• 原子量和普通变量相比，主要体现在读写的线程安全上。对原子量的是原子的(比如多线程下的共享变量i++就不是原子的)，由CAS操作保证原子性。对原子量的读可以读到最新值，由volatile关键字来保证可见性。
• 原子量多用于数据统计(如接口调用次数)、一些序列生成(多线程环境下)以及一些同步数据结构中。

/**      Implementation of class sun.misc.Unsafe*/
...
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);jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);return (jint)(Atomic::cmpxchg(x, addr, e)) == e;
UNSAFE_END

    {CC"putOrderedInt",      CC"("OBJ"JI)V",             FN_PTR(Unsafe_SetOrderedInt)},...
// The non-intrinsified versions of setOrdered just use setVolatile
UNSAFE_ENTRY(void, Unsafe_SetOrderedInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint x))UnsafeWrapper("Unsafe_SetOrderedInt");SET_FIELD_VOLATILE(obj, offset, jint, x);
UNSAFE_END

  do_intrinsic(_putOrderedInt, sun_misc_Unsafe,putOrderedInt_name, putOrderedInt_signature,F_RN)

    {CC"compareAndSwapLong", CC"("OBJ"J""J""J"")Z",      FN_PTR(Unsafe_CompareAndSwapLong)},...
UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapLong(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jlong e, jlong x))UnsafeWrapper("Unsafe_CompareAndSwapLong");Handle p (THREAD, JNIHandles::resolve(obj));jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));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;}
UNSAFE_END

Cpp代码  

1. {CC"putOrderedLong",     CC"("OBJ"JJ)V",             FN_PTR(Unsafe_SetOrderedLong)},
2. ...
3. UNSAFE_ENTRY(void, Unsafe_SetOrderedLong(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jlong x))
4. UnsafeWrapper("Unsafe_SetOrderedLong");
5. #if defined(SPARC) || defined(X86)
6. // Sparc and X86 have atomic jlong (8 bytes) instructions
7. SET_FIELD_VOLATILE(obj, offset, jlong, x);
8. #else
9. // Keep old code for platforms which may not have atomic long (8 bytes) instructions
10. {
11. if (VM_Version::supports_cx8()) {
12. SET_FIELD_VOLATILE(obj, offset, jlong, x);
13. }
14. else {
15. Handle p (THREAD, JNIHandles::resolve(obj));
16. jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));
17. ObjectLocker ol(p, THREAD);
18. *addr = x;
19. }
20. }
21. #endif
22. UNSAFE_END

Cpp代码  

1. do_intrinsic(_putOrderedLong,           sun_misc_Unsafe,        putOrderedLong_name, putOrderedLong_signature, F_RN)  \

Cpp代码  

1. case vmIntrinsics::_putOrderedLong:
2. return inline_unsafe_ordered_store(T_LONG);
3. ...
4. ol LibraryCallKit::inline_unsafe_ordered_store(BasicType type) {
5. // This is another variant of inline_unsafe_access, differing in
6. // that it always issues store-store ("release") barrier and ensures
7. // store-atomicity (which only matters for "long").
8. if (callee()->is_static())  return false;  // caller must have the capability!
9. ...//忽略不重要部分
10. // Ensure that the store is atomic for longs:
11. bool require_atomic_access = true;
12. Node* store;
13. if (type == T_OBJECT) // reference stores need a store barrier.
14. store = store_oop_to_unknown(control(), base, adr, adr_type, val, type);
15. else {
16. store = store_to_memory(control(), adr, val, type, adr_type, require_atomic_access);
17. }
18. insert_mem_bar(Op_MemBarCPUOrder);
19. return true;

Cpp代码  

1. Node* GraphKit::store_to_memory(Node* ctl, Node* adr, Node *val, BasicType bt,
2. int adr_idx,
3. bool require_atomic_access) {
4. assert(adr_idx != Compile::AliasIdxTop, "use other store_to_memory factory" );
5. const TypePtr* adr_type = NULL;
6. debug_only(adr_type = C->get_adr_type(adr_idx));
7. Node *mem = memory(adr_idx);
8. Node* st;
9. if (require_atomic_access && bt == T_LONG) {
10. st = StoreLNode::make_atomic(C, ctl, mem, adr, adr_type, val);
11. } else {
12. st = StoreNode::make(_gvn, ctl, mem, adr, adr_type, val, bt);
13. }
14. st = _gvn.transform(st);
15. set_memory(st, adr_idx);
16. // Back-to-back stores can only remove intermediate store with DU info
17. // so push on worklist for optimizer.
18. if (mem->req() > MemNode::Address && adr == mem->in(MemNode::Address))
19. record_for_igvn(st);
20. return st;
21. }

Java代码  

1. public class AtomicReference<V>  implements java.io.Serializable {
2. private static final long serialVersionUID = -1848883965231344442L;
3. private static final Unsafe unsafe = Unsafe.getUnsafe();
4. private static final long valueOffset;
5. static {
6. try {
7. valueOffset = unsafe.objectFieldOffset
8. (AtomicReference.class.getDeclaredField("value"));
9. } catch (Exception ex) { throw new Error(ex); }
10. }
11. private volatile V value;

Java代码  

1. /**
2. * Atomically sets the value to the given updated value
3. * if the current value {@code ==} the expected value.
4. * @param expect the expected value
5. * @param update the new value
6. * @return true if successful. False return indicates that
7. * the actual value was not equal to the expected value.
8. */
9. public final boolean compareAndSet(V expect, V update) {
10. return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
11. }

Cpp代码  

1. {CC"compareAndSwapObject", CC"("OBJ"J"OBJ""OBJ")Z",  FN_PTR(Unsafe_CompareAndSwapObject)},
2. ...
3. UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapObject(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jobject e_h, jobject x_h))
4. UnsafeWrapper("Unsafe_CompareAndSwapObject");
5. oop x = JNIHandles::resolve(x_h);
6. oop e = JNIHandles::resolve(e_h);
7. oop p = JNIHandles::resolve(obj);
8. HeapWord* addr = (HeapWord *)index_oop_from_field_offset_long(p, offset);
9. if (UseCompressedOops) {
10. update_barrier_set_pre((narrowOop*)addr, e);
11. } else {
12. update_barrier_set_pre((oop*)addr, e);
13. }
14. oop res = oopDesc::atomic_compare_exchange_oop(x, addr, e);
15. jboolean success  = (res == e);
16. if (success)
17. update_barrier_set((void*)addr, x);
18. return success;
19. UNSAFE_END

Cpp代码  

1. inline oop oopDesc::atomic_compare_exchange_oop(oop exchange_value,
2. volatile HeapWord *dest,
3. oop compare_value) {
4. if (UseCompressedOops) {
5. // encode exchange and compare value from oop to T
6. narrowOop val = encode_heap_oop(exchange_value);
7. narrowOop cmp = encode_heap_oop(compare_value);
8. narrowOop old = (narrowOop) Atomic::cmpxchg(val, (narrowOop*)dest, cmp);
9. // decode old from T to oop
10. return decode_heap_oop(old);
11. } else {
12. return (oop)Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value);
13. }
14. }

Cpp代码  

1. inline intptr_t Atomic::cmpxchg_ptr(intptr_t exchange_value, volatile intptr_t* dest, intptr_t compare_value) {
2. return (intptr_t)cmpxchg((jlong)exchange_value, (volatile jlong*)dest, (jlong)compare_value);
3. }
4. inline jlong    Atomic::cmpxchg    (jlong    exchange_value, volatile jlong*    dest, jlong    compare_value) {
5. bool mp = os::is_MP();
6. __asm__ __volatile__ (LOCK_IF_MP(%4) "cmpxchgq %1,(%3)"
7. : "=a" (exchange_value)
8. : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
9. : "cc", "memory");
10. return exchange_value;
11. }