[Java基础要义] Java语言中Object对象的hashCode()取值的底层算法是怎样实现的？

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Java语言中，Object对象有个特殊的方法：hashcode(), hashcode()表示的是JVM虚拟机为这个Object对象分配的一个int类型的数值，JVM会使用对象的hashcode值来提高对HashMap、Hashtable哈希表存取对象的使用效率。

关于Object对象的hashCode()返回值，网上对它就是一个简单的描述：“JVM根据某种策略生成的”，那么这种策略到底是什么呢？我有一个毛病，遇到这种含糊其辞的东西，就想探个究竟，所以，本文就将hashCode()本地方法的实现给扒出来，也给大家在了解hashCode()的过程中提供一点点帮助吧。

本文将根据openJDK 7源码，向展示Java语言中的Object对象的hashCode() 生成的神秘面纱，我将一步一步地向读者介绍Java Object 的hashcode()方法到底底层调用了什么函数。为了更好地了解这个过程，你可以自己下载openJDK 7 源码，亲自查看和跟踪源码，了解hashCode()的生成过程：

         openJDK 7 下载地址1：http://download.java.net/openjdk/jdk7 （官网，下载速度较慢）

         openJDK 7 下载地址2 ：openjdk-7-fcs-src-b147-27_jun_2011.zip (csdn 网友提供的资源，很不错)

1.查看openJDK 关于 java.lang.Object类及其hashcode()方法的定义：

进入openjdk\jdk\src\share\classes\java\lang目录下，可以看到 Object.java源码，打开，查看hashCode()的定义如下所示：

public native int hashCode();

即该方法是一个本地方法，Java将调用本地方法库对此方法的实现。由于Object类中有JNI方法调用，按照JNI的规则，应当生成JNI 的头文件，在此目录下执行javah -jni java.lang.Object 指令，将生成一个java_lang_Object.h头文件，该头文件将在后面用到它

java_lang_Object.h头文件关于hashcode方法的信息如下所示：

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

2. Object对象的hashCode()方法在C语言文件Object.c中实现

打开openjdk\jdk\src\share\native\java\lang**目录，查看Object.c文件，可以看到hashCode()的方法被注册成有JVM_IHashCode**方法指针来处理：

#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}, }; JNIEXPORT void JNICALL Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls) { (*env)->RegisterNatives(env, cls, methods, sizeof(methods)/sizeof(methods0)); } 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方法指针在 openjdk\hotspot\src\share\vm\prims\jvm.cpp中定义，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方法中调用了ObjectSynchronizer::FastHashCode方法

4. ObjectSynchronizer::fashHashCode方法的实现：

ObjectSynchronizer::fashHashCode()方法在 openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp 文件中实现，其核心代码实现如下所示：

// 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方法的实现，该方法最终会返回我们期望已久的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 ，看到这句话，就特别兴奋，WE FINALLY GET THE HASH!!!! return hash; }

好了，经过上述如此复杂步骤，终于生成了我们的hashcode了，上述的代码是使用的C++实现的，我是看不懂啦，不过有一点可以确定：

**Java 中Object对象的hashcode()返回值一定不会是Object对象的内存地址这么简单！**

       即hashcode()返回的不是对象在内存中的地址。