http://www.bkjia.com/ASPjc/919437.html

 Java語言中，Object物件有個特殊的方法：hashcode(), hashcode()表示的是JVM虛擬機器為這個Object物件分配的一個int型別的數值，JVM會使用物件的hashcode值來提高對HashMap、Hashtable雜湊表存取物件的使用效率。

      關於Object物件的hashCode()返回值，網上對它就是一個簡單的描述：“JVM根據某種策略生成的”，那麼這種策略到底是什麼呢？我有一個毛病，遇到這種含糊其辭的東西，就想探個究竟，所以，本文就將hashCode()本地方法的實現給扒出來，也給大家在瞭解hashCode()的過程中提供一點點幫助吧。

      本文將根據openJDK 7原始碼，向展示Java語言中的Object物件的hashCode() 生成的神祕面紗，我將一步一步地向讀者介紹Java Object 的hashcode()方法到底底層呼叫了什麼函式。為了更好地瞭解這個過程，你可以自己下載openJDK 7 原始碼，親自檢視和跟蹤原始碼，瞭解hashCode()的生成過程：

         openJDK 7 下載地址1：http://download.java.net/openjdk/jdk7 （官網，下載速度較慢）

         openJDK 7 下載地址2 ：openjdk-7-fcs-src-b147-27_jun_2011.zip (csdn 網友提供的資源，很不錯)

1.檢視openJDK 關於 java.lang.Object類及其hashcode()方法的定義：

   進入openjdk\jdk\src\share\classes\java\lang 目錄下，可以看到 Object.java原始碼，開啟，檢視hashCode()的定義如下所示：

public native int hashCode();

   java_lang_Object.h標頭檔案關於hashcode方法的資訊如下所示：

/*
 * Class:     java_lang_Object
 * Method:    hashCode
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode
  (JNIEnv *, jobject);

2. Object物件的hashCode()方法在C語言檔案Object.c中實現

  開啟openjdk\jdk\src\share\native\java\lang\目錄，檢視Object.c檔案，可以看到hashCode()的方法被註冊成有JVM_IHashCode方法指標來處理：

#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的方法指標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);
    }
}

3.JVM_IHashCode方法指標在 openjdk\hotspot\src\share\vm\prims\jvm.cpp中定義，如下：

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

  如上可以看出，JVM_IHashCode方法中呼叫了ObjectSynchronizer::FastHashCode方法

4. ObjectSynchronizer::fashHashCode方法的實現：

     ObjectSynchronizer::fashHashCode()方法在openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp 檔案中實現，其核心程式碼實現如下所示：

// 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;
}
//   ObjectSynchronizer::FastHashCode方法的實現，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  ，看到這句話，就特別興奮，WE FINALLY GET THE HASH!!!!
  return hash;
}

       好了，經過上述如此複雜步驟，終於生成了我們的hashcode了，上述的程式碼是使用的C++實現的，我是看不懂啦，不過有一點可以確定：

           Java 中Object物件的hashcode()返回值一定不會是Object物件的記憶體地址這麼簡單！

       即hashcode()返回的不是物件在記憶體中的地址。