【JVM】底层实现（四）：Object#hashCode 返回的是对象内存地址？（转）

àì夳堔傛蜴生んèń 2022-11-09 14:21 139阅读 0赞

> 原文链接：[https://www.jianshu.com/p/be943b4958f4][https_www.jianshu.com_p_be943b4958f4]

### 基于OpenJDK 8 ###

一直以为 Java Object.hashCode() 的结果就是通过对象的内存地址做相关运算得到的，但是无意在网上看到有相应的意见争论，故抽时间从源码层面验证了剖析了 hashCode 的默认计算方法。

> 先说结论：OpenJDK8 默认hashCode的计算方法是通过和当前线程有关的一个随机数+三个确定值，运用Marsaglia’s xorshift scheme随机数算法得到的一个随机数。和对象内存地址无关。

下面通过查找和分析OpenJDK8源码实现来一步步分析。

### 1. 查找java.lang.Object.hashCode()源码 ###

public native int hashCode();

### 2. 导出Object的JNI头文件 ###

> 切换到Object.class文件所在目录，执行 javah -jni java.lang.Object，得到java\_lang\_Object.h文件，文件内容如下：

/* DO NOT EDIT THIS FILE - it is machine generated */
    #include <jni.h>
    /* Header for class java_lang_Object */
    
    #ifndef _Included_java_lang_Object
    #define _Included_java_lang_Object
    #ifdef __cplusplus
    extern "C" {
        
    #endif
    /*
     * Class:     java_lang_Object
     * Method:    registerNatives
     * Signature: ()V
     */
    JNIEXPORT void JNICALL Java_java_lang_Object_registerNatives
      (JNIEnv *, jclass);
    
    /*
     * Class:     java_lang_Object
     * Method:    getClass
     * Signature: ()Ljava/lang/Class;
     */
    JNIEXPORT jclass JNICALL Java_java_lang_Object_getClass
      (JNIEnv *, jobject);
    
    /*
     * Class:     java_lang_Object
     * Method:    hashCode
     * Signature: ()I
     */
    JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode
      (JNIEnv *, jobject);
    
    /*
     * Class:     java_lang_Object
     * Method:    clone
     * Signature: ()Ljava/lang/Object;
     */
    JNIEXPORT jobject JNICALL Java_java_lang_Object_clone
      (JNIEnv *, jobject);
    
    /*
     * Class:     java_lang_Object
     * Method:    notify
     * Signature: ()V
     */
    JNIEXPORT void JNICALL Java_java_lang_Object_notify
      (JNIEnv *, jobject);
    
    /*
     * Class:     java_lang_Object
     * Method:    notifyAll
     * Signature: ()V
     */
    JNIEXPORT void JNICALL Java_java_lang_Object_notifyAll
      (JNIEnv *, jobject);
    
    /*
     * Class:     java_lang_Object
     * Method:    wait
     * Signature: (J)V
     */
    JNIEXPORT void JNICALL Java_java_lang_Object_wait
      (JNIEnv *, jobject, jlong);
    
    #ifdef __cplusplus
    }
    #endif
    #endif

### 3 . 查看Object的native方法实现 ###

OpenJDK源码链接：[http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/3462d04401ba/src/share/native/java/lang/Object.c][http_hg.openjdk.java.net_jdk8u_jdk8u_jdk_file_3462d04401ba_src_share_native_java_lang_Object.c] ，查看Object.c文件，可以看到hashCode()的方法被注册成由JVM\_IHashCode方法指针来处理。

static JNINativeMethod methods[] = {
          
        {
        "hashCode",    "()I",                    (void *)&JVM_IHashCode},//hashcode的方法指针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},  
    };

而JVM\_IHashCode方法指针在 openjdk\\hotspot\\src\\share\\vm\\prims\\jvm.cpp中定义为：

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

从而得知，真正计算获得hashCode的值是ObjectSynchronizer::FastHashCode

### 4 . ObjectSynchronizer::fashHashCode方法的实现 ###

openjdk\\hotspot\\src\\share\\vm\\runtime\\synchronizer.cpp 找到其实现方法。

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;
    }

该方法中

// 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);
    ...
    }

对hash值真正进行了计算，查看get\_next\_hash方法源码[http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/synchronizer.cpp\#l555][http_hg.openjdk.java.net_jdk8u_jdk8u_hotspot_file_87ee5ee27509_src_share_vm_runtime_synchronizer.cpp_l555]

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 = cast_from_oop<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 = cast_from_oop<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;
    }

对于OpenJDK8版本，其默认配置[http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/globals.hpp\#l1127][http_hg.openjdk.java.net_jdk8u_jdk8u_hotspot_file_87ee5ee27509_src_share_vm_runtime_globals.hpp_l1127] 为：

product(intx, hashCode, 5,                                                \
              "(Unstable) select hashCode generation algorithm")                \

其对应的hashCode计算方案为：

// 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 ;

其中Thread->\_hashStateX, Thread->\_hashStateY, Thread->\_hashStateZ, Thread->\_hashStateW在[http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/thread.cpp\#I263][http_hg.openjdk.java.net_jdk8u_jdk8u_hotspot_file_87ee5ee27509_src_share_vm_runtime_thread.cpp_I263] 有定义：

// thread-specific hashCode stream generator state - Marsaglia shift-xor form
      _hashStateX = os::random() ;
      _hashStateY = 842502087 ;
      _hashStateZ = 0x8767 ;    // (int)(3579807591LL & 0xffff) ;
      _hashStateW = 273326509 ;

所以，JDK8 的默认hashCode的计算方法是通过和当前线程有关的一个随机数+三个确定值，运用Marsaglia’s xorshift scheme随机数算法得到的一个随机数。对xorshift算法有兴趣可以参考原论文：[https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf][https_www.jstatsoft.org_article_view_v008i14_xorshift.pdf] 。

xorshift是由George Marsaglia发现的一类伪随机数生成器，其通过移位和与或计算，能够在计算机上以极快的速度生成伪随机数序列。其算法的基本实现如下：

unsigned long xor128(){
        
    static unsigned long x=123456789,y=362436069,z=521288629,w=88675123;
    unsigned long t;
    t=(xˆ(x<<11));x=y;y=z;z=w; return( w=(wˆ(w>>19))ˆ(tˆ(t>>8)) );

这就和上面计算hashCode的OpenJDK代码对应了起来。

### 5 . 其他几类hashCode计算方案： ###

> **可以通过在JVM启动参数中添加-XX:hashCode=4，改变默认的hashCode计算方式**

1）hashCode == 0

此类方案返回一个Park-Miller伪随机数生成器生成的随机数（OpenJdk 6 &7的默认实现）

[http://hg.openjdk.java.net/jdk7u/jdk7u/hotspot/file/5b9a416a5632/src/share/vm/runtime/globals.hpp\#l1100][http_hg.openjdk.java.net_jdk7u_jdk7u_hotspot_file_5b9a416a5632_src_share_vm_runtime_globals.hpp_l1100]  
[http://hg.openjdk.java.net/jdk6/jdk6/hotspot/file/5cec449cc409/src/share/vm/runtime/globals.hpp\#l1128][http_hg.openjdk.java.net_jdk6_jdk6_hotspot_file_5cec449cc409_src_share_vm_runtime_globals.hpp_l1128]

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() ;
      }

2）hashCode == 1

此类方案将对象的内存地址，做移位运算后与一个随机数进行异或得到结果

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 = cast_from_oop<intptr_t>(obj) >> 3 ;
         value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
      }

3）hashCode == 2

此类方案返回固定的1

if (hashCode == 2) {
        
         value = 1 ;            // for sensitivity testing
      }

4）hashCode == 3

此类方案返回一个自增序列的当前值

if (hashCode == 3) {
        
         value = ++GVars.hcSequence ;
      }

5）hashCode == 4

此类方案返回当前对象的内存地址

if (hashCode == 4) {
        
         value = cast_from_oop<intptr_t>(obj) ;
      }

> 再放一个写的不错的参考链接：[https://www.javazhiyin.com/55723.html][https_www.javazhiyin.com_55723.html]…

[https_www.jianshu.com_p_be943b4958f4]: https://www.jianshu.com/p/be943b4958f4
[http_hg.openjdk.java.net_jdk8u_jdk8u_jdk_file_3462d04401ba_src_share_native_java_lang_Object.c]: https://link.jianshu.com?t=http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/3462d04401ba/src/share/native/java/lang/Object.c
[http_hg.openjdk.java.net_jdk8u_jdk8u_hotspot_file_87ee5ee27509_src_share_vm_runtime_synchronizer.cpp_l555]: https://link.jianshu.com?t=http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/synchronizer.cpp#l555
[http_hg.openjdk.java.net_jdk8u_jdk8u_hotspot_file_87ee5ee27509_src_share_vm_runtime_globals.hpp_l1127]: https://link.jianshu.com?t=http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/globals.hpp#l1127
[http_hg.openjdk.java.net_jdk8u_jdk8u_hotspot_file_87ee5ee27509_src_share_vm_runtime_thread.cpp_I263]: https://link.jianshu.com?t=http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/thread.cpp#I263
[https_www.jstatsoft.org_article_view_v008i14_xorshift.pdf]: https://link.jianshu.com?t=https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf
[http_hg.openjdk.java.net_jdk7u_jdk7u_hotspot_file_5b9a416a5632_src_share_vm_runtime_globals.hpp_l1100]: https://link.jianshu.com?t=http://hg.openjdk.java.net/jdk7u/jdk7u/hotspot/file/5b9a416a5632/src/share/vm/runtime/globals.hpp#l1100
[http_hg.openjdk.java.net_jdk6_jdk6_hotspot_file_5cec449cc409_src_share_vm_runtime_globals.hpp_l1128]: https://link.jianshu.com?t=http://hg.openjdk.java.net/jdk6/jdk6/hotspot/file/5cec449cc409/src/share/vm/runtime/globals.hpp#l1128
[https_www.javazhiyin.com_55723.html]: https://www.javazhiyin.com/55723.html