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

淡淡的烟草味﹌ 2022-11-28 13:39 225阅读 0赞

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？  
（顺带回顾一下JNI）

# 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™ 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();
     
      
     
      
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`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);
    }
     
      
     
      
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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));
        }
    }
     
      
     
      
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## 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
     
      
     
      
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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);  
    }
     
      
     
      
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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*:  
`user@host:/usr/lib/jvm/java-7-openjdk-amd64$ 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：  
`$ java TestNativeUtils`  
居然出错了：

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

## All in one —— Makefile ##

上面的多个命令可以写到一个Makefile里，可以实现“一键执行”：

test: runtest
    
    all: libnative-utils.so
    
    JNI_INCLUDE=/usr/lib/jvm/java-7-openjdk-amd64/include
    
    NativeUtils.class: NativeUtils.java
        javac NativeUtils.java
    
    TestNativeUtils.class: TestNativeUtils.java
        javac TestNativeUtils.java
    
    NativeUtils.h: NativeUtils.java
        javah -jni NativeUtils 
    
    libnative-utils.so: NativeUtils.cc NativeUtils.h
        g++ -shared -I${JNI_INCLUDE} -o libnative-utils.so NativeUtils.cc
    
    runtest: TestNativeUtils.class libnative-utils.so
        @echo "run test:"
        java -Djava.library.path=. TestNativeUtils
    
    clean:
        rm -v *.class *.so *.h
     
      
     
      
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# 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);
        }
     
      
     
      
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（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);
     
      
     
      
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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());
    }
     
      
     
      
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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;
    }
     
      
     
      
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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;
    }
     
      
     
      
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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*][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);
        }
    }
     
      
     
      
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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.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
     
      
     
      
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这里只是一个包装，实际计算hashCode的是`ObjectSynchronizer::FastHashCode`，位于[*synchronizer.cpp*][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;
    }
     
      
     
      
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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;
    }
     
      
     
      
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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）。

# 结论 #

前面通过JNI验证已经能够得到很显然的结论，hashCode返回的并不一定是对象的（虚拟）内存地址，具体取决于运行时库和JVM的具体实现。

**我的运行环境：**  
OS：  
Ubuntu 12.04 64bit Desktop |

JDK：  
java version “1.7.0\_55”  
OpenJDK Runtime Environment (IcedTea 2.4.7) (7u55-2.4.7-1ubuntu1~0.12.04.2) \\  
OpenJDK 64-Bit Server VM (build 24.51-b03, mixed mode)

[Object.c]: http://hg.openjdk.java.net/jdk7/jdk7/jdk/file/9b8c96f96a0f/src/share/native/java/lang/Object.c
[jvm.cpp]: http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/9b0ca45cd756/src/share/vm/prims/jvm.cpp
[synchronizer.cpp]: http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/9b0ca45cd756/src/share/vm/runtime/synchronizer.cpp