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面试必问的HashCode技术内幕

2022-08-03    是啊超ya
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hashCode的内幕

tips:面试常问/常用/常出错

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<Integer> integerList = new ArrayList<Integer>();
        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)

 

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)

 

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:

 

64位:

image.png

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

 


 

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

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?

 

srcsharevmprimsjvm.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

 


 

先说生成流程,留个印象:

 

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 = ((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() ;//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<intptr_t>(obj) >> 3 ;
     value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;//随机数位移异或计算
  } else  if (hashCode == 2) {
     value = 1 ;            // 返回1
  } else  if (hashCode == 3) {
     value = ++GVars.hcSequence ;//返回一个Sequence序列号
  } else  if (hashCode == 4) {
     value = cast_from_oop<intptr_t>(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 = 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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