JDK 1.8 sun.misc.Unsafe类CAS底层实现

在java.util.concurrent包下面的很多类为了追求性能都采用了sun.misc.Unsafe类中的CAS操作，从而避免使用synchronized等加锁方式带来性能上的不足。

在sun.misc.Unsafe中CAS方法如下：

    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);

在JDK1.8中只有上述三个CAS方法，其方法参数含义为：var1为待修改的field对象；var2为field对象偏移量，为long型；var4为期望值；var5或var6为替换值，当var1[offset] == var4则设置var1[offset] = var5(var6)。

这三个方法都是native方法，可以查看hotspot源码查看其底层实现：(hotspot/src/share/vm/prims/unsafe.cpp)

#define FN_PTR(f) CAST_FROM_FN_PTR(void*, &f)

{CC"compareAndSwapObject", CC"("OBJ"J"OBJ""OBJ")Z",  FN_PTR(Unsafe_CompareAndSwapObject)},
{CC"compareAndSwapInt",  CC"("OBJ"J""I""I"")Z",      FN_PTR(Unsafe_CompareAndSwapInt)},
{CC"compareAndSwapLong", CC"("OBJ"J""J""J"")Z",      FN_PTR(Unsafe_CompareAndSwapLong)},

 

UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapObject(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jobject e_h, jobject x_h))
  UnsafeWrapper("Unsafe_CompareAndSwapObject");
  oop x = JNIHandles::resolve(x_h); // 更新值
  oop e = JNIHandles::resolve(e_h); // 期望值
  oop p = JNIHandles::resolve(obj); // 更新对象
  HeapWord* addr = (HeapWord *)index_oop_from_field_offset_long(p, offset); // 根据偏移量offset获取内存中的具体位置
  oop res = oopDesc::atomic_compare_exchange_oop(x, addr, e, true); // 调用方法执行CAS操作
  jboolean success  = (res == e);  // 如果返回值res==e则表明满足compare条件，swap成功
  if (success)
    update_barrier_set((void*)addr, x); // 更新memory barrier
  return success;
UNSAFE_END

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

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

先来看下Unsafe_CompareAndSwapObject方法，该方法通过调用index_oop_from_field_offset_long方法找到需要执行CAS对象的具体地址，然后调用atomic_compare_exchange_oop方法执行CAS操作。

继续深入atomic_compare_exchange_oop方法看一下，源码如下

// 声明在hotspot/src/share/vm/oops/oop.hpp
static oop atomic_compare_exchange_oop(oop exchange_value,
                                       volatile HeapWord *dest,
                                       oop compare_value,
                                       bool prebarrier = false);

// 定义在hotspot/src/share/vm/oops/oop.inline.hpp
inline oop oopDesc::atomic_compare_exchange_oop(oop exchange_value,
                                                volatile HeapWord *dest,
                                                oop compare_value,
                                                bool prebarrier) {
  if (UseCompressedOops) {
    if (prebarrier) {
      update_barrier_set_pre((narrowOop*)dest, exchange_value);
    }
    // encode exchange and compare value from oop to T
    narrowOop val = encode_heap_oop(exchange_value);
    narrowOop cmp = encode_heap_oop(compare_value);

    narrowOop old = (narrowOop) Atomic::cmpxchg(val, (narrowOop*)dest, cmp);
    // decode old from T to oop
    return decode_heap_oop(old);
  } else {
    if (prebarrier) {
      update_barrier_set_pre((oop*)dest, exchange_value);
    }
    return (oop)Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value);
  }
}

在atomic_compare_exchange_oop方法中，核心的CAS操作最终是调用了Atomic::cmpxchg(val, (narrowOop*)dest, cmp)函数或者Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value)函数。

 Atomic::cmpxchg(val, (narrowOop*)dest, cmp)函数虽然有很多重载函数，但最终都是调用的下面的函数：

// hotspot/src/share/vm/runtime/Atomic.cpp
jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte* dest, jbyte compare_value) {
  assert(sizeof(jbyte) == 1, "assumption.");
  uintptr_t dest_addr = (uintptr_t)dest;
  uintptr_t offset = dest_addr % sizeof(jint);
  volatile jint* dest_int = (volatile jint*)(dest_addr - offset);
  jint cur = *dest_int; // 对象当前值
  jbyte* cur_as_bytes = (jbyte*)(&cur);  // 当前值cur的地址
  jint new_val = cur;
  jbyte* new_val_as_bytes = (jbyte*)(&new_val);  // new_val地址
  // new_val存exchange_value，后面修改则直接从new_val中取值
  new_val_as_bytes[offset] = exchange_value;
  // 比较当前值与期望值，如果相同则更新，不同则直接返回
  while (cur_as_bytes[offset] == compare_value) {
    // 调用汇编指令cmpxchg执行CAS操作，期望值为cur，更新值为new_val
    jint res = cmpxchg(new_val, dest_int, cur);
    if (res == cur) break;
    cur = res;
    new_val = cur;
    new_val_as_bytes[offset] = exchange_value;
  }
  // 返回当前值
  return cur_as_bytes[offset];
}

Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value)函数在不同系统中都有各自的声明，但是最终都是调用的下面的函数：

// hotspot/src/os_cpu/solaris_x86/vm/Atomic_solaris_x86.inline.hpp

  // This is the interface to the atomic instruction in solaris_i486.s.
  jlong _Atomic_cmpxchg_long_gcc(jlong exchange_value, volatile jlong* dest, jlong compare_value, int mp);

  inline jlong _Atomic_cmpxchg_long(jlong exchange_value, volatile jlong* dest, jlong compare_value, int mp) {
#ifdef AMD64
    __asm__ __volatile__ (LOCK_IF_MP(%4) "cmpxchgq %1,(%3)"
                        : "=a" (exchange_value)
                        : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
                        : "cc", "memory");
    return exchange_value;
#else
    return _Atomic_cmpxchg_long_gcc(exchange_value, dest, compare_value, os::is_MP());

    #if 0
    // The code below does not work presumably because of the bug in gcc
    // The error message says:
    //   can't find a register in class BREG while reloading asm
    // However I want to save this code and later replace _Atomic_cmpxchg_long_gcc
    // with such inline asm code:

    volatile jlong_accessor evl, cvl, rv;
    evl.long_value = exchange_value;
    cvl.long_value = compare_value;
    int mp = os::is_MP();

    __asm__ volatile ("cmp $0, %%esi\n\t"
       "je 1f \n\t"
       "lock\n\t"
       "1: cmpxchg8b (%%edi)\n\t"
       : "=a"(cvl.words[0]),   "=d"(cvl.words[1])
       : "a"(cvl.words[0]), "d"(cvl.words[1]),
         "b"(evl.words[0]), "c"(evl.words[1]),
         "D"(dest), "S"(mp)
       :  "cc", "memory");
    return cvl.long_value;
    #endif // if 0
#endif // AMD64
  }

在这个方法中废弃了32位系统的cmpxchg8b指令实现CAS操作方式，只提供了AMD64位操作系统cmpxchgq指令实现方式。

从上面可以看出无论是Atomic::cmpxchg(val, (narrowOop*)dest, cmp)函数或者Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value)函数，二者最终都是通过一条汇编指令实现CAS操作的。

Unsafe_CompareAndSwapInt和Unsafe_CompareAndSwapLong两个方法都是调用Atomic::cmpxchg(val, (narrowOop*)dest, cmp)函数实现的，这个函数上面已经解释过。

综合上面的源码分析，可以知道sun.misc.Unsafe类中的CAS都是通过一条汇编指令实现的，这也就不难理解为什么这个操作可以保证原子性了。

 

参考文章：

http://blog.csdn.net/qqqqq1993qqqqq/article/details/75211993

https://www.cnblogs.com/dennyzhangdd/p/6734933.html