unsafe中对应拥有三个方法 compareAndSwapObject ,compareAndSwapInt和compareAndSwapLong ，他们都被标记为nativecompareAndSwapObject它的核心实现为oop res = oopDesc::atomic_compare_exchange_oop(x, addr, e);实现核心如下inline oop oopDesc::atomic_compare_exchange_oop(oop exchange_value, volatile HeapWord dest, oop compare_value) { if (UseCompressedOops) { narrowOop val = encode_heap_oop(exchange_value); narrowOop cmp = encode_heap_oop(compare_value); narrowOop old = (narrowOop) Atomic::cmpxchg(val, (narrowOop)dest, cmp); return decode_heap_oop(old); } else { return (oop)Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value); }}UseCompressedOops: 32位平台运行的程序在64位上会占用更大的长度，可以使用 -XX:+UserCompressedOops压缩指针，达到节约内存的目的。compareAndSwapInt核心代码如下return (jint)(Atomic::cmpxchg(x, addr, e)) == e;compareAndSwapLong核心代码如下if (VM_Version::supports_cx8()) return (jlong)(Atomic::cmpxchg(x, addr, e)) == e;else { jboolean success = false; ObjectLocker ol(p, THREAD); if (addr == e) { addr = x; success = true; } return success;}supports_cx8:判断硬件是不是支持8-byte compare-exchange ， x86架构中通过cpuid指令来获取是否试支持，CMPXCHG8指令 ；SPARC架构也是看 (_features & v9_instructions_m)指令的支持情况Atomic::cmpxchg无论是那个调用，最终都归结到了Atomic上，Atomic.hpp中函数声明如下//比较当前的值和目的地址的值，如果比较成功，就把目的地址的值更改为exchange_value，并返回原来存的值static jbyte cmpxchg (jbyte exchange_value, volatile jbyte dest, jbyte compare_value);static jint cmpxchg (jint exchange_value, volatile jint dest, jint compare_value);static jlong cmpxchg (jlong exchange_value, volatile jlong* dest, jlong compare_value);static unsigned int cmpxchg(unsigned int exchange_value, volatile unsigned int* dest, unsigned int compare_value);static intptr_t cmpxchg_ptr(intptr_t exchange_value, volatile intptr_t* dest, intptr_t compare_value);static void* cmpxchg_ptr(void* exchange_value, volatile void* dest, void* compare_value);从Atomic.cpp可以看到在不同的操作系统中有不同的实现在 windows_x86中，一种实现如下inline jint Atomic::cmpxchg (jint exchange_value, volatile jint* dest, jint compare_value) { int mp = os::is_MP(); //查看是否是多核 __asm { mov edx, dest mov ecx, exchange_value mov eax, compare_value LOCK_IF_MP(mp) cmpxchg dword ptr [edx], ecx }}linux_x86中，实现如下inline jint Atomic::cmpxchg (jint exchange_value, volatile jint* dest, jint compare_value) { int mp = os::is_MP(); asm volatile (LOCK_IF_MP(%4) “cmpxchgl %1,(%3)” : “=a” (exchange_value) : “r” (exchange_value), “a” (compare_value), “r” (dest), “r” (mp) : “cc”, “memory”); return exchange_value;}可以看到最终都是使用操作系统对应的指令来完成都在哪儿用了可以看到Atomic的实现就是用的CAS，比如AtomicInteger的incrementAndGetpublic final int incrementAndGet() { for (;;) { int current = get(); int next = current + 1; if (compareAndSet(current, next)) return next; }}这种一直循环的操作也称作自旋CAS的缺点如果一直没有成功，则一直循环，给CPU带来很大的开销。只能是一个变量ABA问题。一个变量取值为A，恰巧另一个线程将它换成了B然后又换回来了，这个时候再读取还是A，实际上是改变了值。java自身提供了AtomicStampedReference来解决这个问题，原理是添加一个额外的版本来做判断源码来自jdk1.7