前言：在工作中使用==埋下的坑这篇博文的最后，我想到了两个问题，其中一个是——为什么 int int1=99;int int2=99;int1和int2的identityHashCode是一样的哪？为什么float float1=99;float float2=99;float1和float2的identityHashCode是不一样的哪？那就需要了解identityHashCode的生成规则了，需要了解一下java的内存地址分配规则了。
今天的事情不多，我就查了查资料，找到了对应的底层实现的方式，并且也验证了hashCode和identityHashCode 的关系这篇博文中的部分观点。
本文将根据openJDK 6源码，向你展示Java语言中的Object对象的hashCode() 方法和System对象的identityHashCode()方法的底层实现的神秘面纱，我将一步一步地向你介绍Java Object对象的hashCode() 方法和System对象的identityHashCode()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 6 源码，亲自查看和跟踪源码，了解Object对象的hashCode() 方法和System对象的identityHashCode()方法的生成过程：
openJDK 6 下载地址：http://download.java.net/openjdk/jdk6/
1：java.lang.System.java类的identityHashCode()方法如下所示——在 openjdk-6-src-b27-26_oct_2012jdksrcshareclassesjavalang 目录下，它是一个静态的本地方法
/** * 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类的identityHashCode()方法的本地c语言的实现——System.c——在 openjdk-6-src-b27-26_oct_2012jdksrcshare ativejavalang 目录下，他调用的是JVM_IHashCode()方法
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_2012hotspotsrcsharevmprimsjvm.cpp文件中，它又调用ObjectSynchronizer::FastHashCode()方法
// 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_2012hotspotsrcsharevm untimesynchronizer.cpp文件中，它是最终实现hashCode()和identityHashCode()方法的方法，核心的实现代码如下，我们从这里也可以看得到，还是比较复杂的，并不是简单取一个对象的引用地址那么简单。
// 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类的hashCode()方法如下所示——在 openjdk-6-src-b27-26_oct_2012jdksrcshareclassesjavalang 目录下，它是一个本地方法
/** * 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类的hashCode()方法的本地c语言的实现——Object.c——在 openjdk-6-src-b27-26_oct_2012jdksrcshare ativejavalang 目录下，他调用的也是JVM_IHashCode()方法，由此可见我们在hashCode和identityHashCode 的关系中的观点，在此处也得到了再次的验证
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}, };
7：如上所示，经过一步步的分析，我们已经了解到了 hashCode和identityHashCode底层到底是怎么生成的，不过有些事情这里要在下面补充一下。
7-1：本地方法是什么东西？
本地方法是指用本地程序设计语言，比如：c或者c++，来编写的特殊方法。在java语言中通过native关键字来修饰，通过Java Native Interface(JNI)技术来支持java应用程序来调用本地方法。
7-2：本地方法的特点是什么？
本地方法在本地语言中可以执行任意的计算任务，并返回到java程序设计语言中。
7-3：本地方法的用途是有哪些？
从历史上看本地方法主要有三种用途。
1）提供“访问特定于平台的机制”的能力，比如：访问注册表和文件锁。
2）提供访问遗留代码库的能力，从而可以访问遗留数据。
3）可以通过本地语言，编写应用程序中注重性能的部分，以提高系统的性能。
7-4：使用本地方法的优缺点是什么？
总体来讲使用本地方需要格外的谨慎，因为本地代码中的一个bug就有可能破坏掉整个应用程序。
使用本地代码的优点是：提高系统性能，访问特定于平台的机制。
使用本地代码的缺点是：
1）因为本地语言是不安全的，所以，使用本地方法的应用程序也不能免受内存毁坏错误的影响。
2）因为本地语言是与平台相关的，使用本地方法的应用程序也不再是可自由移植的。
3）使用本地方法的应用程序更难调试
4）在进入和退出本地代码时，需要相关的固定开销，所以，如果本地代码时做的少量的工作，本地方法就可能降低性能。
5）需要“胶合代码”的本地方法编写起来的单调乏味，并且难以阅读。
8：参考