java中hashCode()源码小记

2021-08-11 03:51:59

java如何生成hashCode

JDK8 Object基类的hashCode()方法的源码如下。hashCode方法是一个本地native方法，这里主要看它给的注释。

/**
 * Returns a hash code value for the object. This method is
 * supported for the benefit of hash tables such as those provided by
 * {@link java.util.HashMap}.
 * <p>
 * The general contract of {@code hashCode} is:
 * <ul>
 * <li>Whenever it is invoked on the same object more than once during
 *     an execution of a Java application, the {@code hashCode} method
 *     must consistently return the same integer, provided no information
 *     used in {@code equals} 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 {@code equals(Object)}
 *     method, then calling the {@code hashCode} 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 {@code hashCode} 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 hash tables.
 * </ul>
 * <p>
 * As much as is reasonably practical, the hashCode method defined by
 * class {@code Object} 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&trade; programming language.)
 *
 * @return  a hash code value for this object.
 * @see     java.lang.Object#equals(java.lang.Object)
 * @see     java.lang.System#identityHashCode
 */
public native int hashCode();

hashCode()方法依据对象的内存地址为对象返回一个哈希值，这个哈希值会在一些哈希表中使用，比如HashMap，HashTable, HashSet。并且这个方法在Object基类中定义。

注释中对这个hashCode()这个方法提出了如下几个要求（contract）：

在同一个java进程中，对同一个对象多次调用这个方法得到的返回值必须一样。
如果两个对象通过equals() 比较后相等，那么他们的hashCode()返回必须一样。
如果两个对象equals() 比较不相等，他们的hashCode()返回不需要不一样。

提出几个问题

既然hashCode返回的值与对象的内存的地址有关，那在JVM中发生垃圾回收处理（GC)后，对象的内存地址必然发生了变化，那hashCode的返回值是否会改变？

不会。

第一次调用hashCode()方法，会根据对象的内存地址计算得到一个int类型数值，并保存在对象头中。之后再调用，直接取用这个值。当然这只对没有被我们重写的hashCode方法。重写过的hashCode每次调用都会进行计算，这也要求我们重写hashCode()方法时注意返回的值不能随意改变。

下图截取自周志明的《深入理解java虚拟机》

为什么equals和hashCode方法要求一起重写

主要是由于 hashMap等集合类中对hashCode方法和equals方法的使用，如果不规范随意的重写这两个方法可能会导致hashMap集合类不能正常使用，重写hashCode()和equals()方法要严格遵守源码注释中提到的 Contract。

从openjdk中看hashCode实现

由于Oracle jdk的开源协议问题，由c/c++实现的本地方法代码文件被压解成dll库文件，我们不能直接查看，选择下载 openjdk查看

openjdk官网

hashCode()方法在openjdk中的实现应该是synchronizer.cpp文件，路径是 hotspot/src/share/vm/runtime/synchronizer.cpp

贴出一部分代码：

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() ;
  } 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;
}