前言
哈希算法又称摘要算法，核心在于有一个 hash 函数，将输入映射到一个固定位数的值，这个值就叫哈希值或哈希码（HashCode）。哈希的目的在于将原本复杂、不规则的数据转化为简洁的、固定长度的值，使得数据的存储和检索更加高效。广泛应用于数据存储（如哈希表）、数据完整性验证（如 MD5 、 SHA ）、密码学（加密）以及区块链等领域。
下面我们就顺着源码，一步两步（一步一步似爪牙）探索是怎么实现的吧！
一、Java 中 hashCode 是什么？
Java 中的 hashCode ，是 Object 下的一个 native 方法
/**
* 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 {@link
* equals(Object) equals} 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 equals(Object) equals} 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>
*
* @implSpec
* As far as is reasonably practical, the {@code hashCode} method defined
* by class {@code Object} returns distinct integers for distinct objects.
*
* @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();
根据注释可知
- 该方法主要用于支持哈希表，如 HashMap
- 对于同一个对象如果中用于比较的字段未被修改，那么多次调用 hashCode() ，应返回相同的哈希值。
- 如果两个对象根据 equals() 相等，那么哈希值也应该相等
- 如果两个对象根据 equals() 不相等，则不做要求
二、native 具体实现
1.1首先下载源码
jdk 8 源码 hg.openjdk.org/jdk8u/jdk8u… hotspot 8源码 hg.openjdk.org/jdk8u/jdk8u… jdk17及以上版本 github.com/openjdk/jdk
2.1、入口点
OpenJDK的 src/share/vm/prims/jvm.h
/*************************************************************************
PART 1: Functions for Native Libraries
************************************************************************/
/*
* java.lang.Object
*/
JNIEXPORT jint JNICALL
JVM_IHashCode(JNIEnv *env, jobject obj);
2.2、JVM_ENTRY
hotspot 中的 \src\share\vm\prims\jvm.cpp
// java.lang.Object ///////////////////////////////////////////////
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
2.3、FastHashCode
hotspot 中的 src\share\vm\runtime\synchronizer.cpp FastHashCode 方法有点长核心在于下面这行，如果对象头里没有就尝试生成
hash = get_next_hash(Self, obj);
源码如下
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()) {
// Box and unbox the raw reference just in case we cause a 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() ||
!SafepointSynchronize::is_at_safepoint(), "invariant") ;
assert (Universe::verify_in_progress() ||
Self->is_Java_thread() , "invariant") ;
assert (Universe::verify_in_progress() ||
((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 = (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.
} 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 = (markOop) Atomic::cmpxchg_ptr(temp, monitor, 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;
}
2.4、get_next_hash
hotspot 中的 src\share\vm\runtime\synchronizer.cpp 恭喜看官，这里就是真正的hash逻辑啦，支持六种模式 0. 随机数 1. 内存地址做移位再和一个随机数做异或 2. 固定值1 3. 自增序列的当前值 4. 内存地址 5. 当前线程有关的一个随机数+三个确定值，运用xorshift随机数算法得到的一个随机数
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;
}
线程状态的定义 hotspot \src\share\vm\runtime\thread.cpp
// thread-specific hashCode stream generator state - Marsaglia shift-xor form
_hashStateX = os::random() ;
_hashStateY = 842502087 ;
_hashStateZ = 0x8767 ; // (int)(3579807591LL & 0xffff) ;
_hashStateW = 273326509 ;
2.5、那么，默认是哪种模式呢
hotspot src\share\vm\runtime\globals.hpp
product(intx, hashCode, 5,
"(Unstable) select hashCode generation algorithm")
可以看到默认是5，第六种策略即xorshift基于异或和位移的随机数生成算法，当然也可以通过在启动参数中添加-XX:hashCode=4，改变默认hashCode计算策略 ps:源码都是基于jdk1.8的，jdk-17，以及最新的jdk-24 默认也是此策略
三、String hashcode() 实现
public static int hashCode(byte[] value) {
int h = 0;
for (byte v : value) {
h = 31 * h + (v & 0xff);
}
return h;
}
31 * h 进行散列 v & 0xff ，将v转换为一个正值 (然而调用 String hashCode() 会返回负值么？评论区见) 最后追加 31 * h + (v & 0xff) 特点就是快速高效，内容相同的字符串，产生的哈希值相同
四、总结
1、Java 中默认 hashcode() 实现是基于 Xorshift 的伪随机数生成算法，受到当前线程有关的一个随机数+三个确定值影响。内容相同的对象，每次new对象，哈希值都不同 2、String 类重写了 hashcode()，对于内容相同字符串，会产出同样的哈希值 3、如果对象需要存到哈希表中，如 HashMap、HashSet，一定要注意重写 equals()和 hashCode() 方法
