hashCode和identityHashCode底層是怎麼生成的工做中使用==埋下的坑 hashCode和identityHashCode 的關係 hashCode和identityHashCod

時間 2019-12-12

標籤 hashcode identityhashcode 底層怎麼生成使用埋下關係 identityhashcod 简体版

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前言：在工做中使用==埋下的坑這篇博文的最後，我想到了兩個問題，其中一個是——爲何 int int1=99;int int2=99;int1和int2的identityHashCode是同樣的哪？爲何float float1=99;float float2=99;float1和float2的identityHashCode是不同的哪？那就須要瞭解identityHashCode的生成規則了，須要瞭解一下java的內存地址分配規則了。html

今天的事情很少，我就查了查資料，找到了對應的底層實現的方式，而且也驗證了hashCode和identityHashCode 的關係這篇博文中的部分觀點。java

本文將根據openJDK 6源碼，向你展現Java語言中的Object對象的hashCode() 方法和System對象的identityHashCode()方法的底層實現的神祕面紗，我將一步一步地向你介紹Java Object對象的hashCode() 方法和System對象的identityHashCode()方法到底底層調用了什麼函數。爲了更好地瞭解這個過程，你能夠本身下載openJDK 6 源碼，親自查看和跟蹤源碼，瞭解Object對象的hashCode() 方法和System對象的identityHashCode()方法的生成過程：c++

openJDK 6 下載地址：http://download.java.net/openjdk/jdk6/算法

1：java.lang.System.java類的identityHashCode()方法以下所示——在 openjdk-6-src-b27-26_oct_2012\jdk\src\share\classes\java\lang 目錄下，它是一個靜態的本地方法安全

    /**
     * Returns the same hash code for the given object as
     * would be returned by the default method hashCode(),
     * whether or not the given object's class overrides
     * hashCode().
     * The hash code for the null reference is zero.
     *
     * @param x object for which the hashCode is to be calculated
     * @return  the hashCode
     * @since   JDK1.1
     */
    public static native int identityHashCode(Object x);

2：java.lang.System.java類的identityHashCode()方法的本地c語言的實現——System.c——在 openjdk-6-src-b27-26_oct_2012\jdk\src\share\native\java\lang 目錄下，他調用的是JVM_IHashCode()方法app

JNIEXPORT jint JNICALL
Java_java_lang_System_identityHashCode(JNIEnv *env, jobject this, jobject x)
{
    return JVM_IHashCode(env, x);
}

3：JVM_IHashCode()方法在 openjdk-6-src-b27-26_oct_2012\hotspot\src\share\vm\prims\jvm.cpp文件中，它又調用ObjectSynchronizer::FastHashCode()方法less

// 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

4：ObjectSynchronizer::FastHashCode()方法在 openjdk-6-src-b27-26_oct_2012\hotspot\src\share\vm\runtime\synchronizer.cpp文件中，它是最終實現hashCode()和identityHashCode()方法的方法，核心的實現代碼以下，咱們從這裏也能夠看獲得，仍是比較複雜的，並非簡單取一個對象的引用地址那麼簡單。jvm

//
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;
}

5：java.lang.Object.java類的hashCode()方法以下所示——在 openjdk-6-src-b27-26_oct_2012\jdk\src\share\classes\java\lang 目錄下，它是一個本地方法async

    /**
     * Returns a hash code value for the object. This method is
     * supported for the benefit of hashtables such as those provided by
     * <code>java.util.Hashtable</code>.
     * <p>
     * The general contract of <code>hashCode</code> is:
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during
     *     an execution of a Java application, the <tt>hashCode</tt> method
     *     must consistently return the same integer, provided no information
     *     used in <tt>equals</tt> comparisons on the object is modified.
     *     This integer need not remain consistent from one execution of an
     *     application to another execution of the same application.
     * <li>If two objects are equal according to the <tt>equals(Object)</tt>
     *     method, then calling the <code>hashCode</code> method on each of
     *     the two objects must produce the same integer result.
     * <li>It is <em>not</em> required that if two objects are unequal
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
     *     method, then calling the <tt>hashCode</tt> method on each of the
     *     two objects must produce distinct integer results.  However, the
     *     programmer should be aware that producing distinct integer results
     *     for unequal objects may improve the performance of hashtables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by
     * class <tt>Object</tt> does return distinct integers for distinct
     * objects. (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<font size="-2"><sup>TM</sup></font> programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.util.Hashtable
     */
    public native int hashCode();

6：java.lang.Object.java類的hashCode()方法的本地c語言的實現——Object.c——在 openjdk-6-src-b27-26_oct_2012\jdk\src\share\native\java\lang 目錄下，他調用的也是JVM_IHashCode()方法，因而可知咱們在hashCode和identityHashCode 的關係中的觀點，在此處也獲得了再次的驗證ide

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},
};