Java CAS底层实现原理实例详解

//var1为CAS操作的对象，offset为var1某个属性的地址偏移值，expected为期望值，var2为要设置的值，利用JNI来完成CPU指令的操作
public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);
public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);
public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);
public native Object getObjectVolatile(Object var1, long var2);
public native void putObjectVolatile(Object var1, long var2, Object var4);

Hotspot源码中关于unsafe的实现hotspot\src\share\vm\prims\unsafe.cpp
UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint e, jint x))
 UnsafeWrapper("Unsafe_CompareAndSwapInt");
 oop p = JNIHandles::resolve(obj);根据偏移量,计算value的地址。这里的offset就是 AtomaicInteger中的valueOffset
 jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);
 return (jint)(Atomic::cmpxchg(x, addr, e)) == e;
UNSAFE_END\hotspot\src\share\vm\runtime\atomic.cppunsigned Atomic::cmpxchg(unsigned int exchange_value,
             volatile unsigned int* dest, unsigned int compare_value) {
 assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
 return (unsigned int)Atomic::cmpxchg((jint)exchange_value, (volatile jint*)dest,
                    (jint)compare_value);
}根据操作系统类型调用不同平台下的重载函数，这个在预编译期间编译器会决定调用哪个平台下的重载

linux_x86底层实现\hotspot\src\os_cpu\linux_x86\vm\atomic_linux_x86.inline.hpp
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;
}

windows_x86底层实现
hotspot\src\os_cpu\windows_x86\vmatomic_linux_x86.inline.hpp
inline jint   Atomic::cmpxchg  (jint   exchange_value, volatile jint*   dest, jint   compare_value) {
 // alternative for InterlockedCompareExchange
 int mp = os::is_MP();
 __asm {
  mov edx, dest
  mov ecx, exchange_value
  mov eax, compare_value
  LOCK_IF_MP(mp)
  cmpxchg dword ptr [edx], ecx
 }
}