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面试必问的HashCode技术内幕( 二 )

HashCode


4) 怎么生成的？接上文 , 我们追究一下 ， 它详细的生成及移动过程 。
我们都知道 ， 这货是个本地方法
public native int hashCode();那就需要借助上面提到的办法 ， 通过JVM虚拟机源码 ， 查看hashcode的生成
1）先从Object.c开始找hashCode映射
srcsharenativejavalangObject.c
JNIEXPORT void JNICALL//jni调用//全路径：java_lang_Object_registerNatives是java对应的包下方法Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls){//jni环境调用；下面的参数methods对应的java方法(*env)->RegisterNatives(env, cls,methods, sizeof(methods)/sizeof(methods[0]));}JAVA--------------------->C++函数对应
//JAVA方法（返回值）----->C++函数对象static JNINativeMethod methods[] = {//JAVA方法返回值（参数）c++函数{"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},};JVM_IHashCod在哪里呢？
2）全局检索JVM_IHashCode
完全搜不到这个方法名 ， 只有这个还凑合有点像 ， 那这是个啥呢？

面试必问的HashCode技术内幕

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srcsharevmprimsjvm.cpp
/*JVM_ENTRY is a preprocessor macro thatadds some boilerplate code that is common for all functions of HotSpot JVM API.This API is a connection layer between the native code of JDK class library and the JVM.JVM_ENTRY是一个预加载宏 ， 增加一些样板代码到jvm的所有function中这个api是位于本地方法与jdk之间的一个连接层 。所以 ， 此处才是生成hashCode的逻辑！*/JVM_ENTRY(jint, JVM_IHashCode(JNIEnv* env, jobject handle))JVMWrApper("JVM_IHashCode");//调用了ObjectSynchronizer对象的FastHashCode return handle == NULL ? 0 : ObjectSynchronizer::FastHashCode (THREAD, JNIHandles::resolve_non_null(handle)) ;JVM_END3）继续 ， 
ObjectSynchronizer::FastHashCode
面试必问的HashCode技术内幕

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面试必问的HashCode技术内幕

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先说生成流程 ， 留个印象：
面试必问的HashCode技术内幕

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intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {//是否开启了偏向锁(Biased：偏向 ， 倾向)if (UseBiasedLocking) {//如果当前对象处于偏向锁状态if (obj->mark()->has_bias_pattern()) {Handle hobj (Self, obj) ;assert (Universe::verify_in_progress() ||!SafepointSynchronize::is_at_safepoint(),"biases should not be seen by VM thread here");//那么就撤销偏向锁（达到无锁状态 ， revoke：废除）BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());obj = hobj() ;//断言下 ， 看看是否撤销成功（撤销后为无锁状态）assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");}}// ……ObjectMonitor* monitor = NULL;markOop temp, test;intptr_t hash;//读出一个稳定的mark;防止对象obj处于膨胀状态；//如果正在膨胀 ， 就等他膨胀完毕再读出来markOop mark = ReadStableMark (obj);//是否撤销了偏向锁（也就是无锁状态）（neutral：中立 ， 不偏不斜的）if (mark->is_neutral()) {//从mark头上取hash值hash = mark->hash();//如果有 ， 直接返回这个hashcode（xor）if (hash) {// if it has hash, just return itreturn hash;}//如果没有就新生成一个(get_next_hash)hash = get_next_hash(Self, obj);// allocate a new hash code//生成后 ， 原子性设置 ， 将hash放在对象头里去 ， 这样下次就可以直接取了temp = mark->copy_set_hash(hash); // merge the hash code into header// use (machine word version) atomic operation to install the hashtest = (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.//如果已经升级成了重量级锁 ， 那么找到它的monitor//也就是我们所说的内置锁(objectMonitor) ， 这是c里的数据类型//因为锁升级后 ， mark里的bit位已经不再存储hashcode ， 而是指向monitor的地址//而升级的markword呢？被移到了c的monitor里} else if (mark->has_monitor()) {//沿着monitor找header ， 也就是对象头monitor = mark->monitor();temp = monitor->header();assert (temp->is_neutral(), "invariant") ;//找到header后取hash返回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())) {//轻量级锁的话 ， 也是从java对象头移到了c里 ， 叫helpertemp = mark->displaced_mark_helper(); // this is a lightweight monitor ownedassert (temp->is_neutral(), "invariant") ;hash = temp->hash();// by current thread, check if the displaced//找到 ， 返回if (hash) {// header contains hash codereturn hash;}}......略


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