深入理解 Java HashCode

1. hashcode
2.使用JOL查看对象信息

1. hashcode

1.Object的hashcode

   默认调用object的hashcode的话，会调用native方法，与本地相关。

public native int hashCode();

 <br />    来自 ： [https://zhuanlan.zhihu.com/p/33915892](https://zhuanlan.zhihu.com/p/33915892)<br />     下载完整的jdk呗[（](https://github.com/dmlloyd/openjdk/tree/jdk8u/jdk8u)[openJDK1.8](https://github.com/dmlloyd/openjdk/tree/jdk8u/jdk8u)[）](https://github.com/dmlloyd/openjdk/tree/jdk8u/jdk8u)。找到[Object.c](https://github.com/dmlloyd/openjdk/blob/jdk8u/jdk8u/jdk/src/share/native/java/lang/Object.c)文件，查看上面的方法映射表发现，hashCode被映射到了一个叫JVM_IHashCode上去了。

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},
};

 顺藤摸瓜去看看JVM_IHashCode到底干了什么？熟悉的味道，在jvm.h里面有方法声明，那实现一定在jvm.cpp里面。不过[jvm.cpp](https://link.zhihu.com/?target=https%3A//github.com/dmlloyd/openjdk/blob/jdk8u/jdk8u/hotspot/src/share/vm/prims/jvm.cpp)对于JVM_IHashCode的实现调用的是ObjectSynchronizer::**FastHashCode**的方法。

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

 发现声明在[synchronizer.hpp](https://link.zhihu.com/?target=https%3A//github.com/dmlloyd/openjdk/blob/jdk/jdk/src/hotspot/share/runtime/synchronizer.hpp) 实现在这里[synchronizer.cpp](https://link.zhihu.com/?target=https%3A//github.com/dmlloyd/openjdk/blob/jdk/jdk/src/hotspot/share/runtime/synchronizer.cpp)<br />

// 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 global Park-Miller 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 = cast_from_oop<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 = cast_from_oop<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;
}
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()) {
      // Handle for oop obj in case of 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() || DumpSharedSpaces ||
         !SafepointSynchronize::is_at_safepoint(), "invariant");
  assert(Universe::verify_in_progress() || DumpSharedSpaces ||
         Self->is_Java_thread() , "invariant");
  assert(Universe::verify_in_progress() || DumpSharedSpaces ||
         ((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 = obj->cas_set_mark(temp, 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, inflate_cause_hash_code);
  // 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 = Atomic::cmpxchg(temp, monitor->header_addr(), 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;
}

2.Java Object.hashCode()返回的是对象内存地址？不是！

OpenJDK8 默认hashCode的计算方法是通过和当前线程有关的一个随机数+三个确定值，运用Marsaglia’s xorshift scheme随机数算法得到的一个随机数。和对象内存地址无关。

  当然也可以自己实现hashcode方法，关于hashcode() 与 equals()。主要是利用hashcode 可以判断2个对象的**不等**，hashcode相等，对象不一定相等，但hashcode不等，可以肯定2个对象不相等。在很多地方判断对象等不等。如果equals 定义了2个对象是相等的，需要注意hashcode还是不是相等的。

3.String类的hasCode()

public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;
            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

2.使用JOL查看对象信息

maven 依赖

<dependency>
                <groupId>org.openjdk.jol</groupId>
                <artifactId>jol-core</artifactId>
                <version>0.9</version>
</dependency>

为了内存比较“整齐”，关闭压缩指针，启动参数加上-XX:-UseCompressedOops

1.测试对象

class AAA{
    private int number;
}

2.测试方式1

public static void main(String[] args) {
        AAA aaa = new AAA();
        System.out.println("---------before invoke hascode()-----------------");
        System.out.println(ClassLayout.parseInstance(aaa).toPrintable());
        /*invoke hashcode() 转换成16进制*/
        System.out.println(Integer.toHexString(aaa.hashCode()));
        System.out.println("------------after invoke hascode()-----------------");
        System.out.println(ClassLayout.parseInstance(aaa).toPrintable());
        synchronized (aaa){
            System.out.println("---------in synchronized() func--------------");
            System.out.println(Integer.toHexString(aaa.hashCode()));
            System.out.println(ClassLayout.parseInstance(aaa).toPrintable());
            System.out.println(Integer.toHexString(aaa.hashCode()));
        }
        System.out.println("---------after invoke  synchronized()--------------");
        System.out.println(Integer.toHexString(aaa.hashCode()));
        System.out.println(ClassLayout.parseInstance(aaa).toPrintable());
    }

输出

---------before invoke hascode()-----------------
AAA object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           a8 35 85 1c (10101000 00110101 10000101 00011100) (478492072)
     12     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
     16     4    int AAA.number                                0
     20     4        (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
7e0b37bc
------------after invoke hascode()-----------------
AAA object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           01 bc 37 0b (00000001 10111100 00110111 00001011) (188201985)
      4     4        (object header)                           7e 00 00 00 (01111110 00000000 00000000 00000000) (126)
      8     4        (object header)                           a8 35 85 1c (10101000 00110101 10000101 00011100) (478492072)
     12     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
     16     4    int AAA.number                                0
     20     4        (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
---------in synchronized() func--------------
7e0b37bc
AAA object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           b0 f4 34 03 (10110000 11110100 00110100 00000011) (53802160)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           a8 35 85 1c (10101000 00110101 10000101 00011100) (478492072)
     12     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
     16     4    int AAA.number                                0
     20     4        (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
7e0b37bc
---------after invoke  synchronized()--------------
7e0b37bc
AAA object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           01 bc 37 0b (00000001 10111100 00110111 00001011) (188201985)
      4     4        (object header)                           7e 00 00 00 (01111110 00000000 00000000 00000000) (126)
      8     4        (object header)                           a8 35 85 1c (10101000 00110101 10000101 00011100) (478492072)
     12     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
     16     4    int AAA.number                                0
     20     4        (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

可以看到，在调用一次 hashcode之后，就会在object header 中生成hashcode。注意区分大小端模式。
搜狗截图20191011144038.png

object header 在有锁的情况下会发生变化，但是不会改变hashcode的值。

为什么有锁的状态下，头部的hashcode会变。无锁后又变回来了？

另外，调用了一次hashcode 就会在栈帧中存在hashcode，但是与偏向锁的字段发生冲突，因此jvm采用调用过hashcode的，不会存在偏向锁。为什么其它锁可以呢？因为保存了指向栈中锁记录的指针，可以记录在里面。
搜狗截图20191011162436.png