 10.14.18 PM.png

public native int hashCode();

即該方法是一個(gè)本地方法，Java將調(diào)用本地方法庫(kù)對(duì)此方法的實(shí)現(xiàn)。由于Object類中有JNI方法調(diào)用，按照J(rèn)NI的規(guī)則，應(yīng)當(dāng)生成JNI 的頭文件，在此目錄下執(zhí)行javah -jni java.lang.Object 指令，將生成一個(gè)java_lang_Object.h頭文件，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(duì)象的hashCode()方法在C語(yǔ)言文件Object.c中實(shí)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(shí)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(shí)現(xiàn)，該方法最終會(huì)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)，在openJDK中實(shí)現(xiàn)了多種不同的hash值計(jì)算策略，只有一種是直接返回對(duì)象的內(nèi)存地址。