面试必问的HashCode技术内幕

3 hashCode的内幕

tips:面试常问/常用/常出错
【面试必问的HashCode技术内幕】hashCode到底是什么?是不是对象的内存地址?
1) 直接用内存地址? 目标:通过一个Demo验证这个hasCode到底是不是内存地址
public native int hashCode();

com.hashcode.HashCodeTest
package com.hashcode; import org.openjdk.jol.vm.VM; import java.util.ArrayList; import java.util.List; public class HashCodeTest { //目标:只要发生重复,说明hashcode不是内存地址,但还需要证明(JVM代码证明) public static void main(String[] args) { List integerList = new ArrayList(); int num = 0; for (int i = 0; i < 150000; i++) { //创建新的对象 Object object = new Object(); if (integerList.contains(object.hashCode())) { num++; //发生重复(内存地址肯定不会重复) } else { integerList.add(object.hashCode()); //没有重复 } } System.out.println(num + "个hashcode发生重复"); System.out.println("List合计大小" + integerList.size() + "个"); } }

15万个循环,发生了重复,说明hashCode不是内存地址(严格的说,肯定不是直接取的内存地址)
面试必问的HashCode技术内幕
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思考一下,为什么不能直接用内存地址呢?
  • 提示:jvm垃圾收集算法,对象迁移……
那么它到底是什么?如何生成的呢
2) 不是地址那在哪里? 既然不是内存地址,那一定在某个地方存着,那在哪里存着呢?
答案:在对象头里!(画图。类在jvm内存中的布局)
面试必问的HashCode技术内幕
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对象头分为两部分,一部分是上面指向class描述的地址Klass,另一部分就是Markword
而我们这里要找的hashcode在Markword里!(标记位意义,不用记!)
32位:
面试必问的HashCode技术内幕
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64位:
面试必问的HashCode技术内幕
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3) 什么时候生成的? new的瞬间就有hashcode了吗??
show me the code!我们用代码验证
package com.hashcode; import org.openjdk.jol.info.ClassLayout; import org.openjdk.jol.vm.VM; public class ShowHashCode {public static void main(String[] args) { ShowHashCode a = new ShowHashCode(); //jvm的信息 System.out.println(VM.current().details()); System.out.println("-------------------------"); //调用之前打印a对象的头信息 //以表格的形式打印对象布局 System.out.println(ClassLayout.parseInstance(a).toPrintable()); System.out.println("-------------------------"); //调用后再打印a对象的hashcode值 System.out.println(Integer.toHexString(a.hashCode())); System.out.println(ClassLayout.parseInstance(a).toPrintable()); System.out.println("-------------------------"); //有线程加重量级锁的时候,再来看对象头 new Thread(()->{ try { synchronized (a){ Thread.sleep(5000); } } catch (InterruptedException e) { e.printStackTrace(); } }).start(); System.out.println(Integer.toHexString(a.hashCode())); System.out.println(ClassLayout.parseInstance(a).toPrintable()); }}

结果分析
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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结论:在你没有调用的时候,这个值是空的,当第一次调用hashCode方法时,会生成,加锁以后,不知道去哪里了……
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
src\share\native\java\lang\Object.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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src\share\vm\prims\jvm.cpp
/* JVM_ENTRY is a preprocessor macro that adds 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_END

3)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 it return 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 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. //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helper 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 //找到,返回 if (hash) {// header contains hash code return hash; } }......略

问:
为什么要先撤销偏向锁到无锁状态,再来生成hashcode呢?这跟锁有什么关系?
答:
mark word里,hashcode存储的字节位置被偏向锁给占了!偏向锁存储了锁持有者的线程id
(参考上面的markword图)
扩展:关于hashCode的生成算法(了解)
// hashCode() generation : // 涉及到c++算法领域,感兴趣的同学自行研究 // 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 = https://www.it610.com/article/((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 = https://www.it610.com/article/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 = https://www.it610.com/article/os::random() ; //返回随机数 } 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. //和地址相关,但不是地址;右移+异或算法 intptr_t addrBits = cast_from_oop(obj) >> 3 ; value = https://www.it610.com/article/addrBits ^ (addrBits>> 5) ^ GVars.stwRandom ; //随机数位移异或计算 } elseif (hashCode == 2) { value = https://www.it610.com/article/1 ; // 返回1 } elseif (hashCode == 3) { value = ++GVars.hcSequence ; //返回一个Sequence序列号 } elseif (hashCode == 4) { value = cast_from_oop(obj) ; //也不是地址 } else { //常用 // 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. //马萨利亚教授写的xor-shift 随机数算法(异或随机算法) 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 = https://www.it610.com/article/v ; }

5)总结 通过分析虚拟机源码我们证明了hashCode不是直接用的内存地址,而是采取一定的算法来生成
hashcode值的存储在mark word里,与锁共用一段bit位,这就造成了跟锁状态相关性
  • 如果是偏向锁:
一旦调用hashcode,偏向锁将被撤销,hashcode被保存占位mark word,对象被打回无锁状态
  • 那偏偏这会就是有线程硬性使用对象的锁呢?
对象再也回不到偏向锁状态而是升级为重量级锁。hash code跟随mark word被移动到c的object monitor,从那里取
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