Java 的 Object.hashCode() 的返回值到底是不是对象内存地址？

发布于 2024-01-25 02:51:29 字数 21481 浏览 33 评论 0

刚学 Java 的时候我也有过这种怀疑，但一直没有验证；最近在 OSCHINA 上看到有人在回答问题时也这么说，于是萌生了一探究竟的想法—— java.lang.Object.hashCode() 的返回值到底是不是对象内存地址？

hashCode 契约

说到这个问题，大家的第一反应一定和我一样——去查 Object.hashCode 的源码，但翻开源码，看到的却是这样的（Oracle JDK 8）：

/**
  * Returns a hash code value for the object. This method is
  * supported for the benefit of hash tables such as those provided by
  * {@link java.util.HashMap}.
  * <p>
  * The general contract of {@code hashCode} is:
  * <ul>
  * <li>Whenever it is invoked on the same object more than once during
  *     an execution of a Java application, the {@code hashCode} method
  *     must consistently return the same integer, provided no information
  *     used in {@code equals} 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 {@code equals(Object)}
  *     method, then calling the {@code hashCode} 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 {@code hashCode} 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 hash tables.
  * </ul>
  * <p>
  * As much as is reasonably practical, the hashCode method defined by
  * class {@code Object} 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&trade; programming language.)
  *
  * @return  a hash code value for this object.
  * @see     java.lang.Object#equals(java.lang.Object)
  * @see     java.lang.System#identityHashCode
  */
public native int hashCode();

Object.hashCode 是一个 native 方法，看不到源码（Java 代码，Oracle 的 JDK 是看不到的，OpenJDK 或其他开源 JRE 是可以找到对应的 C/C++代码）。

上面这段注释指出了 Object.hashCode() 在 JRE（Java Runtime Library）中应该遵循的一些契约（contract）：
（PS：所谓契约当然是大家一致达成的，各个 JVM 厂商都会遵循）

一致性（consistent），在程序的一次执行过程中，对同一个对象必须一致地返回同一个整数。
如果两个对象通过 equals(Object) 比较，结果相等，那么对这两个对象分别调用 hashCode 方法应该产生相同的整数结果。（PS：这里 equals 和 hashCode 说的都是 Object 类的）
如果两个对象通过 java.lang.Object.equals(java.lang.Ojbect) 比较，结果不相等，不必保证对这两个对象分别调用 hashCode 也返回两个不相同的整数。

实际上 java.lang 包里面的类，都是 JRE 必须的，属于运行时库（Runtime Library），这也是为什么很多 JRE 下该类的 class 文件被打包到rt.jar中的原因（应该是 Runtime 的简写）。

而这些运行时库一般都是跟 JDK/JRE 一起发布的；所以，对于不同的 JRE 环境，问题的答案未必相同。

考虑到具体 JVM 厂商实现的 Object.hashCode 相对较复杂，下面先通过另一思路对开头提出的问题进行探究。

最后我们再找一些开源 JRE 的 Object.hashCode 的具体实现作简要分析。

Java 中如何获得对象内存地址？

看不到 Object.hashCode 的源码，反过来，我们可以得到对象的内存地址和 Object.hashCode 比较，也能得出结论。

要验证这个问题自然需要一种得到对象内存地址的方法，但 Java 本身并没有提供类似的方法；这也是我在初学 Java 时没有验证这个问题的原因。

“内存地址”在 Java 里得不到，但在 C/C++中却很容易得到。于是，我们想到——通过 JNI 让 Java 代码调用一段 C/C++代码来得到对象内存地址。

这里可能需要考虑的还有一点——用 Java 什么类型能放得下 C/C++的指针？
在 64 位机器上，C/C++的指针是 8 字节；32 位是 4 字节。
嗯(⊙_⊙)~ 不管怎样 Java 的 long 都是 8 字节，足矣~

Think in Java——接口和测试

假设我们已经有了一个 NativeUtils.java ：

class NativeUtils {
    public native static long getNativePointer(Object o);
}

并且已经实现了 getNativePointer 方法。

那么验证开头提出的问题就变得异常简单了：

class TestNativeUtils {
    public static void main(String args[]) {
        Object o = new Object();

        long nptr = NativeUtils.getNativePointer(o);
        long hash = o.hashCode();

        System.out.println(String.format("hash: %x, nptr: %x", hash, nptr));
    }
}

Think in C++——实现 native 方法

好了说干就干，现在就差那个 native 的 getNativePointer 了。

用 javah 生成对应的 .h 文件：
$ javah NativeUtils
该命令执行后生成了 NativeUtils.h ：

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class NativeUtils */

#ifndef _Included_NativeUtils
#define _Included_NativeUtils
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     NativeUtils
 * Method:    getNativePointer
 * Signature: (Ljava/lang/Object;)J
 */
JNIEXPORT jlong JNICALL Java_NativeUtils_getNativePointer
  (JNIEnv *, jclass, jobject);

#ifdef __cplusplus
}
#endif
#endif

接着实现这个 Java_NativeUtils_getNativePointer ，文件命名为 NativeUtils.cc ：

#include "NativeUtils.h"

JNIEXPORT jlong JNICALL
Java_NativeUtils_getNativePointer(JNIEnv *env, jclass clazz, jobject o)
{
    return reinterpret_cast<jlong>(o);  
}

编译为动态库：

$ g++ -shared -o libnative-utils.so NativeUtils.cc

可能因为找不到 jni.h ，报错：

NativeUtils.h:2:17: fatal error: jni.h: No such file or directory

在 JDK 安装目录下查找 jni.h :

$ find . -name jni.h ./include/jni.h

知道 jni.h 路径后，用-I 选项加到编译命令上再次编译：

$ g++ -shared -I/usr/lib/jvm/java-7-openjdk-amd64/include/ -o libnative-utils.so NativeUtils.cc

OK，编译成功，生成了 libnative-utils.so 文件。

Run in shell——在 shell 环境运行

下面就让 TestNativeUtils 在 shell 环境执行起来。

首先，编译 NativeUtils 和 TestNativeUtils :

$ javac NativeUtils.java
$ javac TestNativeUtils.java

分别生成了 NativeUtils.class 和 TestNativeUtils.class

好了，就差临门一脚了——执行 class 文件：

Exception in thread "main" java.lang.UnsatisfiedLinkError: NativeUtils.getNativePointer(Ljava/lang/Object;)J
    at NativeUtils.getNativePointer(Native Method)
    at TestNativeUtils.main(TestNativeUtils.java:5)

加载动态库到我们的程序中

到目前为止，我们的 Java 代码并没有实现 NativeUtils.getNativePointer ；所以，会有上面的错误。
必须在调用 NativeUtils.getNativePointer 前，将我们编译好的动态库加载上。可以用 static 代码块，以保证在调用前完成加载；修改后的 NativeUtils ：

class NativeUtils {
    static {
        System.loadLibrary("native-utils");
    }

    public native static long getNativePointer(Object o);
}

让 JVM 能找到动态库

再次编译、执行：

$ javac NativeUtils.java
$ java TestNativeUtils

又有错误，但已经和刚才不一样了：

Exception in thread "main" java.lang.UnsatisfiedLinkError: no native-utils in java.library.path
    at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1886)
    at java.lang.Runtime.loadLibrary0(Runtime.java:849)
    at java.lang.System.loadLibrary(System.java:1088)
    at NativeUtils.<clinit>(NativeUtils.java:3)
    at TestNativeUtils.main(TestNativeUtils.java:5)

这次错误是：没有在 java.library.path 中找到 native-utils ，可以在 javac 命令-D 参数上将当期目录加到 java.library.path 上，让其能够找到 native-utils ：

$ java -Djava.library.path=. TestNativeUtils
hash: 4f5f1ace, nptr: 7f223a5fb958

几个 JRE 的具体实现

说道开源的 Java 运行环境，首先能想到的自然是 OpenJDK 和 Android，下面分别简要分析。

hashCode on Android

Android 的 Object.hashCode 和 Oracle JDK 的略有不同：

private transient int shadow$_monitor_;

public int hashCode() {
    int lockWord = shadow$_monitor_;
    final int lockWordMask = 0xC0000000;  // Top 2 bits.
    final int lockWordStateHash = 0x80000000;  // Top 2 bits are value 2 (kStateHash).
    if ((lockWord & lockWordMask) == lockWordStateHash) {
        return lockWord & ~lockWordMask; // 
    }
    return System.identityHashCode(this);
}

（PS：估计 shadow$_monitor_ 应该是 hash 值的一个 cache，第一次需要计算一下，以后都不用计算了）
第一次执行时， shadow$_monitor_ 的值为 0，将会调用 System.identityHashCode ：

public static native int identityHashCode(Object anObject);

而它是一个 native 的方法，对应代码 java_lang_System.cc ：

static jint System_identityHashCode(JNIEnv* env, jclass, jobject javaObject) {
  if (UNLIKELY(javaObject == nullptr)) {
    return 0;
  }
  ScopedFastNativeObjectAccess soa(env);
  mirror::Object* o = soa.Decode<mirror::Object*>(javaObject);
  return static_cast<jint>(o->IdentityHashCode());
}

很显然，这里的关键在于 mirror::Object::IdentityHashCode ：

int32_t Object::IdentityHashCode() const {
  mirror::Object* current_this = const_cast<mirror::Object*>(this);
  while (true) {
    LockWord lw = current_this->GetLockWord(false);
    switch (lw.GetState()) {
      case LockWord::kUnlocked: { // kUnlocked 是 LockWord 的默认 State 值
        // Try to compare and swap in a new hash, if we succeed we will return the hash on the next
        // loop iteration.
        LockWord hash_word(LockWord::FromHashCode(GenerateIdentityHashCode()));
        DCHECK_EQ(hash_word.GetState(), LockWord::kHashCode); 
        if (const_cast<Object*>(this)->CasLockWordWeakRelaxed(lw, hash_word)) {
          return hash_word.GetHashCode();
        }
        break;
      }
      case LockWord::kThinLocked: {
        // Inflate the thin lock to a monitor and stick the hash code inside of the monitor. May
        // fail spuriously.
        Thread* self = Thread::Current();
        StackHandleScope<1> hs(self);
        Handle<mirror::Object> h_this(hs.NewHandle(current_this));
        Monitor::InflateThinLocked(self, h_this, lw, GenerateIdentityHashCode());
        // A GC may have occurred when we switched to kBlocked.
        current_this = h_this.Get();
        break;
      }
      case LockWord::kFatLocked: {
        // Already inflated, return the has stored in the monitor.
        Monitor* monitor = lw.FatLockMonitor();
        DCHECK(monitor != nullptr);
        return monitor->GetHashCode();
      }
      case LockWord::kHashCode: { // 以后调用
        return lw.GetHashCode();
      }
      default: {
        LOG(FATAL) << "Invalid state during hashcode " << lw.GetState();
        break;
      }
    }
  }
  LOG(FATAL) << "Unreachable";
  return 0;
}

这段代码可以看出——ART 的 Object.hashCode 确实是有 cache 的，对于同一个 Ojbect，第一次调用 Object.hashCode 将会执行实际的计算并记入 cache，以后直接从 cache 中取出。真正计算 hashcode 的是 GenerateIdentityHashCode：

int32_t Object::GenerateIdentityHashCode() {
  static AtomicInteger seed(987654321 + std::time(nullptr));
  int32_t expected_value, new_value;
  do {
    expected_value = static_cast<uint32_t>(seed.LoadRelaxed());
    new_value = expected_value * 1103515245 + 12345;
  } while ((expected_value & LockWord::kHashMask) == 0 ||
      !seed.CompareExchangeWeakRelaxed(expected_value, new_value));
  return expected_value & LockWord::kHashMask;
}

从 GenerateIdentityHashCode 可以看出，ART 的 Object.hashCode 的返回值和对象的地址并没有直接的关系。

hashCode on OpenJDK

OpenJDK 项目首页： openjdk.java.net
TODO: OpenJDK 上的 hashCode 具体实现和简要分析 Object.c

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},
};

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);
    }
}

这段代码指出了 Object.hashCode 对应的 C 函数为 JVM_IHashCode ，下面需要找到 JVM_IHashCode 的代码 jvm.cpp ：

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

这里只是一个包装，实际计算 hashCode 的是 ObjectSynchronizer::FastHashCode ，位于 synchronizer.cpp ：

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(); // 取出缓存
  if (hash == 0) {
    hash = get_next_hash(Self, obj); // 实际计算
    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;
}

又是假牙，实际计算 hashCode 的是 get_next_hash ，代码和 ObjectSynchronizer::FastHashCode 相邻：

// 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() ; // 随机数
  } 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.
     // 地址基础上 hack
     intptr_t addrBits = intptr_t(obj) >> 3 ; 
     value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
  } else
  if (hashCode == 2) {
     value = 1 ;            // for sensitivity testing， 实际不会使用
  } 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;
}

这段代码可以看出 OpenJDK 一共实现了 5 中不同的计算 hash 值的方法，通过
这段代码中 hashCode 进行切换。其中 hashCode == 4 的是直接使用地址的（前面的实验说明 OpenJDK 默认情况下并没有使用这种方式，或许可以通过运行/编译时参数进行选择）。